CR, calcium sources and bone

[Guest guest]

Hi All,

The below is I thought a good review of CR

and its effect and recommendation for calcium

from various sources to prevent loss of bone

tissue.

Vitamin D appears to be downplayed for having

a role in bone loss.

It may be relevant that " Intake levels for

calcium during caloric restriction studies range from

1000 to 1800 mg/d. "

Table 2 can be located in the below pdf excerpts

using the search function. It evaluates different

strategies to achieve higher calcium levels in

comparing the benefits and deficiencies of using

supplements, dairy foods or plant diets to obtain

adequate levels and prevent bone loss.

Regarding calcium in plant-based diets,

http://www.ajcn.org/cgi/reprint/70/3/543S

is a good review pdf-available to all comparing

alternative vegetarian and dairy choices. It also

seems to have a nice representation of how

protein intakes affect our calcium requirements.

" In this review, the choice of the calcium food source

and the influence of salt, protein,

and caffeine on calcium retention,

and ultimately calcium requirements, are discussed. "

Below is the current paper and its pdf excerpts.

Nutr Rev. 2004 Dec;62(12):468-81.

Caloric restriction and calcium's effect on bone metabolism and

body composition

in overweight and obese premenopausal women.

Radak TL.

... reduction of body weight has been correlated with a reduction

in bone mass,

.... Most studies

demonstrate a positive relationship between calcium intake and bone

mass.

However, during caloric restriction, which is commonly used for

weight loss,

calcium intake has shown mixed results. Calcium from dairy sources

has received

additional attention, beyond its importance to bone, for its role in

regulating

body weight and composition. Dairy foods are perceived as high fat,

and

therefore, are generally minimized or avoided during caloric

restriction. The

current calcium intake for premenopausal women is significantly below

recommendations, and even if met during caloric restriction, may not

be

adequate. This review underscores the need for maintaining at least

adequate

intake levels of calcium, if not more, during weight loss regimens to

minimize

potential long-term detrimental effects on bone metabolism.

PMID: 15648822 [PubMed - in process]

...

Osteoporosis and related diseases are increasing.

Approximately 40% of women at 50 years of age will

experience an osteoporotic fracture during their life-times.

7 Osteoporosis is a disease that causes low bone

mass, resulting in increased susceptibility to various

fractures. It has been estimated that over a women's

lifetime, about one-half of her trabecular bone and one-third

of her cortical bone will be lost.8 The prevalence of

osteoporosis is expected to increase over the coming

decades,9 and is likely to become the most common

disorder in the aging population.10

Caloric restriction is a typical approach employed by

individuals to reduce excess body weight. A significant

proportion of the public engages in weight loss activi-ties,

11 which are recommended to address the health

risks associated with obesity.12-14 The positive effects of

weight loss on chronic diseases have been well docu-mented

both in patient health and in health care

costs.15,16

Obese women are thought to be at decreased risk for

osteoporosis because of the mechanical influence of

increased load on bone mass. A reduction in body weight

has been correlated with a reduction in bone mass, which

could be a catalyst for future osteoporotic disease. It has

not been fully elucidated whether this reduction in bone

mass with weight loss is due to: 1) the weight loss itself,

2) measurement error, or 3) inadequate calcium intake.

Calcium is one of the most investigated nutrients in

relation to bone health. Current calcium intake in pre-menopausal

women is well below national recommenda-

tions. Major calcium food sources, such as dairy prod-ucts,

are perceived as high fat and are therefore generally

minimized or avoided during caloric restriction. This

could have implications for bone health. However, cal-cium

from dairy sources has received additional attention

for its role in regulating body weight and composition,

particularly fat mass, and simultaneously may have a

positive effect on bone health.17,18 This review will

examine caloric restriction for reducing body weight and

its effect on chronic disease and obesity, the effect of

caloric restriction on bone mass, and the role of calcium

in weight loss, body composition, and bone health during

caloric restriction. Considerations among the various

sources of calcium and the effect on other diseases will

also be examined. Suggestions for intake levels and

calcium sources during caloric restriction will be pro-vided

based on available studies.

