A recent study conducted on overweight elderly men who had experienced muscle loss due to aging sought to determine the effects of a resistance training program combined with protein supplementation on their body composition. Over 16 weeks, the study examined how these interventions influenced muscle gain and fat loss. Contrary to what one might expect, the results provide an intriguing perspective on how weight changes can occur even when the body is in an energy deficit. 

The study’s participants experienced a net weight gain of 1.3 pounds (0.6 kg) throughout the 16-week study period. Initially, this may seem to suggest that they were in an energy surplus. However, a closer examination reveals a different story. In fact, the participants were in a net energy deficit of approximately 7,300 kcal throughout the study.

This outcome can be attributed to the changes in muscle and fat composition among the participants. Specifically, the study found that they lost 2.4 pounds (1.1 kg) of body fat while gaining an average of 3.7 pounds (1.7 kg) of muscle mass. This interplay between muscle gain and fat loss led to a complex picture of energy balance.

How is it possible to gain weight while on a calorie deficit? When your body burns 1 pound (about 0.5 kg) of pure body fat to offset an energy deficit, it releases approximately 4,270 kcal. Similarly, gaining 1 pound (0.5 kg) of pure body fat requires a surplus of around 4,270 kcal without adding any lean body mass. You might be wondering why the commonly quoted value is 3,500 kcal. That’s because 3,500 kcal is the energy content of 1 pound of adipose tissue, including pure fat, water, and some minerals.

When monitoring weight changes at home, the “3,500 calorie rule” serves as a helpful guideline. However, in lab settings, measurements focus on changes in pure body fat rather than adipose tissue and a pound of pure body fat contains more energy. 

The loss of body fat represents an energy expenditure of approximately 10,400 kcal, as the body used stored fat to compensate for the energy deficit. Meanwhile, the gain in muscle mass required an energy surplus of only around 3,100 kcal. When these values are calculated together, the net energy deficit over the study period can be estimated as 10,400 kcal minus 3,100 kcal, equaling approximately 7,300 kcal. Dividing this net deficit by the 112-day study duration results in an average daily energy deficit of roughly 65 kcal.