Basic Physiology of Obesity

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Obesity is the result of excess calories, in the form of triglycerides stored in billions of fat cells or adipocytes. When the calories in versus calories out equation favors excess calories in, then the patient gains weight as fat cells fill up with triglycerides. Excess calories, ingested from carbohydrates, proteins, or fats, are not melted away, eliminated through the kidneys, or passed through the colon. The math is simple. A weight increase of one pound is the result of 3500 extra calories consumed, and the loss of one pound of weight is the expenditure of 3500 calories.

If the caloric seesaw tips towards a negative balance, then the body turns to the adipocytes for release of stored energy. This process is called lipolysis. Stored triglycerides are broken down into glycerol and non-esterified free fatty acids (FFAs) and released into the circulation to be used by various cells for energy. If enough lipolysis occurs, the fat cells shrink and the patient loses weight.

Obesity is a chronic disease based on the fact that fat cells shrink or expand but they never go away. When communicating this to patients, I use the analogy that adipocytes are like balloons. Without water in them, they have little weight, but when filled with water, balloons weigh as much as the water placed in them. In terms of weight lost and weight regain, fat cells weigh as little or as much as the triglycerides stored in them.

Most obese patients have lost and regained weight over the years but never knew this simple fact about adipocytes. Repeated weight loss followed by weight gain is easily understood using the balloon analogy. It is this basic physiology that explains why obesity is a chronic, recurrent disease driven by the seesaw balance of the calories in versus calories out equation.

Before the body stores excess calories as triglycerides, it tries to use the ingested calories as energy. It does this in three ways: basal metabolic rate (BMR), thermogenesis, and physical activity [26]. Like the idling of an engine, the basal metabolic rate is the body's constant conversion on the cellular level of ATP to ADP for energy. This continuous utilization of energy accounts for 70% of the body's daily caloric expenditure. BMR is influenced by thyroid conditions. BMR slows with hypothyroid and increases with hyperthyroid conditions. Consequently, people with hypothyroidism are often obese because of a slow metabolism, and people with hyperthyroidism are commonly thin.

The muscular mass of an individual influences the caloric needs of the person. Muscular individuals of the same weight as an obese person burn more calories because muscle tissue utilizes more calories per pound than does fatty tissue per pound.

Gender, weight loss, and age impact caloric requirements. Males typically have more muscle mass and therefore expend more calories than females. This is why the average daily caloric requirements for males are higher than for females. As people age they need fewer calories, in part because they have less muscle mass [26]. For both genders, weight loss reduces caloric demands by as much as 30%.

Thermogenesis accounts for about 15% of the body's caloric utilization. Ingested food creates heat by increasing sympathetic tone, raising catecholamine levels, and increasing insulin levels. Thermogenesis is decreased with aging and possibly in insulin-resistant conditions [26].

Physical activity accounts for approximately 15% of calories burned each day. While some people enjoy exercising daily, others have an aversion to the thought of doing jumping jacks, swimming, running, or lifting weights. For whatever reason, the number of individuals who dislike exercising is significant. According to the US Surgeon General's 1996 report on physical activity and health, about 60% of the American population is not regularly physically active, and about 25% is not active at all [27]. Many obese patients are either physically handicapped or else physical activity is too dangerous for them until they lose weight and can avoid injury.

Though there is a wide range in the amount of physical activity among individuals, each person's daily physical activity routine is fairly constant. Consequently, obese patients who have a high level of physical activity are not likely to greatly increase that level in order to lose weight. Physical inactivity is not the cause of their obesity; therefore activity is not the answer either. On the other hand, those who are not physically active are not likely to dramatically increase their level of activity for the reasons already mentioned.

Reduced physical activity or physical labor characterizes American workplaces and home environments. Labor on farms has been replaced by huge machines or seasonal migrant workers. Factory assembly lines utilize robots or outsource products to other countries. Home electronic conveniences from garage openers, remote controls, or riding lawnmowers lessen the need for physical labor. As a result, physical activity in both the home and work environment is less today than ever before.

Reduced physical activity lessens the need for caloric intake. Couple this trend with an increase in consumption of calories through larger food portion sizes and conditions promote obesity. This process takes place when adipocytes store excess calories by expanding through hypertrophy, which results in obesity. Once the condition of obesity exists, the triglycerides (TGs) stored in adipocytes can cause serious health consequences.

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Dieting Dilemma and Skinny Solutions

Dieting Dilemma and Skinny Solutions

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