In the past, the first law of thermodynamics was often used to explain the control of body weight. Simply put, if energy intake (food) exceeds energy expenditures (exercise/activity), then weight gain occurs. Conversely, reducing intake and increasing expenditures was believed to be the key to weight loss. For years, doctors and researchers believed this simple theory to be the answer. We now know, however, that other factors are involved. Some people can exercise religiously, reduce food intake and still not lose weight. And, of course, we all know people who can eat whatever they want and never gain a pound.
Weight gain and obesity are complex conditions, dependent upon various lifestyle, hormonal, biochemical, metabolic and genetic factors. Some of the most important factors include:
Basal Metabolic Rate
Your BMR rate at which your body burns calories at rest. This rate is dependent on several of the factors listed below, such as activity level and thyroid function.
Overeating and consuming more calories than your body uses for energy can result in weight gain – regardless of whether those calories come from fat, carbohydrates or protein.
Quality of food
Eating too much saturated fat, sugar, processed food and fast food is associated with weight gain.
Your activity level is the major player in weight balance. Inactivity causes loss of muscle mass, a reduced metabolic rate and increased body fat. Conversely, regular exercise can improve muscle mass and boost metabolism. As we exercise, our muscles utilize calories for energy and generate heat, which promotes the burning of fat.
Exposure to chronic stress can cause weight gain, particularly around the mid-section. This occurs because stress increases the production and release of Cortisol, a hormone that increases body fat storage.
The thyroid gland plays a vital role in controlling metabolism. If your thyroid is sluggish and not functioning optimally, this can reduce your metabolic rate and cause weight gain.
When insulin levels are high, the body stores more fat and is not able to use fat as a source of energy – the reason insulin is also known as “the fat storage hormone.” This can be a problem for those with insulin resistance who often develop hy-perinsulinemia (high insulin levels).
Genetics may be responsible for about 25 percent of obesity cases, but experts agree that having a genetic predisposition towards obesity does not mean that this is your fate. Several studies have shown that lifestyle factors are a more important determinant.
High estrogen levels are associated with weight gain. Yet, many women find that they gain weight during menopause while their estrogen levels are lower. This happens because as estrogen levels decline in menopause, as a compensatory mechanism, the fat cells take over the production of estrogen. In order to meet the growing demand during menopause, they increase in size and number.
Testosterone helps the body maintain lean muscle mass and burn fat. A deficiency of this hormone can cause the loss of muscle mass and fat gain. This is a significant contributor to fat gain in older men.
Human growth hormone (HGH)
By increasing lean muscle mass and reducing body fat storage, human growth hormone regulates body weight. Levels decline with age, particularly after age 50, causing a shift in our body composition. As HGH decreases, we gain body fat and lose muscle mass.
Serotonin is a chemical messenger in the brain that regulates satiety. When levels are low, we feel hungry and when they are high, we feel satisfied. Certain weight-loss products work by elevating serotonin to promote satiety and reduce cravings for food. Leptin Satiety is also regulated by leptin, a hormone produced by body fat. Researchers have found that some people become resistant to their own leptin. To compensate for this the body produces more and more of the hormone, but the “satisfied” message is not property received by the brain.