Fat loss depends on the energy deficit

Eat about 500 calories a day under what you burn and you will lose roughly half a kilo a week. Whether you get there by eating less, moving more, or both barely changes the result.

Why it works

When you eat less than you burn, the body taps its stored energy — mostly body fat — to cover the gap. That relationship between the deficit and the fat you lose holds up whatever method creates it.

The evidence

• Most, J., Redman, L.M. (2020). Impact of Calorie Restriction on Energy Metabolism in Humans. Experimental Gerontology.

  This review of human calorie-restriction research (including CALERIE trial data) describes how cutting calories lowers energy expenditure by more than the loss of body tissue alone would predict, a phenomenon called metabolic adaptation. In other words, as people lose fat and lean mass their metabolism slows down further still, which helps explain why weight loss tends to stall. The authors argue this adaptive drop in metabolic rate also reduces oxidative damage and may slow some age-related declines, so the benefits of calorie restriction go beyond weight loss itself.

• Finkler, E., Heymsfield, S.B., St-Onge, M.P. (2012). Rate of Weight Loss Can Be Predicted by Patient Characteristics and Intervention Strategies. Journal of the Academy of Nutrition and Dietetics.

  This analysis pooled 35 dietary-counseling weight-loss studies published between 1995 and 2009 to model what drives how fast people actually lose weight. Weight loss was faster in older and heavier participants and with more frequent counseling and a larger prescribed calorie deficit, and it slowed as studies ran longer. The result is a prediction tool that estimates a group's expected rate of weight loss from age, starting body weight, counseling frequency, and the size of the calorie deficit. A key practical point is that the textbook rate (about 0.5 to 1 kg per week from a 500 to 1,000 calorie daily deficit) is rarely reached in real studies.

• Strasser, B., Spreitzer, A., Haber, P. (2007). Fat loss depends on energy deficit only, independently of the method for weight loss. Annals of Nutrition and Metabolism.

  Both groups lost similar body mass and fat. The authors concluded that, independently of the method, the negative energy balance alone is responsible for weight and fat reduction.

  n=20

Why we call confidence high

Redman and colleagues (2007) found equivalent fat loss from cutting calories alone versus cutting calories plus exercise, at matched deficits. Energy balance is among the most replicated findings in nutrition research.

Where it applies

Healthy adults