There is no one single gene that causes obesity. Weight gain is the byproduct of multiple systems acting to increase hunger, reduce metabolism, and absorb and store fatty acids. If there is a “fat gene”, it is most likely a set of interacting genes and will be different for each person depending on which system is erratic.
Leptin, grelin, and neuropeptide Y are three proteins involved in hunger. Produced by the brain, the stomach, and in response to the sympathetic nervous system these proteins tell a person that they are in need of sustenance or tells them when they are full. There are also stretch receptors in the stomach to aid in the full feeling. For many reasons – receptor dysfunction, sensitivity to one particular protein, or a lack of a particular protein – a person may eat beyond their body’s requirements. Over time the stomach may become larger and the “full” feedback system becomes disrupted, leaving hunger to the overactive proteins or receptors. This system alone would involve more than 6 genes.
Metabolism is a complex system of chemical reactions involved in using energy for life functions. Individuals with a “slower” metabolism, due to genetics, sedentary lifestyle, disease, or lack of activity, need to take in less energy and are less likely to utilize stored energy. The problem is that certain vitamins and elements are needed to be taken in through the diet and are often accompanied by the calories that will not be used. Instead the calories are stored as fat, though the person did not overeat. Their metabolism is simply not working enough to use that energy. Metabolism is also affected by hormones such as those produced by the thyroid and by blood pressure and blood sugar. Low caloric intake has been said to decrease metabolism thus making dieting an inefficient solution.
Lipid metabolism is the process of absorbing, storing, and using fatty acids. Lipids are necessary for vitamin absorption, adipose tissue that cushions the organs, cell membranes, and cholesterol production – even the good kind HDL. Excess glucose is stored as fatty acids so both sugars and fats contribute to obesity. The stored energy is essential to being alive and active. The number of genes potentially responsible for obesity due to metabolism is in the dozens.
Problems with the gall bladder, or its previous removal, have been implicated with fat digestion problems. Inability to properly absorb fats would lead to increased signaling within the body that sustenance is needed. This would lead to overeating which would lead to stretching of the stomach and problems in the feedback system. The extra calories taken in with the needed fats, since cell membranes require their intake or production, would lead to weight gain. A deranged signaling system that has been created would then make the problem worse and so on. Obesity would develop due to a cascade of problems that may begin with a glandular problem (thyroid, gall bladder) or lack of nutrition (not enough fats, too little calories) and lead to a metabolic slowdown or a receptor signaling problem, or vice versa. It will be different for each person.
It’s too complex to be solved by locating a single gene, or even a dozen.
