Alas, most dieters are destined to regain the weight they lost and even some extra weight.

Does dieting make you fat or is dieting a response to a genetic predisposition to gain weight?

To find out, researchers analyzed data from 4,129 individual twins who took part in the FinnTwin16 cohort. There were 542 identical twins, 516 same sex “fraternal” twins, and 554 opposite sex twins.

Weight and height was measured when the twins were ages 16, 17, 18, and 25. At age 25 the twins were asked how many episodes of intentional weight loss (IWL) of at least 5 kg they had up to that time.

There was a dose-dependent relationship between the number of IWL episodes and the odds of being overweight at age 25.

Compared to no IWL, one IWL was associated with 80% higher odds among men and 2.7-fold higher odds among women.

Two or more IWLs were associated with a 2.0-fold higher risk among men and a 5.2-fold higher risk among women.

Data from fraternal twins suggest that dieting is a response to a genetic predisposition toward weight gain, so people who diet are susceptible to future weight gain.

But analysis of data from the identical twins suggests that dieting per se was associated with a small weight gain, independent of genetic propensity. So, it appears that dieting reflects both a susceptibility to gain weight and increases the risk of weight gain.