The underlying causes of obesity can vary between people and typically involve a mix of both genetic and environmental factors.

Obesity is a chronic health condition defined as the accumulation of excess body fat to the point where it becomes a health risk. While this can mean something different for everyone, obesity is typically screened for using body mass index (BMI), a calculation of your weight in relation to your height.

BMI isn’t a fail-proof indicator of obesity. It doesn’t directly measure fat or health, and it can be inaccurate among individuals with higher levels of muscle mass, such as bodybuilders. It does, however, provide a universal way for doctors and scientists to monitor weight among the general public.

Obesity by BMI is defined as a score of 30 or higher. It’s then subdivided into classes indicating the severity of obesity:

  • Class 1: 30 to <35 BMI
  • Class 2: 35 to <40 BMI
  • Class 3 (severe obesity): 40 or higher BMI

According to the latest obesity data from the Centers for Disease Control and Prevention (CDC), between the years of 2017 and 2020, obesity prevalence among the sampled adult U.S. population was 41.9%.

Genetics can contribute to the causes of obesity both directly, as in Prader-Willi syndrome, and indirectly by influencing metabolic pathways, neural networks, and appetite control centers in the body.

According to a 2022 clinical review, more than 500 obesity-related genes have been identified in humans.

These genes can affect body weight in many ways. They can create changes in insulin metabolism, inflammatory responses, blood pressure, fat deposition, and the level of circulating fat in the bloodstream. They can cause you to want to consume more energy (i.e., food) but can prevent you from using it effectively.

Sometimes the link between certain genes and obesity is distant but still relevant. For example, certain genes may predispose you to conditions that increase the risk for obesity, such as Cushing syndrome.

Research from 2021 indicates that 40–50% of the variability in body weight can be traced back to genetics, with genes playing a more significant role (as high as 80%) among people living with obesity.

This doesn’t mean genetics definitely leads to weight gain. For most people, it means genetics may make it more likely for you to gain weight, particularly in certain circumstances.

Is there an obesity gene?

There’s no single gene universally responsible for obesity, though single gene variations may underlie obesity for some people.

Overall, hundreds of genes have been linked to the accumulation of excess body fat, and the genetic variations responsible can range drastically among people living with obesity. You may live with one genetic variation or many.

Genetic obesity is categorized into several types based on the way genes are involved:

  • Monogenic obesity: Excessive weight caused by a mutation in a single gene. The MC4R gene is the most commonly affected gene related to obesity.
  • Polygenic obesity: Variations in multiple genes contribute to the susceptibility to obesity in small ways. Polygenic obesity is the most common form of genetic obesity.
  • Syndromic obesity: Genetic changes in specific diseases, like Prader-Willi syndrome, can directly lead to obesity.

You can develop obesity if you regularly consume more calories than you use, with or without any influence from genetics. This energy imbalance is often attributed to external variables known as environmental factors.

Environmental factors can be everything from the size of your food portions to the built environment around you. They affect the amount of energy you take in and how your body uses it.

The most common environmental factors linked to obesity include:

Social determinants of health

Social determinants of health (SDOH) are a collective group of environmental factors that also influence obesity. Your SDOH are the conditions that make up the environment around you. Income level, local crime rate, accessibility of quality food, and discrimination exposure are all examples of SDOH.

These factors can promote lifestyle choices that contribute to obesity. If you live in a high-crime neighborhood, for example, you may be less likely to go outside for exercise.

For most people living with obesity, excess fat accumulation is the result of both genetics and environmental factors.

Environmental factors set the stage for lifestyle choices that promote obesity, while genetics can increase how prone you are to putting on excess weight in those circumstances.

The relationship can be bi-directional, however, meaning it goes both ways. Just as genetics can increase the effect of environmental factors, environmental factors can affect genetics through epigenetic changes — changes in the expression of genes.

For example, research indicates environmental factors like high intake of fried foods, poor sleep, and a sedentary lifestyle can cause epigenetic changes that promote obesity.

Exposure to endocrine disruptors, chemicals that impair hormone signaling in your body, can alter your intestinal microbiome, which has also been linked to changes in genetic expression.

Obesity can be challenging to treat, but it typically starts with lifestyle changes related to balanced nutrition, calorie reduction, and increased physical activity.

For many people, treating obesity is not as simple as restricting calories consumed and increasing calories burned. If environmental factors like stress, sleep habits, and SDOH can’t be easily resolved, losing weight can be extremely difficult.

In addition to energy balancing, many treatment plans for obesity also include:

  • nutritional counseling
  • behavioral strategies
  • medication
  • surgery
  • psychotherapy
  • support groups

Can genetic obesity be treated?

Obesity can often be treated even when genetics play a significant role. Successful management may require a team of medical professionals, including pediatricians, endocrinologists, obstetricians, nutritionists, geneticists, genetic counselors, and psychologists.

With the guidance of these experts, lifestyle modifications, medications, and surgery form the foundation of genetic obesity treatment. Your individual plan will depend on how genetics influences your body fat accumulation.

Treatment can vary in genetic obesity, for example:

  • If you live with syndromic obesity, treating the underlying condition may improve the symptoms of obesity.
  • If you live with obesity related to endocrine disruptor exposure, an endocrinologist may suggest certain lifestyle changes, specifically related to what you put in and on your body.
  • When genetic obesity is monogenetic and hereditary, lifestyle changes implemented in childhood may improve health outcomes in adulthood.

Obesity can be environmental, genetic, or both. But that doesn’t mean having genes associated with obesity will always lead to weight gain.

For most people, obesity can be traced back to underlying genetic factors that increase the likelihood of gaining weight under certain circumstances. The relationship also works in the opposite direction — environmental factors can alter how your genes work, which then influences fat accumulation in the body.

Obesity from any cause is treated with lifestyle changes, medications, and surgical interventions. When genetics are a significant factor, additional experts, like a geneticist, may become a part of your medical team.