When it comes to obesity across the generations, are we really destined to be overweight if our parents are? The answer to this question involves distinguishing between the risk factors associated with inherited obesity, linked to DNA, and those that relate to a person's environment in which junk food habits are ingrained, and how these factors themselves influence our genes as they pass from one generation to the next. “If a child has both parents who are obese, they have a 60% risk of becoming obese too. If only one parent is obese, the risk is 30%,” explains Philippe Froguel, a geneticist specialising in obesity and diabetes. “This is above average.”
As of 2024, 18.1% of French people are obese, which equates to nearly 10 million, up from 8.5% in 1997. And the proportion continues to grow. Obesity affects 21% of manual workers compared to just 12.7% of those in managerial positions. Among French people experiencing financial difficulties, 22% are obese.
Due to a lack of publicly-funded studies, these figures come from the OFÉO study – it stands for the Observatoire Français d’Épidémiologie de l’Obésité - which was published in June. Conducted by Odoxa, it was commissioned by France's Ligue Nationale Contre l’Obésité (LCO) and funded for the first time by Novo Nordisk. This Danish pharmaceutical company is closely monitoring this issue, and for good reason: it is set to launch its new cash cow, the appetite suppressant Wegovy which is already available in the United States, in France.
The regions most afflicted by obesity are France's overseas territories and the north of France, the only areas where the rate exceeds 22%. The consumption of sugary and fatty foods is a significant factor. For instance, in some overseas territories, commercially-sold yoghurt contains more sugar than that available in mainland France.
As for the prevalence of obesity in the north of France, “in colder regions, people eat more; it's a long way from the Mediterranean diet”, comments Annick Fontbonne, epidemiologist and head of the OFÉO study. “Moreover, there's significant poverty, and the poorest classes are the most obese. Since the industrialisation of food, ultra-processed foods have become cheap.” In 2017, twice as many adolescent children of manual workers were obese compared to the children of white-collar workers, according to a study carried out by the health statistical organization the Direction de la Recherche, des Études, de l’Évaluation et des Statistiques (DREES): obesity is no longer a disease of the wealthy.
In the long term, obesity, defined by a body mass index (BMI) over 30, leads to other associated pathologies. “The trouble starts when the body's fat storage capacity reaches its limit. It goes into the liver, muscles, pancreas...” explains Philippe Froguel, director of the European Genomic Institute for Diabetes. Obese individuals are more likely to develop diabetes, high blood pressure and sleep apnoea. This underscores the importance of limiting obesity as much as possible - whether in its onset or severity, regardless of its cause.
The gene behind a minority of obesity cases
So, are people obese due to genetic inheritance, diet, or lack of physical activity? “In most cases, it is a multifactorial disease caused by obesogenic environments, psychosocial factors, and genetic variants,” explains the state health authority the Haute Autorité de Santé in a guide to the subject.
A minority, around 1 to 2% of those who have developed severe obesity, with a BMI over 40, do so because of a specific gene. This type of obesity can be identified very early in childhood, often before the age of six. “These individuals suffer from an uncontrollable biological hunger - they're constantly, genuinely hungry. The neurons involved in the regulation of food intake do not function properly, meaning they need to eat much more to feel satisfied,” explains Karine Clément, a professor in the nutrition department at the Pitié-Salpêtrière Hospital in Paris.
Carrying such a load in their DNA, can they escape it? Philippe Froguel, professor of genomic medicine at Imperial College London, has studied the risk of becoming obese for individuals with this particular gene across three generations. The findings revealed that the grandchildren had an 80% chance of developing obesity, compared to 40% for the grandparents.
“The older generation experienced rationing after World War II and did not have access to unlimited food. They were constantly hungry but lived with it, and did not necessarily develop obesity despite their strong genetic predisposition. The younger generation, however, live in an obesogenic environment where food is readily available,” says the doctor.
Ultra-processed foods do not match with our internal signals of feeling sated; this has been studied by agri-food industries. We eat them without hunger, like lab rats.
Aside from this rare form of specific obesity, we are not all equal when it comes to gaining weight, due to the genetic predispositions we inherit. A landmark study from Quebec observed a group of twelve pairs of identical twins, who share the same genetic make-up. Over three months, the twins were given 1,000 extra calories per day on top of their usual intake, while their physical activity was limited to a daily 30-minute walk. Within each pair, the researchers observed similar weight gain and fat distribution.
