Is Your Weight in Your Genes? Twins, DNA and Everyday Life

“It runs in my family” is one of the most common explanations people give for their weight. Researchers have studied it carefully, in twins, in families and in the DNA of hundreds of thousands of people. This guide walks through the evidence and what it does and does not mean. It is general education, not medical advice.

What “heritable” means

When scientists say body mass index (BMI) is heritable, they mean that in a group of people, a share of the differences between them is linked to differences in their genes. It is not a percentage of any one person’s weight, and it is not a prediction. Heritability also changes with the age group studied and with the surrounding environment.

Twins and families

A 2012 review by Elks and colleagues pooled 88 estimates from twin studies and 27 from family studies. Twin estimates ranged widely, from about 0.47 to 0.90, which means that in those studies genes accounted for roughly half to nine-tenths of the variation in BMI. They were generally higher than the family estimates. Differences in study design explained nearly half of that variation, and the genetic influence looked stronger in childhood than in adulthood.

Reading the DNA

• Many small effects. A large 2015 study of up to 339,224 people found 97 places in the genome linked to BMI. Together they explained only about 2.7 percent of the variation, although the authors estimated that common genetic variation overall could account for more than 20 percent.
• A “risk score” helps a little. A 2019 study in the journal Cell combined millions of small variants into a single score. As reported by the Broad Institute and others, adults in the top tenth of scores weighed on average about 13 kg (30 pounds) more than those in the bottom tenth, and the difference showed up in early childhood.
• Rare, strong forms exist. In a small share of people, particularly those with severe obesity beginning in early childhood, a single gene change in the brain pathways that control hunger can be the main cause.

A 2022 review by Loos and Yeo points out that these common and rare forms share a theme: many of the genes act in the brain, shaping appetite and how full a person feels.

Why genes are only part of it

Obesity rates have roughly tripled over the past four decades, according to that same review. Our genes have not changed that quickly, so the change likely involves the environment, for example food that is cheap and easy to overeat, less movement in daily life and less sleep. Genes load the dice, and the environment shapes how they roll. This also fits what we know about the body defending a weight; see our guide to set point theory and to the hormones behind hunger.

Making sense of it

• Weight is not only willpower. People differ in how hungry they feel and how strongly food draws them. That is partly biology, which is one reason blaming people for their weight, the subject of our guide to weight stigma, is not supported by the evidence.
• Genes are not destiny. A genetic risk shifts the odds. It does not decide what a person can do about sleep, activity, food or medical care.
• Consumer gene tests do not predict weight well. The studies above used enormous groups. A single person’s score gives only a clue, and you do not need a test to start a healthy habit.
• Family history is useful to share. A clinician can use it, along with your other health information, when talking about options.

The takeaway

Genes play a real, substantial part in differences in body weight between people, but through thousands of small effects and a few rare strong ones, mostly in the brain’s appetite systems. The same genes sit in a very different food environment from the one our grandparents had. Both things are true, and neither one is a verdict.

Sources

• Elks CE, den Hoed M, Zhao JH, et al. Variability in the heritability of body mass index: a systematic review and meta-regression. Frontiers in Endocrinology, 2012;3:29.
• Locke AE, Kahali B, Berndt SI, et al. Genetic studies of body mass index yield new insights for obesity biology. Nature, 2015.
• Khera AV, Chaffin M, Wade KH, et al. Polygenic prediction of weight and obesity trajectories from birth to adulthood. Cell, 2019 (figures as reported by the Broad Institute and Harvard Gazette; the full paper was not reviewed).
• Loos RJF, Yeo GSH. The genetics of obesity: from discovery to biology. Nature Reviews Genetics, 2022;23(2):120-133.