A dose theory of exercise: overcoming holiday overeating

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A dose theory of exercise: overcoming holiday overeating.

Physical Activity and Health: The Evidence Explained, 2003, Routledge, London,

pp. 122-130, 141-146

As we all know, a stable body weight results from being in energy balance:

energy intake must match energy expenditure. Overeating leads to an increase in

fat storage and consequently body weight. Overeating is seldom intentional; so

why does it occur so frequently in today's culture? It is especially pertinent

to ask this question steeped in the merry season of holiday baked goods,

constant snacking, extravagant meals, and prolonged sitting.

Weight gain often goes unnoticed until it has already occurred. One major reason

for this is thought to be the role of a high-fat diet, too often present during

lavish holiday family cooking. So how, exactly, does a high-fat diet contribute?

A high-fat diet can lead to unconscious weight gain in a few different ways. For

starters, converting dietary fat into body fat is a very efficient process, much

more so than protein and carbohydrates. Second, fat is more energy dense than

other nutrients, which means that you need to eat many less grams of it to

obtain the same amount of fuel for use than you do carbohydrate or protein.

Because we ordinarily monitor our food intake based on bulk and not energy

density, it is very easy to overeat fatty foods. Third, under normal conditions,

carbohydrate and protein are not converted to fat at all. Increases in intake of

these nutrients stimulates an increase in oxidation. This is because there is

limited potential to store these nutrients in the body--unlike fat, which has

enormous potential for bodily storage. Fat intake does not necessarily stimulate

for oxidation.

The role of exercise

The most obvious benefit of exercise as a countermeasure to holiday overeating

is the increase in energy expenditure, the linchpin of weight control. To

restore the balance of calories eaten versus calories burned, increase the burn

along with the eating. But there are other, more subtle contributions to weight

control that exercise offers, and to understand them is to understand what a

unique weapon exercise is in the arsenal.

The first of these is something we might call resting metabolic rate elevation.

There are three categories of energy expenditure that make up one's overall

caloric burn each day. The first and largest of these is resting metabolic rate

(RMR), which can be defined as the energy required to sustain life while

resting.

RMR accounts for between 60 and 75 percent of the energy expended over a 24-hour

period. All forms of activity, including willful exercise, account for another

15 to 30 percent. The rest is the fuel burned during food intake, absorption,

and storage--another 10 percent or so. Since RMR accounts for such a large

portion of daily energy expenditure, it's been hypothesized that elevating it is

the easiest path to energy balance. Does a low RMR increase obesity risk, and

does exercise increase RMR?

The data is positive, but the answer is not quite what you might think. Current

research suggests that a low RMR is not really a cause of obesity. Obesity is

more directly caused by inactivity and energy imbalance. Furthermore, exercise

appears to elevate RMR only temporarily, in the hours immediately after a

workout. This, though, turns out to be very useful indeed.

Because exercise elevates RMR for several hours after exercise, one can, by

moderately exercising regularly and frequently (e.g., walking for an hour in the

morning, and then for an hour later that night), achieve a near-permanent

elevation in RMR. Dieting, on the other hand, causes a reduction in RMR. While a

low resting metabolic rate may not be a direct cause of obesity, a

near-permanently high one is certainly a boon to energy balance, as so much of a

person's caloric expenditure is accounted for by RMR. As we have already begun

to examine elsewhere in this issue, the value of the acute effects of a single

workout are surprising and can hardly be overstated.

Postprandial Lipoprotein Metabolism--nature's bonus gift

Exercise works to offset the effects of a large, high-fat meal in a second way.

The prevalence of fat in the plasma after eating is reduced--in one study by as

much as 43 percent!--in endurance trained athletes, compared to sedentary

controls. But even more interestingly, acute bouts of exercise significantly

reduce the effects of a high-fat meal consumed eight hours later.

Researchers first observed this after finding that when training in endurance

athletes is disrupted for even a few days, triglycerides increased significantly

in the blood following a meal. Researchers began to notice, then, that one

characteristic of endurance-trained people is that they have always just

exercised recently. It turns out that a single session of exercise reduces

plasma triglyceride response to a subsequent high-fat meal by as much as

one-third. Furthermore, as noted in " Fearing the Other IRS, " exercise intensity

is not the key factor--overall energy expenditure is. Walking briskly for 90

minutes at 60 percent VO2max decreased post-meal triglycerides, lowered insulin

resistance, and increased whole-body fat oxidation by a third, equaling the

results of walking 180 minutes at 30 percent VO2max.

The best approach to holiday eating may be to take a " dose " of exercise in the

hours before a large, high-fat meal. Not choosing high-fat foods at all, of

course, is a better approach to health and weight control. But as a practical

matter, high-fat foods encroach upon us; it is a fact of American culture. The

idea of walking or jogging long distances before settling down to a meal is

appealing; it reverberates with the natural hunter-gather state of our ancestry.

It is not surprising that we are hard-wired to move our bodies long distances

and then consume an (often large) meal. And by focusing on the acute effects of

exercise, somehow the psychological edge is taken off of starting and

maintaining a long-term regimen. This can seem a steep slope to clime, but when

viewed as a daily choice with immediate value, energy balance seems easier to

maintain, even through the tempting winter months.

Physical Activity and Health: The Evidence Explained, 2003, Routledge, London,

pp. 122-130, 141-146