People's weights are determined by multiple factors including their genetic background, eating habits, metabolic rate and their general activity level.
- Genes. Genes influence body weight by setting basic parameters on the body's metabolic efficiency (the efficiency with which the body burns calories). Fatter people tend to have very efficient metabolisms; they require less fuel to run than thin people, and store excess fuel in the form of fat. Genes governing metabolism run in families. If one of your parents is obese you are several times more likely to be obese yourself than someone who has parents of a healthy weight. If both of your parents are obese you are at a very high risk of becoming obese yourself. As powerful as they are, genes only determine a tendency towards higher or lower metabolic efficiency; they do not by themselves determine what actual body metabolism will be. Lifestyle choices such as exercising also influence a person's metabolism and thus their weight.
- Metabolic rate. Apart from genetics, people's metabolic rates are largely determined by how active they are. We are told that for every ten years beyond our early to mid-twenties our metabolism slows about 10%. While a reduction in metabolism is observable as we age, such a reduction may be more due to a sedentary lifestyle than to mere aging. Muscle tissue is metabolically active compared to fat, and thus our metabolic level at any moment is in large part due to the state of our muscle mass. Inactivity accelerates loss of muscle tissue over time which decreases metabolism, making it all but certain that weight will be gained. Activity, on the other hand, reduces muscle loss, or even increases muscle mass, with the effect of increasing metabolism and making it easier to lose weight.
Eating patterns. People's food related habits and cultural expectations are also important determinants of their weight, influencing the types and amounts of foods consumed. For instance, families favoring high-fat, high calorie food (such as lasagna) served 'family style' (so that anyone can take as much as they like) are at greater risk for eating too much food and gaining weight than are families serving smaller portions of lean meats, steamed vegetables and brown rice. Similarly, families who push members to eat, or who keep high fat snacks and deserts handy are at greater risk for weight problems than are families that promote sensible portion sizes and save treats for special occasions. The speed at which people learn to eat and the consciousness with which they do so are important too. People who eat quickly tend to eat more than people who eat slowly as it takes a few minutes for your stomach to tell your brain it is full. Fast eaters sometimes finish their plates before getting the stomach's fullness message.
Ever larger portion size expectations are increasingly part of the problem. Americans have embraced restaurant food (sit-down and take-out both) as offering convenient solutions for meal-preparation. Restaurants, in turn, have taken to offering large and 'super sized' portions of high-calorie food at inexpensive prices as a means of attracting customers. Over time larger portions have come to be considered normal and expected and the overall amount of food being consumed has risen. Weight gain has been the result.
- Exercise. Exercise is an essential component of a healthy weight management program. Where a person's genetics are more or less set at conception, the amount of energy a person expends in physical activity is under voluntary control. People can choose to be more active, and becoming more active will help people to lose weight. Exercise builds lean muscle mass and burns up fat reserves. Muscles are very metabolically active. Adding muscle mass through strength training raises metabolism (the rate at which the body burns calories) which makes it easier to lose weight. A significant loss of muscle mass, on the other hand, which is what happens when people are sedentary, reduces resting metabolic rate and makes it harder to lose weight. Adding muscle mass helps people to look firmer and slimmer because muscle takes up less space than fat. Careful strength training reduces the risk of accidental injury, improves bone density, helps with digestion and assists in lowering blood pressure, cholesterol and triglyceride levels (all predictors of disease when elevated).