For decades, a high proportion of doctors and dietitians have worked to an incorrect assumption that cutting 500 calories of energy intake per day will result in steady weight loss of about one pound per week. This assumption ignores changes in the body’s metabolism and leads to unrealistic expectations for diet plans. The third paper of The Lancet Obesity Series introduces a new web-based bodyweight simulation model*, that incorporates metabolic adaptations that occur with decreasing bodyweight. The paper is by Dr Kevin Hall, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health (NIH), Bethesda, MD, USA, and colleagues.
The model simulates physiological differences between people based on sex, age, height, and weight, and helps explain why some people may lose weight faster compared to others, even when they eat the same diet and do the same exercise. The model also provides a rough dieting rule for a typical overweight adult: every 10 calories per day reduction in energy intake will result in a bodyweight loss of about 1 pound over 3 years, with half of that occurring in the first year. So, cutting a habitual daily 250 calorie chocolate bar will lead to about 25 pounds of weight loss over the next three years. This is much less than the 78-pound weight loss predicted by the old dieting assumption, but far more realistic.
Furthermore, the new model incorporates both a dynamic assessment of how energy expenditure changes over time, as well as how energy imbalances are partitioned between storage or mobilisation of body fat and lean tissue. It demonstrates how bodyweight slowly changes in response to a diet–and that heavier people can expect greater weight change with the same change in diet, though reaching a stable body weight will take longer compared to those who weighed less to begin with. Model simulations can be used by clinicians to help design personalised weight management programmes that address individual goals and tailor the pace of weight loss.
The authors also look at the evidence surrounding success of different diet plans. They say that the body adapts rapidly to changes in relative protein/fat/carbohydrate in a reduced-calorie diet, meaning that all diets with similarly reduced energy content have a similar effect on body-fat loss in the short term. But the authors add: “Little is known about the long-term effects of diets that vary in macronutrient composition, since present methods prohibit [accurate] quantitative assessment of food intake and diet adherence in an outpatient setting.”
Outpatient weight-loss programmes are often noted to result in maximum weight loss within 6 to 8 months and such bodyweight plateaus are often interpreted as resulting from slowed metabolism. However, the authors’ model shows that metabolic changes result in a weight-loss plateau only after much longer periods of time. Model simulations revealed that the typical outpatient bodyweight plateau is instead likely due to a rapid and progressive loss of diet adherence.
Finally, the authors discuss the energy imbalance that has caused the average 20-pound increase of average adult bodyweight over the past 30 years in the USA, which amounts to just 10 calories a day. This seemingly small increase in caloric intake associated with significant weight gain over time gives rise to the misconception that very small reductions in caloric intake could reverse the obesity epidemic. But, in fact, an average 220 extra calories per day extra are required to maintain people at this higher weight (known as the maintenance energy gap). For adults with a BMI greater than 35 kg/m2, representing 14% of the US population, permanent calorie reductions of more than 500 calories per day would be needed to return them to the average bodyweight seen in the USA in 1978. This change would take around three years for those moderately obese, and longer for those even more obese.
The authors conclude: “Accurate mathematical models of human metabolism are needed to properly assess the quantitative effect of interventions at both the individual and population levels. Widespread past use of erroneous rules for estimation of human bodyweight change have led to unrealistic expectations about the potential effect of both behavioural and policy interventions. By modeling the quantitative physiology of human bodyweight change and providing easy access to a web-based simulation tool, we believe that health-care and health-policy practitioners will be in a position to make better informed decisions.”