However, during the experiment, some of the pairs of twins gained 4 kilos each, while others gained as much as 13 kilos. This is because some bodies simply burn more calories than others - a genetic injustice that leads many obese individuals to feel discouraged, with a sense of “what's the point?” Yet it does matter. “We’ve had this genetic make-up for years, but obesity is on the rise because we live in an obesogenic environment,” says Annick Fontbonne.
If you put a rat on a “cafeteria diet” rich in sugars and fats, giving it crisps, it will start snacking around the clock and gain weight. “For humans, it’s the same. We munch through an entire packet of crisps and don’t feel any sense of feeling sated. These ultra-processed foods don’t match with our internal signals of feeling sated; this has been studied by agri-food industries. We eat them without hunger, like the lab rats,” explains Annick Fontbonne.
Enlargement : Illustration 2
The worst part is that this obesogenic environment imprints itself on our bodies and alters how our genes function. This is 'epigenetics': on top of the inheritance of DNA, the consequences of poor dietary habits are passed down from generation to generation. However, not all is lost; the ever-increasing numbers on the scales can still be reversed. Limiting excessive consumption of junk food can reduce the risks of developing obesity in the future.
“There's a genetic predisposition, but with prevention, we can avoid a catastrophic situation with more and more obesity, including its most severe form, morbid obesity. We must find the means to stop this transmission. It won’t happen immediately - one or two generations are going to be sacrificed - but we need to think further ahead,” urges Anne-Sophie Joly, founder of the Collectif National des Associations d’Obèses (CNAO) which campaigns on the issue.
Prevention works
This underscores the importance of information campaigns and preventive measures, which have a significant impact on the most vulnerable populations, as they are more likely to suffer from obesity. “Families with the fewest resources have fewer sources of comfort available. Sometimes, the only way to bring joy to their children is by giving them foods high in fats and sugars, which they love because sugar is addictive,” observes Serge Ahmed, an addiction specialist at the University of Bordeaux.
In France the wealthiest 15% of households eat 12 kilos more vegetables per year compared to the poorest, notes economist Nathalie Mathieu-Bolh in 'Économie de l’obésité' published by La Découverte this year. Today, the less well-off consume more meat, but it’s usually processed meats, which are worse for health, whereas the affluent tend to favour chicken and lamb.
“Healthy foods are relatively more expensive. When their prices rise, the poorest households, who are more constrained by their budgets and need to feed their families at the lowest cost, are driven to buy food with low nutritional value but high caloric content as a form of substitution,” explains the economist.
However, there is she says also a “peer effect. Households with a higher social status, who are wealthier and more educated, tend to place more value on healthy food, which serves as a symbol of social status.” This raises concerns that the gap may widen further between the poor, who are increasingly obese, and the rich, who are more inclined to restrict their children’s sugar intake.
Despite genetic predispositions and differing family eating habits, not all children of obese parents who grew up with cupboards full of food treats end up becoming obese themselves. In the same way that “not all children of alcoholics become alcoholics in adulthood. Some, having witnessed first-hand the damage caused by obesity in their loved ones, decide to radically change their diet and lifestyle,” says Serge Ahmed.
Alongside what we eat, physical activity helps burn calories and therefore limits weight gain. Yet again, we are not all equal in this regard. American researchers followed more than 3,000 adults over five years. Their study, published in March 2024, shows that to avoid obesity, those most at risk due to their genetic heritage need to walk between 3,500 and 6,300 steps more per day compared to those least predisposed, equating to an extra 2 to 4 kilometres—or between thirty minutes and an hour of additional daily walking. The gap is even larger for those whose BMI is already close to obesity.
“Physical activity can prevent obesity, but for some with the heaviest genetic baggage, it’s almost impossible to escape it through this means alone. For them, it’s like cycling uphill while others can ride on the flat,” says Philippe Froguel.
Nevertheless, another study conducted in Denmark and Finland on identical twins shows that the development of body fat is significantly reduced by physical activity, which the scientists suggested is “able to modify the action of the genes responsible for predisposition to obesity”. Starting from an equal genetic footing, sport makes a difference.
Importantly, the earlier prevention begins, through physical activity and diet, the more likely it is to be effective. “We manage to cure about half of childhood obesity cases,” says Philippe Froguel. There is a key period, between the ages of 5 and 12, when it is possible to counteract increased sedentary behaviour, something which again disproportionately affects the children of less affluent families.
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- The original French version of this article can be found here.
English version by Michael Streeter