There’s no doubt that obesity is on the rise – and public health efforts to slow the rise in obesity have, so far, not been particularly effective. The message to cut back on sugar and eat more fruits and vegetables has seemingly fallen on deaf ears as the number of people who are overweight and obese continues to rise. But, is it appropriate to blame the rise in obesity solely on lifestyle or could it be genetics? Is the obesity epidemic really due to overeating and under-exercising or is there a considerable genetic component to blame for the growing number of people who are an unhealthy body weight?
Is It Genetics or Lifestyle?
You’ve probably heard people say that they eat a healthy diet, watch their portion sizes, exercise regularly and still can’t maintain healthy body weight. Is genetics working against them? No doubt, genetics is a factor in who becomes obese. If you look at a family tree of an obese individual, you would likely discover that a number of family members have problems controlling their weight. Scientists are still trying to identify specific genes and gene mutations that contribute to obesity.
Based on what we know, there seem to be genetic factors that impact appetite and metabolism and predispose an individual to obesity. Fortunately, these genetic factors don’t condemn a person to weight gain but, instead, lower the threshold for putting on weight. In the right environment and with the right motivation, an individual with “bad genetics” can still maintain healthy body weight. In other words, genetics isn’t destiny.
On the other hand, if you place a high-risk individual in a setting where they have easy access to fast food and few opportunities to exercise, they will easily put on substantial amounts of weight. Regardless of genetics, environmental factors have a substantial impact on body weight. Staying a healthy body weight is more of a battle for some people than it is for others and the reasons are multi-factorial.
Genetics of Obesity
Although scientists have yet to identify all of the genes associated with appetite, metabolism, and obesity, one of the best-characterized gene mutations linked with obesity is a mutation in a gene called the ob gene, a gene that codes for leptin. As you might know, leptin is one of the hormones that regulate appetite. This appetite-regulating hormone also has an impact on resting metabolic rate. The brain and leptin maintain a close line of communication – and for good reason! Your brain wants to know how much stored body fat you have at any one time to help you avoid starvation. If you have plenty of body fat on hand, fat cells release leptin into your bloodstream and it tells your brain that you’re in a well-fed state and can stop eating. In response, your brain blunts your appetite and, at the same time, boosts your resting energy expenditure. There’s plenty of fuel available, so it’s okay to burn some.
What does this have to do with the genetic tendency towards obesity? Normally, fat cells produce leptin under the direction of the ob gene. Once the ob gene gives the okay, leptin enters the bloodstream and travels to the hypothalamus in the brain where it boosts the production of chemicals that decrease appetite. As you might imagine, if you can’t produce leptin or can’t make enough of it, your appetite is constantly turned on and you overeat.
In fact, mice that have mutations in the ob gene that completely knock out its activity, eat voraciously and become morbidly obese. Like mice, humans can have mutations in the ob gene that reduce their ability to produce leptin as well as mutations that alter the leptin receptor in the brain. In response, the leptin receptor doesn’t respond as readily to leptin and appetite isn’t turned off as easily. Resting metabolic rate also slows when leptin is low or the receptor doesn’t work properly. So, it’s a double whammy from a weight gain standpoint.
More recently, scientists discovered that obese people are more often leptin resistant than leptin deficient. They have such high circulating levels of leptin due to having too much fat tissue that leptin receptors become desensitized and don’t respond as well to leptin signaling. This leads to overeating and weight gain.
The FTO Gene
Another gene linked with obesity that researchers have identified is a gene variant of a gene named the FTO gene. This genetic variation is surprisingly common with up to 40% of people of European origin carrying a copy of the variant. Having this gene variant doesn’t guarantee that you’ll become overweight or obese but it boosts the odds by 70%. According to studies carried out by researchers at the Medical Research Council (MRC) and King’s College London Institute of Psychiatry, people with this genetic variation produce more of an appetite hormone called ghrelin. Cells that line your stomach pump out ghrelin when you haven’t eaten in a while and it turns on the desire to eat. People with this variation in the FTO gene don’t stay full as long after a meal and prefer higher calorie foods. They’re also are at a disadvantage because their appetite is more voracious due to the activity of ghrelin.
Fighting the Battle Against Obesity
These are only two genetic influences on obesity. There are likely many more that have an impact on appetite and energy expenditure that have yet to be discovered – but here’s the good news. Researchers believe that only about 25% of a person’s body weight is genetically determined. That means other factors, like diet and physical activity, explain most of an individual’s body weight. A study also showed that people with the FTO gene variant that predisposes to obesity can avoid becoming obese by exercising at least an hour daily most days of the week. An hour a day to lower your health risks? That’s a good trade-off!
References:
PLOS One. “FTO Gene Variant and Risk of Overweight and Obesity among Children and Adolescents: A Systematic Review and Meta-Analysis” November 22, 2013.
UCL News. “How ‘obesity gene’ triggers weight gain”
WebMD. “Exercise Can Overcome Obesity Gene”
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