Obesity and Bone Health: Relationship of Body

Composition and Bone Mass

The effect of obesity on bone metabolism is not under-stood,

but a positive relationship exists between body

weight, body mass index (BMI), and bone mass or bone

mineral density (BMD). This relationship has been

shown for both total body bone mass and for regional

sites, e.g. the spine and femur in pre-, peri-, and post-menopausal

women.19-26 Rico et al.19 assert that body

weight is the chief determinant for bone mass in women.

Low body weight is an independent predictor of low

bone mass later in life.27 Percentage of body fat in

premenopausal and older women is also significantly

related to total body BMD.20 In older women, the per-centage

of body fat also correlates well with BMD.28

Possible explanations for the protective effect of obesity,

aside from mechanical load on bone, may be the addi-tional

conversion of estrogen from androstenedione in

adipose tissue or a reduction in sex hormone binding

globulin.23,29-32 As obesity increases, bone mineraliza-tion

increases, thereby reducing the risk for osteoporotic

diseases.

Many but not all 33 cross-sectional and longitudinal

studies have shown bone loss in premenopausal women.

When examining outcomes related to bone mass, a de-crease

in hip fractures as a result of greater body weight

has not been demonstrated in all studies,28,34-36 but it is

clear that osteoporotic fracture risk is higher in women

with lower body weight than in heavier women.22,23,37

Again, however, these differences have not been found

in all studies.28 Interestingly, a prospective study by

Johansson 38 found BMD to be a better predictor of death

than blood pressure and cholesterol. Furthermore, low

bone mass was an independent predictor of survival.

Effect of Caloric Restriction on Bone Mass

It has been established that bone density can predict

future osteoporotic disease, and that low BMD is one of

the strongest risk factors for hip fractures.39 Low peak

bone mass is also an osteoporotic risk factor.40,27 Many

studies investigating changes in bone mass in women

have suggested that bone mass begins to decrease after

peak attainment in the thirties and prior to meno-pause.

33,41,42 One recent six-year prospective study de-termined

that bone loss at the femoral neck began as

early as the mid-twenties.43 Another study estimated that

99% of peak total body BMD occurs in women between

the ages of 19.6 and 24.6 years, and 99% of total body

bone mineral content (BMC) was attained between the

ages of 22.5 and 29.9 years.44

Observational studies have indicated an increased

risk for hip fracture in women who experience weight

loss during early or middle adulthood and later in

life.35,36,45-48 Several studies have indicated that with

weight loss there is a concomitant loss of bone mass

either for the total body or for regional sites.49-57 Weight

loss protocols in these studies ranged from 10 to 24

weeks in duration, with one lasting as long as 12

months.58 Additionally, in studies examining eating be-haviors

of women, Van Loan et al.59 found on average a

12% lower BMC in women with cognitive dietary re-straint

and normal to low body weight. Furthermore,

Bacon et al.60 found that in a group of premenopausal

obese women with histories of chronic dieting behavior,

one-third had either osteopenia or osteoporosis. These

studies clearly demonstrate that dietary restriction is

negatively associated with bone health.

Role of Calcium on Bone Mass

As one of the major minerals in the skeleton, calcium has

been well researched in relation to bone growth, preser-vation,

and health. The adult human body contains

roughly 1000 to 1500 g of calcium, making it the most

abundant mineral, and 99% of it is located in the bones.61

Current recommendations for women 19 to 50 years of

age are 1000 mg/d.62 The 1994–1996 Continuing Survey

of Food Intakes by Individuals (CSFII) estimated the

mean intake of calcium in women 18 to 50 years of age

to be approximately 640 mg/d—only 64% of the recom-mendation.

