Is obesity a disease?
I believe that obesity is a combination between genetics and lifestyle. It is too easy to simply say that genetics, or lifestyle is the sole cause of obesity. There is evidence that both are factors of causing obesity, or even anorexia.
There is recently discovered evidence that suggests genetic predispositions can affect our weight and how much we eat. Three known hormones that affect appetite are ghrelin, leptin, and obestatin. Ghrelin which is secreted by cells lining the stomach and the hypothalamus in the brain, has been linked to causing hunger. When ghrelin levels are high in our body, we feel hungry. After we eat, the ghrelin levels diminish. Leptin and obestatin are hormones that suppress appetite. These hormones are released by adipose tissue. Studies conducted on rats injected with obestatin showed that the the rat's appetites were suppressed. There are other hormones suspected of influencing appetite, and this is a growing scientific field.
Thyroid hormone has been linked to controlling our weight. The thyroid hormone affects our weight by controlling our basal metabolism. It controls how fast, or how efficiently our bodies use our food calories. There can be a 2.5 fold difference in what is considered a low normal thyroid level and a high normal thyroid level in our blood. Someone with a high-level thyroid would also have a fast metabolism. It is conceivable that people with naturally higher thyroid levels would be thinner, and vice versa. As a side note, having a fast metabolism would not be ideal in a situation where food is scarce.
Insulin can also affect our weight. It is needed for glucose to enter cells. Some people may produce more insulin than others and therefore the sugar enters cells faster. They are then hungrier faster. If the sugar is not needed, it will turn into fat.
Other hormones which could affect are weight are the sex hormones. Testosterone can increase muscle and estrogens can increase fat. Growth hormone may also be involved. In conclusion, there are many hormones which can influence body weight and appetite, and genetic predisposition plays a role in how much influence these hormones have for an individual.
I believe our lifestyle definitely affects our weight. In America, I don't think we can blame obesity on some widespread genetic disorder. We are genetically similar to our grandparents, yet they did not suffer widespread obesity. We are genetically similar to our counterparts in Europe, Asia, Africa, yet they don’t suffer from obesity like that present in the USA. We do not have a scarcity of food, and we have machines to do some of the things that used to be done by hand. Our country is also “young”, compared to other countries and many developments are built around the use of cars. New developments do not have shopping areas in walking distance and often there are no sidewalks. We are constantly bombarded with advertisement for tasty fast foods, and at the same time, magic diet programs. While this may be a cause for some individuals to become obese, I still think that obesity can be controlled. People with a slow metabolism can still fight obesity by how they live. It may be harder for them but it is doable. They may be genetically predisposed to be overweight, but if they change their lifestyle, they can at least try to prevent the progression from overweight to obese.
In summary, there is growing evidence that hormones affect our appetites and weight, and we already know that lifestyles, what and how much we eat, lack of exercise, etc affect our weight as well.