Obesity is associated with many health problems, including cardiovascular disease, type 2 diabetes, sepsis, liver disease, and lung disease. Current research points to inflammation as the real culprit. Adipose tissue (commonly referred to as “fat”) is now recognized as more than a place to store energy for times when we need it. This tissue produces a variety of peptides that influence the immune system and serve other functions in the body. And too much stress on the adipose tissue can throw other systems out of balance.
To me, the most significant part of my first reference listed below is the suggestion that adipose cells behave differently when they're under stress from their work: either storing or releasing large amounts of energy in the form of fat. This could explain why yo-yo diets may be worse, in terms of health effects, than a constant level of obesity. This is not to say that constant obesity is safe; just that gaining and losing large amounts of weight rapidly and repeatedly may be a higher risk factor. Inflammation is generally associated with tissue repair, as well as fighting infection. Whenever cells have to work overtime to do their jobs, the inflammatory process may be involved.
Leptin, adiponectin, and resistin are three of the most important peptides produced by adipose tissue. Leptin is best known for its ability to decrease hunger, but it also increases the production of pro-inflammatory TNF-alpha, and promotes the formation of blood clots. Obesity may be associated with an imbalance in leptin production. Adiponectin is an anti-inflammatory agent which also enhances insulin sensitivity. Its concentration in the blood is decreased in the obese state. Resistin does the opposite, and may be over-produced in obese persons.
Too much adipose tissue is believed to contribute to a state of chronic inflammation. The pro-inflammatory peptides produced by fat cells are thought to “prime” the other tissues of the body, making them more sensitive to further inflammatory stimuli. This might be OK in case of infection, but the immune system can do a great deal of damage to healthy tissue when there are no invading microbes to fight.