62 In a more recent NHANES survey (1999 –

2000) calcium intake was about 770 mg/d for women 20

to 59 years of age, up slightly but still only about 75% of

the recommended level of intake.63 Both averages fall

significantly short of current recommendations and are

quite distant from the upper intake level for calcium set

at 2500 mg/d.62 These averages also fall below a require-ment

of 975 mg/d determined in a premenopausal cal-cium

balance study.64 A NIH Consensus Conference in

1994 highlighted calcium as one of two nutrient defi-469

ciencies in the United States that warrant a national effort

to increase average intake levels.65

Low calcium intake can limit bone formation in

early life and cause bone loss in maturity.66 Two epide-miological

studies confirmed this finding and demon-strated

an increased risk for hip fractures in women later

in life.67-68 Supplemental calcium has been shown to

reduce the risk of hip fractures.69 Variations in calcium

intake during youth have been estimated to affect peak

bone mass by only 5% to 10%, whereas the influence of

hip fracture risk later in life may account for 25% to 50%

of risk.70 While other vitamins and minerals have been

looked at, calcium is clearly the most well researched.71

A meta-analysis of 33 cross-sectional, longitudinal,

and intervention studies found a small but significant

positive correlation between calcium intake from either

supplement or diet and either BMD or BMC in women

18 to 50 years of age.72 Mean calcium intakes were

between 436 and 1437 mg/d. A second meta-analysis

using 49 investigations of early postmenopausal women

examined the relationship between calcium intake and

bone mass, and also found a positive correlation.73 More

recent studies 74–76 report conflicting results. Heaney 40

performed the largest meta-analysis to date of studies

using calcium supplements or diet (dairy products) and

reaffirmed the positive relationship between calcium in-take

and bone mass. This finding was consistent in the

139 studies examined, with the exception of two ran-domized,

controlled trials and 21 observational studies;

83% of the studies showed a positive relationship. A

fourth meta-analysis by Weinsier 77 of 46 studies in

which only dairy products were used achieved mixed

results; the authors concluded that there was inadequate

evidence to support a recommendation for daily intake of

dairy foods for bone health. This report prompted an

editorial discussing categorical decisions in Weinsier's

meta-analysis (e.g., classifying an observational study in

a strength category that included randomized, controlled

trials).78,79 Relative to calcium supplements versus dairy

foods and the impact on bone, two of the six dairy studies

in the meta-analysis 78 indicated that supplement use was

significantly better than intake of dairy; however, this

observation was not noted. Furthermore, a clinical trial 80

and one retrospective study 81 did not find a significant

relationship between dairy foods and bone mass, but an

association was observed between supplement use and

bone mass. This finding was also noted in another review

by Gueguen and Pointillart in 2000. 82 Finally, one recent

study not included in the above analyses showed a

protective role of dietary calcium on bone mass by

lowering the rate of bone loss in premenopausal women

25 to 30 years of age.83

Overall, it appears that calcium intake has a statis-tically

significant influence on bone accretion and bone

preservation based on the vast majority of studies. Al-though

research suggests that other nutrients, such as

vitamin D, are also important, it is calcium that has the

greatest bone-preserving and bone-building effect.84 The

issue of calcium intake is of paramount importance,

particularly as the median intake for females falls short

of recommended levels after childhood, even when sup-plemental

calcium intakes are included.85

Role of Calcium on Bone Mass During Energy Restriction

Because of a demonstrated loss of bone mass during

caloric restriction, and because calcium intake has been

shown to increase or preserve bone mass, it has been

recommended that calcium supplementation be given

during weight loss regimens.86 Calcium has been iden-tified

as one of the nutrients at risk in diets used for

weight control.87-89 Studies have been conducted to de-termine

the effect of calcium intake on bone mass during

caloric restriction. Relatively few premenopausal studies

have been reviewed, so postmenopausal studies will be

included as well (Table 1). A caloric restriction study

that randomized pre- and postmenopausal obese women

to a group receiving 1 g/d of calcium or to a control

group for 3 months found a significant difference be-tween

groups in whole body and spine BMC, with the

calcium-supplemented group losing less BMC.53 A

3-month follow-up measurement showed no change in

weight in the supplemented group, but the control group

had gained weight and continued to lose more BMC. The

supplemented group had no change in BMC, suggesting

continued bone preservation for calcium supplement use.

A 6-month caloric restriction study by Shapses et

al.90 randomized obese premenopausal women to groups

receiving: 1) 1 g/d of calcium with caloric restriction, 2)

