Doctors and dieticians have worked for decades assuming that cutting 500 calories from a person's daily diet will result in a steady weight loss of approximately one pound per week, however, this assumption is incorrect, as it does not take metabolic changes into account that can lead to unrealistic expectations for diet plans.
Dr Kevin Hall, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health (NIH) in Bethesda, MD, USA and his team have introduced a new web-based bodyweight simulation model*, that incorporates metabolic adaptations that occur with decreasing bodyweight, published in the third paper of The Lancet Obesity Series.
The model is able to simulate individuals' physiological differences based on sex, age, height, and weight, helping to explain faster weight loss in some people compared to others despite eating the same diet or doing the same exercise. It 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. To make it simple, cutting out the habit of eating a 250-calorie chocolate bar each day will result in losing approximately 25 pounds of weight over the next three years; far more realistic but much less than the 78-pound weight loss predicted by the old dieting assumption.
In addition, the new model incorporates both a continuously changing assessment of how energy expenditure changes over time, as well as how energy imbalances are divided between storage or mobilization of body fat and lean tissue.
The model displays slow changes in bodyweight in response to a diet. It also shows that heavier people can expect greater weight change with the same change in diet, although reaching a stable body weight will take longer compared to those who weighed less to begin with. Clinicians can use the model simulations to assist in developing personalized weight management programs for individuals, addressing their goals and managing the speed of weight loss.
The authors also examined successful evidence of different diet plans and discovered, that because of the body's ability to quickly adapt to changes in relative protein/fat/carbohydrate in a reduced-calorie diet means, that all diets with similarly reduced energy content also have a similar effect on body-fat loss in the short term. They added:
"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 programs often achieve maximum weight loss within 6 to 8 months. Such bodyweight plateaus are often interpreted as resulting from slowed metabolism, however, the model displays, that metabolic changes result in a weight-loss plateau only after much longer periods of time. The simulations indicated, that the typical outpatient bodyweight plateau is instead likely due to a rapid and progressive loss of diet adherence.
The authors also discussed the energy imbalance of just 10 calories per day that caused the average 20-pound increase of average adult bodyweight over the past 30 years in the USA. There is a risk of misconception, that this rather small increase in calories linked to significant weight gain over time could mean that very small reductions in calories could reverse the obesity epidemic. However, in order to maintain this higher weight (maintenance energy gap) people would have to increase their daily calorie intake by 220 calories, meaning, that in order to return to the average bodyweight in the USA in 1978, adults with a BMI greater than 35 kg/m2 (14% of the US population), would have to cut their calories by more than 500 calories daily. The change would take approximately three years for those moderately obese, and longer for those even more obese.
In a concluding statement the authors say:
"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 behavioral and policy interventions. By modelling 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."
Written by Petra Rattue