Contrary to popular belief, body weight is not an accurate indicator of whether someone follows a healthy lifestyle. By Jennifer Bowden.

Question:

I see many articles in the media about obesity, most of which suggest strategies
such as diet and exercise to reduce body weight. I’m 74, and I’ve eaten prodigious quantities of food and imbibed significant amounts of beer daily, yet I’m 180cm tall and weigh just 66kg, placing me in the lowest BMI category. There are lots of people like me. Have researchers studied the fundamental differences between people like me and the obese? Such findings could be practically useful.

Answer:

The irony that some people can eat whatever they like and remain thin while others gain weight on a very low-energy diet is lost on many people in society. Some people still characterize overweight people as lazy or lacking in willpower. Yet if body size was an indicator of willpower and exemplary eating and exercise habits, why do most of us know a person who eats an unhealthy diet, never exercises and yet is thin? Something else is going on.

Unhealthy food environments have significantly driven unplanned weight gain in Western society, but each individual’s response to this varies considerably. Some gain weight while others remain stubbornly thin.

Researchers have investigated the factors behind these varying individual responses, and their findings are thought-provoking. For starters, thinner people do not exercise more than normal-weight people, according to a new study published last month in the journal Cell Metabolism. Indeed, the researchers in China found that thinner adults were 23 percent less active and ate 12 percent less food than normal-weight adults.

Clearly, thinner people are not thin because they exercise more and eat less than average, heavier people. Instead, this study suggests thin people likely eat less food because they are not as hungry.

The reality is that an individual’s genetic profile significantly affects their body weight and how their body responds to living in an unhealthy environment that promotes overconsumption and underactivity.

Researchers have used studies of twins to identify genes linked to body weight. Numerous studies investigating the genetic profile of overweight people have also identified hundreds of genes that increase the chance of a person being overweight. Some of those genes have such an effect that they can cause severe obesity from childhood.

Those genetic alterations affect the individual in different ways. For example, some altered genes may cause an individual to always feel hungry. In other cases, different genetic alterations may affect how the body uses energy from food, and altered genes may cause the body to more readily store excess calories as fat.

If you inherit all of the above genetic alterations, your chances of being overweight are significantly higher. Indeed, a 2019 study published in PLOS Genetics confirmed this by comparing the frequency of these genetic alterations for body weight in thin people compared with others.

Researchers analyzed the DNA of some 14,000 adults who were a mixture of underweight, average weight and overweight to determine the degree to which genetic variants might play a role in thinness versus being overweight. Then, the contribution of different genetic variants was added up to work out a genetic risk score for weight gain. They found that overweight people had a far higher genetic risk score than average people, contributing to their chances of being overweight. “The genetic dice are loaded against them,” says Dr Inês Barroso, whose research team at the Wellcome Sanger Institute in Cambridge, England, contributed to the study.

Conversely, they found thinner people had a much lower genetic risk score; that is, thinner people had fewer genetic variants that increase the chances of an individual being overweight.

Ultimately, the genetic architecture of thin people vastly differs from that of overweight people. So, although some thin people can eat and drink whatever they like without gaining a single gram, many overweight people can have a far healthier diet and be more physically active, yet weigh more.

As the saying goes, never judge a book by its cover, which is undoubtedly true regarding weight. Contrary to popular belief, body weight is not an accurate measure of one’s commitment or adherence to a healthy lifestyle.