placebo with caloric restriction, or 3) no calcium or

caloric restriction (control). Although no significant dif-ferences

were observed, the BMD of the spine tended to

increase in the supplemented group, while the two other

groups tended to lose BMD or BMC from the total body

or lumbar spine. Additionally, markers of bone turnover

were not significantly different between groups. These

findings are in contrast to the above study by Jensen,53

and suggest that calcium supplementation does not im-prove

bone status during caloric restriction. It also sug-gests

that low calcium intakes during weight loss do not

result in a significant loss of bone, and that bone mass is

not adversely affected by moderate weight loss. This

prompted an editorial by Barker and Blumsohn,91 who,

in re-analyzing the data, suggested that the overall

change in lumbar spine BMD was not significantly

different between the groups. In fact, in post hoc testing

the only difference was seen between the calcium and

control groups, indicating that there is no support for

suggesting that the calcium group tended to increase

lumbar BMD. Barker and Blumsohn 91 also suggested

that when extrapolating the change seen in lumbar BMD

in the placebo group to a year, the 95% confidence

interval would increase to a 3.5% loss in bone, which

would be a significant loss for premenopausal women

undergoing caloric restriction. This work supports earlier

findings.

A recent 12-week study conducted by Bowen et al.92

examined bone turnover during caloric restriction in men

and in pre- and postmenopausal women 20 to 65 years of

age, and showed that low intakes of calcium (500

mg/d) resulted in significant increases in bone turnover

compared with the other diet group, who received ap-proximately

1400 mg/d. However, this study did not

show any significant changes in total body BMD in

either group, probably because 12 weeks is too short a

time period to see changes in BMD assessed with dual-energy

X-ray absorptiometry. Recently, Radak et al.93

conducted a 12–week, multicenter, randomized caloric

restriction study in overweight and obese men and pre-menopausal

women. Two groups had total average cal-cium

intakes of 1334 +/- 76 (high calcium) or 1221 +/- 126

(high dairy), while a placebo control group averaged

458 +/- 71 mg/d. Results showed significant decreases in

total body BMD in the placebo group and increases in

femur BMD and lumbar BMC in the high-calcium and

high-dairy groups, respectively. Serum bone alkaline

phosphatase significantly declined in the high calcium

group.

These results suggest that, in the short term, high

calcium intakes during weight loss can suppress bone

turnover, and when consumed over extended periods of

time may preserve bone mass. Interestingly, no decline

in bone alkaline phosphatase was seen in the high-dairy

group, who averaged approximately 115 mg/d less cal-cium

than the high-calcium group; markers of bone

formation and bone turnover remained unchanged. The

lack of a significant change in bone markers for the

high-dairy group was unexpected. Perhaps the 115 mg/d

difference in calcium represents the threshold at which

bone turnover is impacted during weight loss. This con-curs

with a recent 6-week calcium and weight loss study,

which found that 1800 mg/d of calcium provided ade-quate

intestinal absorption (348 +/- 118 mg) that was

approximately 56% greater than with a calcium intake of

1000 mg/d (195 +/- 49 mg), based on an average esti-mated

need of approximately 240 mg/d of calcium for

postmenopausal women.94 These two studies 93-94 pre-liminarily

suggest that optimal calcium intake in pre- and

postmenopausal women during caloric restriction could

be estimated to be at least 1300 mg/d for bone preserva-tion.

Finally, results from short-term studies assessing

bone mass may be confounded by that fact that bone

remodeling is believed to act in cycles, a phenomenon

known as the " bone remodeling transient. " 95 Because

calcium is a threshold nutrient, both the age at intake and

the amount of intake can influence bone mass.96 Most

weight lost typically is regained, but whether a concom-itant

increase in bone density and content follows weight

regain is uncertain.97 One study assessing total body

BMC 6 months post-completion found that subjects who

had lost additional weight also lost additional bone

mineral, but subjects who regained weight also regained

bone mass.50 Compston et al.51 also found that total body

BMC approached initial levels when assessment was 10

months after a weight loss intervention with subsequent

weight gain. However, Avenell et al.98 did not observe a

similar response when subjects regained weight. In sum-mary,

investigations of changes in BMD or BMC during

weight reduction are not consistent in their findings.

Long-term studies involving placebo and control groups

with a post-intervention follow-up are needed to more

thoroughly examine the effect of weight loss on bone

metabolism. These studies should also include markers

of bone turnover and should not rely solely on BMD and

BMC values obtained by dual-energy X-ray absorptio-metry

to more accurately assess changes in bone health.

