Maximum Safe Percentage of Caloric Restriction = 16%?

[Guest guest]

The statistics are clear that CR can help us reduce the risk of

cardiovascular disease, diabetes, cancer, hypertension, and other

maladies associated with aging. We all hope that CR will increase our

lifespan, but there have not bany many discussions about the optimum

level of caloric restriction for humans.

The graphs of the rodent data on p.48 of BT120YD indicate that longer

life can be achieved by greater restriction. However, what is missing

in Walford's book is the U-shaped curve from plotting risk of

mortality vs. different levels of CR. At 100%CR there would be 100%

death within a few weeks. At 80%CR the body would wither more slowly,

but longevity would be physiologically impossible. At 0%CR all the

problems associated with ad libitum diets would limit longevity.

Walford addresses the topic of the best level of CR on pp. 50-51, but

does not provide any concrete guidance.

The rodent experiments show that 40% CR administered after weaning

results in adults weighing only 50% of mice fed ad libitum. This

clearly shows that a 40%CR diet can only support a half-sized body,

and that therefore, a 40%CR diet is too severe when started

in adulthood.

The energy metabolism discussed in the Biosphere-2 paper, below,

offers some clues that may shed light on the issue of maximum safe

percentage of CR. The sleeping metabolic rate (SMR) for the

biospherians was about 7% less than the controls.

Keys' semistarvation studies, which are also discussed in the paper,

showed that severe energy restriction decreased BMR in absolute terms

(39%) and also relative to the weight of metabolically active tissue

(16%). However, the Biosphere paper authors conclude that the severe

energy restriction studied by Keys would have soon led to death by

starvation had the study not been terminated after 6 months.

So what is the maximum safe percentage of restriction? The degree to

which the body can adjust metabolic rate provides a good guideline.

If you feed your body less than the amount to which it can adjust, the

body will try to shrink by reducing fat, muscle, or bone mass. If 16%

is the maximum reduction in BMR for metabolically active tissues

obtained by starvation diets, then an optimum CRON diet should not

exceed 16%CR because the metabolism will not be able to adjust to

fewer calories.

Keys' experiment was carried out with lean subjects, so the reference

point for computing 16%CR shuld be a weight corresponding to a BMI of

around 22.

Tony

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Christian Weyer, Roy L Walford, Inge T Harper, Mike Milner, Taber

MacCallum, P Tataranni and Ravussin,

" Energy metabolism after 2 y of energy restriction: the Biosphere 2

experiment " , American Journal of Clinical Nutrition, Vol. 72, No. 4,

946-953, October 2000.

http://www.ajcn.org/cgi/content/full/72/4/946

" Spontaneous physical activity was significantly lower in the

biospherians than in the control group (Table 3). The unadjusted SMR

was 470 ± 270 kJ/d (7.9 ± 3.8%) lower in the biospherians than in the

control group (NS; Table 3), a difference (of 400 ± 210 kJ/d, or 6.7 ±

3.3%; P = 0.06) that was nearly significant after adjustment for age,

sex, fat-free mass, and fat mass. "

" Some 50 y ago, Keys et al (6–8) showed in a pioneering

semistarvation study (the Minnesota experiment) that 6 mo of severe

energy restriction in 32 lean men led to a marked reduction in EE.

This was due to a reduction in both physical activity and in the

resting metabolic rate (RMR), which decreased not only in absolute

terms (39%) but also when expressed per kilogram of metabolically

active tissue (16%). This form of energy conservation, a biologically

meaningful survival mechanism in the face of dangerously low energy

supplies and stores, has been referred to as metabolic adaptation (6,

9). To date, the Minnesota experiment continues to be the most

comprehensive underfeeding study in humans, and its findings—revisited

recently in detail by Dulloo et al (10–12)—have provided important

insights into our understanding of human energy metabolism and body

weight regulation. However, it is important to remember that the diet

in Keys et al's (6) study was designed to represent the severely

energy-deficient diet in European famine areas during and after World

War II. Consequently, the participating lean men rapidly lost large

amounts of weight ({approx}25% of body weight) and by the end of the

study were severely undernourished with weakness, lethargy, and edema

(6, 7). Clearly, despite the adaptive reduction in EE, such severe

energy restriction would have soon led to death by starvation had the

study not been terminated after 6 mo. "