Today, we carry this same genetic makeup with us, and several animal studies and few small human trials indicate that there may be, for us too, health benefits to alternate-day fasting -- a regimen that somewhat mimics the irregular and unpredictable food intake pattern on which our ancestors evolved.
Evidence has been accruing for some time that chronic calorie restriction, in which daily intake is reduced to between 60% and 85% of an individual's daily needs, appears to have significant health benefits. Such restriction has been shown to reduce risk factors for several chronic diseases in animals and humans and to increase life span in rats, mice, fish, flies, worms and yeast.
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But the effects of alternate feast and fast days on body weight and health have only recently been explored.
In a study published in the American Journal of Clinical Nutrition in 2005, scientists at the Pennington Biomedical Research Center examined the effects of alternate-day fasting on heart disease risk in 16 subjects.
Subjects ate whatever they wanted on feast days but consumed only calorie-free beverages and sugarless gum on fast days. After three weeks, blood levels of triglycerides fell in men, but not in women. Women, but not men, experienced increases in "good," or HDL, cholesterol.
There was no clear explanation for the differing results between men and women, but the same group of scientists have observed other sex-specific effects. In a different report, they measured the rise in insulin and blood sugar levels in response to a meal before and after three weeks of alternate-day fasting.
Men -- but not women -- had increased insulin sensitivity so that they cleared sugar from the bloodstream more efficiently after three weeks on the regime, suggesting that alternate-day fasting may be more beneficial to men than women in reducing the risk of Type 2 diabetes.
Similar improvements in insulin sensitivity were observed after two weeks of alternate-day fasting in a small study of eight male subjects at the University of Copenhagen, published in the Journal of Applied Physiology in 2005.
The effects of alternate-day fasting on body weight differ according to the study (and none have been reported in overweight people). Men in the Copenhagen study maintained a stable body weight over the two week period -- but they were specifically instructed to attempt to do so.
In the Pennington study, subjects were informed that they would need to double their usual intake on non-fasting days in order to maintain their weight. But taking in enough food on the feasting days to avoid weight loss proved difficult. The participants lost about 2.5% of their initial weight and 4% of their initial fat mass.
All in all, the few human studies on alternate-day fasting have been small in size, short in duration and have lacked control groups so more studies are warranted. Still, the animal studies suggest that the effects of alternate-day fasting on chronic disease prevention are similar to those reported for chronic calorie restriction.
How such dietary calorie restriction may impart its benefits is not clear, but it may include increased resistance to stress, a reduction in free-radical production (which in turn reduces cellular damage) or the slowing of certain metabolic processes that might damage the body.
From a practical standpoint, it's unclear whether people could stick to an alternate-day fasting regimen for any length of time. Researchers have suggested that the regimen is easier than daily calorie restriction, but is it easy enough? Many subjects in the Pennington study reported feeling hungry and irritable on the days they fasted -- and that would probably limit the number of people who could sustain this pattern of eating for very long.
Anyone wishing to attempt either chronic or alternate-day calorie restriction needs to remember that consuming nutrient-dense foods is key. There are no extra calories to spare, so every bite of food needs to be packed with nutrition.
But a carefully planned diet based on vegetables and fruits, lean proteins, a few whole grains and small portions -- rather than a continuous and abundant food intake -- may be a better nutritional match for those "thrifty" genes.
Susan Bowerman is a registered dietitian and assistant director of the UCLA Center for Human Nutrition.