Calcium Intake as an Independent Regulator of

Body Weight and Composition

There is now evidence from epidemiological, animal,

and human experimental data to suggest that calcium

may play a role in weight regulation, which could pro-vide

an additional reason to ensure adequate calcium

intake during weight loss. Epidemiologic studies have

indicated a strong inverse relationship between adiposity

and calcium intake.99 Zemel 100 examined NHANES III

data and found that calcium intake was related to body

weight. The relative risk for being in the highest quartile

of adiposity was highest among those with the lowest

calcium intake. This observation persisted when physical

activity and energy intake were controlled. Davies et

al.101 explored the relationship further by retrospectively

analyzing five observational and cross-sectional studies

of pre-, peri-, and postmenopausal women. A significant

negative association between calcium intake and body

weight was observed. For each age group, the odds ratio

for being overweight was 2.25 for young women in the

lower half of calcium intakes within their respective

groups. Heaney et al.18 extended the analysis by adding

an additional randomized, controlled trial to the analysis

and using multiple regression analysis to predict BMI

based on calcium intake and other selected macronutri-ents.

The regression coefficient for calcium intake was

significant (p = –0.003) and equated to the average BMI

being 0.3 kg/m 2 lower for each 100-mg increment in

calcium intake.

Zemel 102 was first aware of the relationship between

calcium intake and body weight after re-analyzing a

previous study that examined the effect of calcium intake

on hypertension in obese African Americans. Increases

in dietary calcium ranged from approximately 400 to

1000 mg/d and continued for a year, resulting in a 4.9-kg

reduction in body fat. Two additional servings of yogurt

were used to increase the dietary calcium intake in the

experimental group compared with the control group.

Research has been conducted to determine the

mechanism responsible for the " anti-obesity " effect of

dietary calcium. Animal studies first studied the agouti

obesity gene found in human adipocytes. The agouti

protein stimulates the influx of calcium into adipocytes,

thereby stimulating fatty acid synthase, an enzyme in-volved

in lipogenesis that inhibits basal and agonist-stimulated

lipolysis in human and murine adipocytes via

a calcium-dependent mechanism.100 Exogenous high

calcium intake suppresses 1,25-(OH 2 )-D and decreases

calcium influx to the adipocyte (Figure 1). Increasing

adipocyte intracellular calcium promotes triglyceride

storage and exerts control over lipogenesis and inhibits

lipolysis. The inhibitory effect of intracellular calcium is

also believed to be partially responsible for the inhibition

of phosphodiesterase.103 Trangenic mice expressing ag-outi

in adipose tissue were placed on low- and high-calcium

diets, with the former exhibiting increases in

lipogenesis, inhibition of lipolysis, and accelerated in-creases

in body weight and fat mass.17 In the same mouse

model, another experiment added a caloric restriction

component in addition to calcium to determine whether

additional fat loss could be created secondary to caloric

restriction. As hypothesized, the low-calcium treatment

caused a two-fold increase in adipocyte intracellular

calcium, a weight gain of 29%, and increase in pad fat

mass, while the high-calcium treatments showed a 50%

decrease in intracellular calcium and greater decreases in

weight loss and fat pad mass. Fatty acid synthase was

reduced significantly by a high calcium intake, with

almost a two-fold change when the calcium was derived

from dairy. Other animal studies confirm the positive

effect of calcium on fat and weight reduction,104 and

suggest that rats on caloric restriction and restricted

calcium intake have an increased bone turnover and

decrease in BMD.105

Human clinical trials have been conducted to assess

the effect of calcium on weight loss and body composi-tion

during caloric restriction. In a study conducted by

Zemel et. al,106 subjects were randomized for 24 weeks

to: 1) placebo pill with </= 1 serving of dairy per day

totaling 400 to 500 mg of calcium, 2) high calcium using

a control diet with an 800-mg calcium supplement, or 3)

high dairy with 3 to 4 servings per day of lowfat dairy for

a total calcium intake of 1200 to 1300 mg per day. All

groups had a balanced deficit diet (–500 kcals). All

groups lost weight, with the control losing 6.4 +/- 2.5% of

body weight, which was increased by 26% in the high-calcium

group, and 70% in the high-dairy group. Fat loss

as assessed by dual-energy X-ray absorptiometry fol-lowed

a similar trend.

