Why some people gain weight and others don't - genetics and environment share the responsibility equally
A question that everyone has asked themselves after watching someone they know devour cake with no apparent effect on their figure, received a detailed answer in a Harvard Health Publishing article dated 24th June 2019.
Body weight is determined by the balance between calories consumed and calories expended, yet this balance is simultaneously shaped by heredity and the conditions of everyday life.
Different people have different baseline settings for metabolism and appetite, and the modern environment further amplifies the risk of weight gain.
The simple arithmetic of energy and fat cells
Weight changes according to three variables: how many calories a person takes in through food, how many are burned through physical activity, and how many the body uses at rest. When expenditure consistently equals intake, weight remains stable. Surplus energy is directed into fat cells, which either expand in size or form new fat depots.
When there is a calorie deficit, the process reverses: fat cells shrink, and stored reserves diminish. The mechanism is essentially universal, but the speed and "eagerness" with which the body accumulates or releases fat is largely determined by genes.
More than 400 genes and a range of influence from 25% to 80%
More than 400 genes have been linked to obesity and excess weight, although only a few dozen play a key role. Their influence manifests through multiple channels: appetite and satiety, metabolic rate, cravings for certain foods, distribution of body fat, and a tendency toward stress eating. In some people, genetics accounts for roughly 25% of predisposition; in others, that share reaches 70–80%.
Markers of a high genetic contribution include the following:
- Excess weight has been present from an early age or for a significant portion of life.
- One or both parents, as well as close blood relatives, have pronounced obesity (when both parents have obesity, the risk in offspring reaches approximately 80%).
- Weight loss does not occur even with prolonged calorie deficits and regular exercise.
Signs of a more modest genetic role are different:
- Weight is strongly influenced by food availability and daily routine.
- Weight loss is quite achievable on a "sensible" diet with moderate physical activity.
- Weight gain is typical during holidays, lifestyle changes, or periods of emotional difficulty, but responds well to correction.
An evolutionary legacy that has become a trap
The so-called thrifty genes once helped ancestors survive periods of famine by enabling efficient energy storage. Today, when food is available around the clock and there is no need to hunt for it, this adaptation has become a risk factor. Researchers estimate that approximately 85% of people carry such genes.
Environment has outpaced genetics: the rise in obesity since the 1970s cannot be attributed to DNA
The human genome does not change over half a century, yet the prevalence of obesity among American adults has been rising since the 1970s and affects all demographic groups. This means that external conditions play the primary role: everything that nudges people to eat more and move less.
These factors begin to operate long before a child's first birthday. Maternal smoking and diabetes during pregnancy are associated with an increased risk of obesity in the child later in life. Breastfeeding for more than three months, by contrast, reduces the likelihood of excess weight in adolescence. Childhood habits — sugary fizzy drinks, processed food, screen entertainment instead of active play — often become entrenched for life.
More calories, larger portions, less movement
According to CDC data, the average man now consumes 168 more calories per day than in 1971, and the average woman 335 more. Food is available everywhere: at gas stations, gyms, and around-the-clock convenience stores. The share of spending on food away from home rose from 27% in 1970 to 46% in 2006. A serving of French fries at McDonald's today contains three times more calories than when the chain was founded. Notably, the proportion of fat in the diet has declined since the 1980s, yet "fat-free" products often compensate for flavour with sugar and end up being more caloric than their regular counterparts.
Fewer than 25% of Americans meet the recommended one hour of moderate or vigorous activity per day. Sedentary work, reduced physical education classes, car-dependent culture, and the mechanisation of daily life have reduced everyday movement to a minimum.
Screens, stress, and sleep - the invisible accelerators of weight gain
Average television viewing time is approximately 4 hours per day. Watching more than 2 hours increases the risk of obesity even in three-year-old children. The problem is not only physical inactivity: roughly 11 food advertisements per hour of airtime provoke snacking, and eating in front of a screen increases calorie consumption.
Television has long ceased to be the only screen in the home. A significant portion of leisure time is now divided among smartphones, laptops, and gaming consoles: people watch series, interact on social media, and play video games. The entertainment industry is increasingly moving toward interactive formats, where the viewer does not merely watch but participates in what is happening. In the evening, someone might tune into a live stream where a live host spins the wheel, and the dream catcher live game format keeps viewers engaged just as effectively as a TV series: rounds are short, and the next one begins almost immediately. Such activities do not necessarily lead to weight gain in themselves, but hours of sitting reduce time spent moving, and a hand quietly reaches for a snack. When assessing obesity risk, it is therefore important to consider not only diet and exercise, but also how everyday leisure is structured — especially if it extends late into the night and cuts into hours of sleep.
Stress, long working hours, and lack of sleep complete the vicious cycle. A study involving more than 1,000 volunteers found that people who slept fewer than 8 hours had a higher proportion of body fat and greater body weight. Irregular eating patterns can disrupt circadian hunger and satiety signals. Anxiety and depression encourage overeating, while unsuccessful attempts to lose weight intensify stress, creating a self-reinforcing cycle.
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