Your body, not your food intake, may determine weight

If you think it’s hard for you to lose weight, you might be right.

Researchers at the National Institutes of Health have found evidence “supporting the commonly held belief that people with certain physiologies lose less weight than others when limiting calories.”

This was a 77-day in-patient trial involving 12 obese subjects. Researchers controlled their food intake and measured calorie expenditure, body weight and composition.

Researchers compared individuals with a “thrifty” phenotype with individuals who have a “spendthrift” phenotype. Individuals with a thrifty phenotype lose less weight when calories are reduced. Weight loss success is influenced by an individual’s energy expenditure response to caloric restrictions.

To determine phenotype, the researchers measured patient response to fasting and overfeeding over a 24-hour period. The volunteer patients then underwent six weeks of 50% caloric restriction.

After accounting for age, sex, race and baseline weight, the researchers found that the people who lost the least weight during the calorie-reduced period were those whose metabolism decreased the most during fasting.

In other words, in thrifty individuals, fasting (eating less) results in a slower metabolism. It also resulted in less weight loss. The researchers concluded:

[We] clearly determined that there is variation in the extent of weight loss in obese humans during 50% [calorie restriction] that is not the result of a lack of adherence but is caused by real biologic inter-individual variation in energy expenditure responses to the same energy deficits, that is, thrifty and spendthrift phenotypes. Whether identification of these phenotypes can be harnessed to prevent weight gain in humans remains to be established.

Scientists know that there is substantial individual difference in resting energy expenditure and 24-hour energy expenditure.

But no one knows the whys or hows of the two phenotypes.

It could be differences in nervous system activity, thyroid and appetite hormones, diet and weight-loss-and-regain history, and/or variable amounts of brown fat. Or something else.

Moreover, the rule-of-thumb about how many calories we have to reduce to lose a pound of weight has also been set on its head:

The pervasive rule that an energy deficit of 3,500 kcal is required to lose 1 lb of body weight is based on calculations assuming that dietary weight loss is mostly a result of energy-dense [fat mass] loss. We found considerably lower values and wide inter-individual variation in the caloric cost of losing 1 lb body mass in both thrifty and spendthrift subjects (mean 2,239 kcal/lb, range 1,558–2,993 kcal/lb)…

“The results corroborate the idea that some people who are obese may have to work harder to lose weight due to metabolic differences,” according to Martin Reinhardt, M.D., lead author and PECRB postdoctoral fellow.

A study of only 12 people cannot be extrapolated to an entire population. However this research corroborates earlier research on the spendthrift and thrifty phenotypes.

Reference: Reinhardt M, Thearle M, Ibrahim M et al. A human thrifty phenotype associated with less weight loss during calorie restriction. Diabetes. 2015.

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