The twins who show why some people are more likely to gain weight

Weight gain is a concern for many people, while many others do not gain weight even if they lead a sedentary life and don’t pay much attention to the calories they consume. These differences have previously been attributed to genetics, but a group of researchers from Finland have found a new approach to the question of why some people are more susceptible to weight gain than others. The research, published in the journal Obesity, is the first of its kind to decipher in a pair of twins with large weight differences, which of the two had acquired a higher body mass index (BMI) than what they were genetically destined to.

The study, carried out by the University of Helsinki, followed for 36 years the BMI trajectory in twins who presented levels lower or higher than expected according to their genetics, with special emphasis on those who showed a great disparity between each other. The author of the study, Bram J. Berntzen, from the Institute of Molecular Medicine of Finland, says that this “novel approach” opens doors to discover the factors that protect or predispose people to weight gain. In addition, he says that this methodology can offer valuable information on how to maintain a healthy weight.

Jaakko Kaprio, a geneticist specializing in genetic epidemiology at the University of Helsinki and co-author of the study, explains that “a key finding” was that the predicted weight based on genetics was sometimes closer to the weight of the thinner twin, so it could be considered that the heavier twin deviates more from their biological disposition. “This suggests that there are environmental reasons for gaining weight that have affected the heavier twin, and these can be studied more closely,” adds the researcher.

The main contribution of their work, the authors explain, is that in previous studies applied to twins with large weight differences, these had not established whether the twin with a higher or lower BMI was the one who deviated from their genetic predisposition. For this particular research, more than 3,000 Finnish same-sex twins were examined. The study began in 1975 and were followed in 1981, 1990 and 2011, providing a unique perspective on how weight patterns evolve over time. The researchers found that the twin with a higher BMI in 1975 was more likely to deviate above the predicted BMI, compared to the twin with a lower BMI, who was more likely to deviate below the prediction. This points to a genetic relationship between baseline BMI and weight trends over decades. Individuals classified within or above prediction in 1975 showed consistent patterns, respectively achieving overweight status and obesity by 2011.

Researchers discovered that some people already had a genetic predisposition in their childhood to gain weight faster, while their twins did not. Once in adulthood, they would gain weight in the same way. Thus, they highlight the importance of studying the reasons why a child can gain much more weight than others. This being the case, the so-called trick of eating whatever you want without gaining weight disappears when that individual becomes an adult. Jennifer Lovejoy, a researcher at Duke University in North Carolina who did not participate in the study, asks another question she considers important: “What protects some people in such a way that we see twins who are thinner? It is not entirely true that it is genetically determined.”

In recent weeks, a Netflix production, You Are What You Eat: A Twin Experiment, has brought into the global conversation the possibility of using twins to study the effects of a disparate diet: omnivore or vegan. In the program, in just eight weeks, improvements in cardiovascular health were seen in those who followed the vegan diet. However, the television program has some scientific weaknesses pointed out by specialists, and even divergences with the results published by Stanford University researchers in a scientific journal.

Therefore, the greatest achievement of the study published in Obesity is not its present conclusions, but the window of opportunity it opens for future research. By following this methodology, scientists explain that in the future it will be possible to study children from birth until they reach adulthood, with the aim of better understanding the factors that cause obesity and thus finding formulas to combat it. In the words of Kaprio, thanks to this discovery, it is now possible to study two different groups: those who gain weight and those who lose weight separately. “This can help researchers understand how people maintain a normal weight and how they sometimes do not.” And this, in turn, could have important implications for public health strategies and personalized treatments.

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