Abstract: Adults consume millions of kilocalories over the course of a few years, but the typical weight gain amounts to only a few thousand kilocalories of stored energy. Furthermore, food intake is highly variable from day to day and yet body weight is remarkably stable. These facts have been used as evidence to support the hypothesis that human body weight is regulated by active control of food intake operating on both short and long time scales. Here, we demonstrate that active control of human food intake on short time scales is not required for body weight stability and that the current evidence for long term control of food intake is equivocal. To provide more data on this issue, we emphasize the urgent need for developing new methods for accurately measuring energy intake changes over long time scales. We propose that repeated body weight measurements can be used along with mathematical modeling to calculate long-term changes in energy intake and thereby quantify adherence to a diet intervention and provide dynamic feedback to individuals that seek to control their body weight.
Alex’s Notes: We are never in balance; energy balance that is. It doesn’t matter if you are losing weight, you will have moments of energy surplus. It doesn’t matter if you are gaining weight, you will have moments of energy deficits. It doesn’t matter if you are weight-stable; we are always in a flux of energy imbalance. In other words, your food/energy intake will never exactly match you body’s needs at the time of consumption, and there will be times when any food you did eat is no longer available to provide energy your body needs (sleep, for example). This raises the simple yet complex question of what is the relevant time scale over which energy is balanced.
Free-living energy intake is known to vary by roughly 20-30% per day in those who don’t meticulously track their food intake, and yet body weight is for the most part stable. In the article at hand, the authors demonstrate via mathematical modeling that,
“Weight variations on the order of 1 kg correspond to random, uncorrelated, daily energy intake variations on the order of 630 kcal/day. Even for a conceivably low value of ε = 10 kcal/kg/day for an extremely sedentary person, day-to-day food intake variations of ~ 400 kcal/day are required to result in body weight fluctuations of 1 kg. Therefore, body weight is remarkably stable in the face of random, uncorrelated fluctuations in energy intake.”
The rationale for this stability is simply that weight-change happens on a slow time scale when one isn’t deliberately trying to gain or lose weight. Therefore, the day-to-day fluctuations in food intake are effectively averaged over a long time. But how can this be so?
We may jump to the conclusion that there exists a feedback system whereby each day’s intake is adjusted to compensate for previous imbalances. This idea garners interesting support. Since the 1970s, the food availability in the U.S. has increased by roughly 750 kcal per person per day, and yet energy intake has risen by only 250 kcal. The dramatic increase in availability and marketing of highly palatable, convenient, inexpensive, and energy-dense foods may have been actively resisted since only about one third of the increased food available was actually eaten. Additionally, in the classic Minnesota-starvation experiment, there was substantial hyperphagia among the subjects at its conclusion, further suggesting a compensatory mechanism. On the flip-side of all this, experimental over-feeding studies show no caloric restriction following the surplus period. Perhaps we are better suited to compensate for a lack of energy than abundance? This would certainly help explain the rising obesity rates.
The unsatiating truth is that we don’t know. More research is definitely needed to establish whether human food intake is under active control or is primarily habitual and follows the changing environment. What we do know is that large daily fluctuations in energy intake are irrelevant to body weight regulation. Although persistent changes can and will lead to substantial weight changes over the long-term, pigging out at your favorite buffet won’t destroy your waistline – at least so long as you don’t make it a habit.