A similar study 93 included subjects randomized to

diets similar to the Zemel study, specifically: 1) placebo

pill with </= 500 mg calcium from either non-dairy or </=

1 servings of dairy per day, 2) high calcium using control

diet with a 900-mg calcium supplement, or 3) high dairy

with <\=3 lowfat dairy servings for a total daily calcium

intake of 1200 to 1300 per day. All groups had a

balanced deficit diet (–500 kcals). All groups lost weight,

with the control losing 2.82 +/- 2.76 kg of body weight,

the high-calcium group losing 2.83 +/- 2.8 kg, and the

high-dairy group losing 4.20 +/- 3.8 kg. Fat loss, as

assessed by dual-energy X-ray absorptiometry, was sig-nificant

for the high-dairy group (p < 0.05).

A randomized, 2-year exercise intervention in

young, normal-weight women performed secondary

analysis to assess any impact of exercise on calcium

intake and body weight and body composition.107 For all

groups pooled together, regression analysis indicated a

negative relationship between calcium intake and total

body weight and body fat when adjusted for energy

intake. Average intake was 781 +/- 212 mg/d of calcium.

The above animal and human studies support a

potential beneficial role for calcium on weight loss and

body composition, with the benefit appearing greatest

with dairy products. Zemel 17 suggests that one of the

additional components responsible for dairy's increased

effect is found in the whey fraction of milk. To examine

the results of these calcium and dairy food studies on

body composition and weight loss, Barr 108 performed a

Medline search of all randomized studies using calcium

or dairy that had data on change in body composition or

weight. Nine studies were found using dairy; seven of

these showed no significant differences in body weight

or composition between control and treatment groups.

Two other studies showed a significantly greater increase

in weight for the dairy-supplemented group versus con-trols.

The author notes that these results were in conflict

with calcium eliciting increased energy utilization, but

the results may be compromised if the subjects compen-sated

in some way for the additional calories from the

dairy supplements. Seventeen other studies using cal-cium

supplements also showed no significant differences

between calcium-supplemented and control groups, ex-cept

for one study, which actually showed greater weight

loss in the calcium-supplemented group. These studies

did not include caloric restriction and did not have body

weight or composition as the primary end points. Tee-garden

and Zemel 109 note that in order for an effect of

calcium to be seen, caloric intake must be factored in.

Additionally, most of the trials included normal-weight

individuals in whom caloric restriction would not be

appropriate. However, a pooled analysis of three caloric

restriction studies investigating calcium supplementation

and weight loss or body fat found no significant differ-ence

between placebo and calcium-supplemented groups

(1 g/d calcium) of pre- and postmenopausal women.110

Clearly, more research is needed to assess calcium's

impact on body composition and weight during caloric

restriction and to determine if dairy calcium exerts an

additional positive effect above that of calcium supple-mentation.

Other Potential Influences on Loss of Bone

During Caloric Restriction

The mechanism responsible for the loss of bone mass

during weight loss is unclear. A number of possible

reasons have been explored. Jensen et al.50 and others

have suggested that the bone loss associated with weight

loss may be due to the decrease in weight applied to

bone, e.g. mechanical load, which influences bone re-modeling.

If this is the case, then the effect of weight loss

should be greater on weight-bearing bones than on non-weight-

bearing bones.19 et al.111 examined this

hypothesis to see if exercise could preserve the loss by

adding a resistance training component to one of the diet

groups, but no significant bone-sparing effect was ob-served.

Svendsen et al.112 found that decreases in lumbar

spine BMD actually were greater in the diet and exercise

group than in the diet-only group. In a 6-year prospective

study in pre- and perimenopausal women, current phys-ical

activity was not correlated with BMD or bone

loss.113 However, et al.114 did find a benefit of

exercise in preserving regional BMD.

McLean et al.115 looked at cognitive change among

dieters and implicated the production of cortisol as a

contributing factor to bone loss during weight loss.

Leptin levels may also influence the rate of bone turn-over

and has been shown to decrease during weight

loss.116 It has also been suggested that leptin regulates

bone formation independent of its influence on body

weight.117 Another factor could be a decrease in adipose

cells resulting in a reduction in estrogen or estrone levels,

as confirmed by Ricci et al.56 Parathyroid hormone levels

tend to be altered during weight loss and this slight

increase may also be a contributor.56,118

There have also been methodological issues raised

when the analytical method used for determining bone

loss is dual-energy x-ray absorptiometry.119-122 The in-creased

thickness of soft tissue in obese subjects may

interfere with bone edge detection and affect the accu-racy

of measurements.8,51 Measurements are also ex-pressed

in areal density as grams per centimeter squared,

and do not factor in the dimension of depth.123,124 Obese

subjects tend to have increased bone size, resulting in a

possible overestimation of BMD. Another potential con-sideration

may be that women with greater bone mass

also lose it at a faster rate than women of lower bone

mass.125 et al.111 noted that because bone mass

is higher in obese women than in normal-weight women,

the bone loss seen from weight loss only serves to bring

those women back to within the " normal " range.

