As you know, we have a system of hormones that adjust our energy intakes — through hunger and satiety signals — to match our energy needs. What happens if we override this control system and overeat? Could it be that our bodies simply increase their energy expenditure to dispose of any excess calories? A couple of recent articles (1,2) make me think that this may indeed be the case.

The first study (1) examined the effects of changes in portion sizes, numbers of daily eating/drinking occasions, and food energy densities on weight gains in adult Americans between 1977-1978 and 2003-2006. The investigators found that daily energy intake increased by an astonishing 570 Kcal during that period. Americans ate more often and they ate larger portions; the energy density of their food, on the other hand, didn′t change much.

The second study (2) estimated the independent contributions of diet, exercise and other lifestyle factors to weight change. More than 120,000 healthy non-obese Americans were followed between 1986 and 2006, and changes in lifestyle and weight were assessed every four years. The investigators found average weight gains of 0.8 lb per year; the top 5% increased their weight by about 3 lb annually. Physical activity in the least and most active 20% surveyed shaved ½ lb and 2½ lb, respectively, off the overall weight gain.

Back to that increase in energy intake of 570 Kcal per day (1). It is actually about twice as high as the 1971 to 2004 increases reported by the American Heart Association (3). According to those estimates, women increased their average daily energy consumption by 344 Kcal (from 1542 to 1886) and men by 243 Kcal (from 2450 to 2693). Even those increases are still substantial. If we assume that Americans ate enough food in the seventies to meet all their daily energy needs then these 243 (or 344 or 570) Kcal have to be considered excess energy. What does the body do with this excess?

Could these extra calories simply be stored as fat? That′s highly unlikely. Suppose that every day you stored 10 Kcal as fat. In one year you would put away an extra 3,650 Kcal, about the energy content of one pound of fat (3,500 Kcal). In other words, for every 10 Kcal stored per day you gain 1 lb per year. If your energy intake increased by 243 Kcal per day (the lowest of the three estimates) you would gain about 25 lb per year; this is about an order of magnitude more than the top weight gain of 3 lb quoted in ref. 2.

An increase in physical activity doesn′t use up this excess energy either. Even the top 20% — the most active — of the participants in the second study only managed to burn about 2½ lb per year (2), or about 25 Kcal per day. Again, this is only about one tenth of the lowest estimated consumption increase.

Since neither fat storage nor increased physical activity — nor for that matter a combination of the two — accounts for that energy excess, what happens to those extra calories? I can only see two other possibilities. Either they simply 'pass through', i.e. the extra food is not digested and absorbed, or the body increases its energy expenditure to use up that excess.

Could energy expenditure really rise in response to increased energy availability? I′ve never come across any discussion on that topic, and the authors of the papers I quoted don′t talk about it either. But if this is correct then it isn′t surprising that diets don′t work — the energy requirements you try to match or beat are a moving target.

Sources:

1. Duffey KJ, Popkin BM, Energy density, portion size, and eating occasions: Contributions to increased energy intake in the United States, 1977-2006.PLoS Medicine 2011;8(6):e1001050.
   http://www.plosmedicine.org/article/info:doi/10.1371/journal.pmed.1001050
2. Mozaffarian D, Hao T, Rimm EB et al. Changes in diet and lifestyle and long-term weight gain in women and men. N Engl J Med 2011;364:2392-2404.
   http://anpron.eu/wp-content/uploads/2011/06/Changes-in-Diet-and-Lifestyle-and-Long-Term-Weight-Gain-in-Women-and-Men.pdf
3. Lloyd-Jones D, Adams RJ, Brown TM et al. Executive summary: Heart disease and stoke statistics – 2010 update: A report from the American Heart Association. Circulation 2010;121(7):948-954.
   http://circ.ahajournals.org/content/121/7/948.full.pdf