There are many genes that have been discovered to play a role in obesity. However, most of these genes interact in complex ways with other genes. We don’t fully know how most of these genes work.
However, there are a few genetic defects that are known directly to cause obesity,
The genetic defect that is now treatable involves leptin, a hormone that is made by fat tissues. It tells the brain how much fat the body has. If the brain does not receive enough leptin, it attempts to make the body gain weight.
Some people have a genetic defect that prevents them from making any leptin at all. Therefore, their brains keep trying to make the body store more fat, even if the person already has a great deal of fat. Also, the brain turns down the immune system and does other things to conserve energy. About half of children who cannot make leptin die from infections before they grow up.
Artificial leptin is now available and has been used to treat this problem. The leptin injections cause a dramatic loss of fat. No dieting or special exercise is necessary, just the injections of leptin
There are studies correlating the weight of children to the weight of their mother during pregnancy, the reason this study is important is all of the children were adopted, so it directly addressed the question of environment vs genes.
That is, children raised in a different family were still more likely to be overweight if their birth mother was overweight, regardless of the fitness level of the adopting family.
But it is not hereditary in the sense of, say, eye color. There isn’t a gene that makes you fat, though there are genes which can contribute, which leave you more at risk of experiencing issues like insulin resistance or poor physical resistance to stress, for example. The correlation between the weight of the child and the weight of the birth mother was one of insulin resistance.
Insulin cannot pass through the barrier between mother and child, but the sugars in the diet can. A child of an overweight mother who over ate sugars and starches was not directly exposed to the insulin levels of the mother, but was directly exposed to their diet, forcing the child in utero to be producing excessively high levels of insulin already.
That means they are already showing signs of chronically elevated insulin levels at birth, and that can influence how their bodies process food. That means even in a healthy weight, fit family, an adopted child will still be subject to the influences they experienced in their mother’s womb.
From what I have observed over the years, a large part of it is genetic. Some of the factors that contribute to obesity are.
1. Disinclination to exercise
Some people are less active than others, preferring to move less and rest more. I think this is a tendency we are born with, much like personality. Exercise here doesn’t have to mean sport. Fidgeting, for example, is also exercise - a constant involuntary activity that burns energy
2. Mental inactivity/calmness
The brain burns a lot of energy. The more you think, worry, fret, stress, etc., the more energy you burn. Some people are born calmer or less mentally active than others, same as with physical activity.
It is well known that some people have higher metabolism than others, and higher metabolism = faster rate of energy burn. Metabolism is partly determined by the combined effect of 1 & 2. i.e., if you have greater physical and mental activity, you burn more energy, but there are also differences in metabolism not accounted for by 1. And 2. Some people’s bodies just run faster and hotter than others.
4. Greater reward response to food
Some people’s brains react more strongly to sweet and fatty tastes. Compared to those who react less strongly, these people find it harder to resist foods containing large amounts of sugar and fat? It is not a moral failing on the part of these people. Their brains are just wired this way. This pleasure reaction to food is nature’s way of ensuring that we don’t starve to death. Hence, those that have a stronger reaction, from nature’s point of view, are superior specimens.
5. High nutrient absorption
Some bodies are more efficient at extracting and storing food energy than others. If two people ate the exact same meal, one person might frugally absorb every bit of nutrient from the food, while the other might absorb a bit and excrete the rest as poo. Again, from nature’s point of view the former is superior, with the ability to weather famines since a little food goes a long way. In fact, it is believed the majority of people in our time fit into the former category, since those in the latter would have died of starvation along the way, and the ‘profligate’ genes would have died out
All of the above factors have a strong genetic component to them. This does not mean that people who inherit these factors are doomed to be obese, but it does mean that they will find it harder to lose weight than others, and will have to put in more effort to achieve the same results