Many individuals undergo repeated episodes of ca-loric

restriction, so the effect of weight cycling on bone

mass and turnover has also been investigated. Results

from these studies are not conclusive.60,96,126 Recently,

however, Bacon et al.60 examined a group of obese

women, each with a history of chronic dieting, and found

that one-third had either osteopenia or osteoporosis.

While other nutrients play a role in bone metabo-lism,

most are within adequate intake levels compared

with levels of calcium intake. However, it is unclear

whether other nutrients are compromised during caloric

restriction to the degree that calcium is, and this is of

particular importance for individuals who engage in

repeated weight loss episodes. Nutrients found in fruit

and vegetables have been suggested as having a positive

association with BMD for late premenopausal women.117

Vitamin D is needed for calcium absorption in the

intestine and also plays a role in bone turnover.127

Positive effects of calcium on bone have been reported

both with and without the inclusion of vitamin D.128 This

may be explained partially because the major source of

vitamin D is cutaneous production via sunlight exposure,

which is geographically variable. Trials looking to dif-ferentiate

the effect of vitamin D or calcium have shown

that the preservation of bone is due primarily to calcium

and not to vitamin D,84 although a deficiency of vitamin

D could have negative consequences for bone metabo-lism.

96 Perhaps the largest contribution to bone loss

during caloric restriction is due to a reduced calcium

intake.

Specific dietary regimens have also been investi-gated.

The DASH (Dietary Approaches to Stop Hyper-tension)

diet was used during a 90-day randomized

crossover trial in middle-aged, mostly overweight men

and women. Markers of bone turnover were significantly

reduced compared with control diets during secondary

analysis.129 A recent review looking at vegetarian-based

diets and bone mass found no significant increase or

decrease in bone mass in the nine studies investigated.130

Considerations for Choices of Calcium Source

While available studies suggest preservation of bone

with adequate calcium during caloric restriction, and that

dairy sources of calcium may yield additional weight/fat

loss during caloric restriction, there are other factors to

consider between supplementation or dietary sources

(Table 2).

...Table 2. Characteristics of Calcium Sources

----------------------------------------------------------------------

-----

Source

Characteristics

Supplements

Positive:

Less expensive

Slightly better absorbed than dairy

Can meet recommendation with one or two pills

Good for lactose-intolerant people

Negative:

Increased risk of kidney stones

Dairy

Positive:

Additional benefit of fat loss during caloric restriction

Contains other vitamins and minerals

Negative:

Sometimes perceived as high fat and avoided

High in cholesterol and saturated fat

Mixed results regarding effect on chronic diseases

Plant-based

Positive:

Excellent fractional absorption

Contains vitamins, minerals, and other phytonutrients

Negative:

Challenging to meet recommendations with just plant-based sources

...Those who try to reduce weight via caloric

restriction commonly strive to consume less fat.17,131-132

Dairy products, a major source of dietary calcium, are

sometimes perceived as fat-containing foods and are

typically not emphasized during periods of caloric re-striction.

133-136 As mentioned, this may have a deleteri-ous

effect on bone health for premenopausal women

already striving to meet calcium intake recommenda-tions.

Various issues have been raised surrounding the

sources of calcium available. Supplements can be easier

to take than dietary sources, less expensive, better ab-sorbed,

and can meet recommendations with one pill, as

well as being more accepted by a sizable portion of the

population who are lactose intolerant.137-139 Those who

follow a strict plant-based diet might benefit from sup-plements,

140 particularly during weight loss. While many

plant-based sources of calcium generally have good

fractional absorption,141-143 with some better than dairy

sources,140,144 their intake alone falls short of meeting

calcium requirements for the majority of the population.

The challenge with plant sources is getting sufficient

quantities of intake. Dairy products remain the most

significant calcium source for the public.145 The National

Health Interview Survey in 1989 quantified calcium

supplement use at roughly 25 percent for women.146 Pill

supplementation comes without the addition of choles-terol

and saturated fats, known risk factors for cardio-vascular

disease, and has been shown to have some

favorable effects on blood lipids but in general has no

significant additional effect.147

Dairy products, as a source of dietary calcium and

other nutrients, have been proposed over supplements for

additional reasons beyond their effect on bone health.144

The CARDIA study indicated a reduction in cardiovas-cular

and type 2 diabetes risk factors with increased dairy

intake.148 A recent prospective study found a modest

reduction in risk of distal colorectal cancer with higher

calcium intake.149 Others note reductions in hypertension

and homocysteine levels in diets such as DASH, which

contain lowfat dairy.150 Other components of dairy, such

as conjugated linoleic acid, have been suggested to be

potentially protective of certain diseases and have exhib-ited

antitumor properties,151,152 although one epidemio-logical

study found conjugated linoleic acid intake to

have a weak but positive relation with breast cancer

incidence.153 Other forms of cancer have been associated

with dairy consumption,154,155 but many studies have

had conflicting results, with some showing dairy con-sumption

to be protective and others indicating a lack of

protective effect.145,156-159 Other studies and one review

have suggested that dairy consumption can be related to

an elevated insulinemic index.160-162 While dairy prod-ucts

contain cholesterol and saturated fats, studies did not

find significant increases with dairy consumption, but

rather decreases in plasma lipid and lipoproteins related

to cardiovascular risk.147 However, in hypercholester-olemic

individuals who had been on a lipid-lowering

diet, the addition of dairy for 6 weeks increased LDL and

decreased HDL, while also increasing lipid peroxida-tion.

163

Dietary sources of calcium have been shown to

decrease the risk of kidney stones, while supplemental

calcium has been shown to increase risk and could

possibly interfere with other minerals.85,164 Lastly, some

follow-up studies have shown that the positive effects of

calcium did not persist years later for pill supplements 85

and dairy calcium,165 although one study suggested that

the effects did persist with dairy.166

Regardless of the choice of calcium sources, the

most important factor is to choose one as a source or

addition to one's total calcium intake, particularly during

caloric restriction.

Conclusions and Applications

It is prudent to suggest that weight loss during premeno-pausal

years is desirable and outweighs the potential

risks to bone health that are now being investigated. The

effects on bone metabolism during this time may impact

bone health in the future, when bone preservation is of

the utmost importance to minimize osteoporotic-related

fractures. Available premenopausal studies show a de-cline

in bone mass with weight loss and low calcium

intakes. However, there are still many unknowns relative

to the cause of bone loss during weight loss. Findings

from studies examining bone health during weight loss

suggest that high calcium intake during energy restric-tion

may attenuate the loss of bone. Intake levels for

calcium during caloric restriction studies range from

1000 to 1800 mg/d. As most women's calcium intake is

well below this recommendation, adding an additional

500 to 1000 mg/d from dietary or supplemental sources

would not exceed the upper intake level and could

provide a benefit to bone metabolism, weight loss, and

fat loss during caloric restriction. The influence of high

calcium intake beyond standard recommendations war-rants

additional investigation for additional weight loss,

fat loss, and preservation of bone mass. If supplements

are recommended, the addition of vitamin D is sug-gested.

At this time, unless there is a known medical

condition or heredity history, dual-energy X-ray absorp-tiometry

scans to assess bone status are not performed in

premenopausal women, but could be beneficial in iden-tifying

women at risk for future osteoporotic disease.

For many women, the stage appears set for possible

future osteoporotic disease due to a combination of

already low calcium intake, normal bone loss due to

aging, the negative influence of weight loss on bone

mass, and potentially an additional insult from weight

cycling. All of these together represent a risk to bone

health and may predispose premenopausal women to

future osteoporotic events. Because calcium intake and

weight loss appear to affect bone mass, it is important for

additional studies to determine the risks and benefits of

both concurrently. Longer-term studies are needed to

evaluate whether the influence of calcium persists during

and after weight loss, and also to evaluate other potential

influences on bone loss in order to ascertain adequate

calcium intake levels.