Causes of obesity and weight gain
Excess body fat accumulates when people take in more food energy than they spend. Why do they do this? Is it environmental? Behavioral? Psychological? Biological? Genetic? Metabolic? Dietary? All of these? Most likely, obesity has many interrelated causes; some experts in the field speak of many different obesities. The next sections summarize current obesity research without coming to a final conclusion as to cause. Several lines of investigation seem promising. Emerging findings suggest that obesity is multifactorial.
Fat cell development
When more energy is consumed than spent for whatever reason, much of the excess energy is stored in fat cells. The amount of fat on a person's body reflects both the number of fat cells and the size of those fat cells. The number of fat cells normally multiplies during the growing years and levels off as people reach adulthood. Fat cell number increases more rapidly in obese children than in lean children, and obese children entering their teen years may already have as many fat cells as do adults of normal weight.
The fat cells enlarge as they store additional fat, expanding eight to ten-fold in size. When the cells reach their maximum size, cell number may increase again. Thus obesity develops when a person's fat cells increase in number, in size, or quite often both.
With fat loss, the fat cells shrink in size, but not in number. For this reason, people with extra fat cells are less likely to lose weight successfully; they may be able to shrink their cells, but not reduce the number. In contrast, people with a normal number of enlarged fat cells may be more successful. Prevention of obesity is most critical, then, during the growing years when cell number is increasing.
Genetics
Several studies have concluded that genetics plays the most important role in determining a person's body weight and body composition. When both parents are obese, the chances that their children will be obese are quite high (80%), whereas when neither parent is obese, the chances are relatively small (less than 10%). Adoption studies find a similarity in obesity between biological parents and their natural children, but not between adoptive parents and their adopted children.
To determine the relative contributions of genetic and environmental factors to body weight and body composition, one group of researchers studied over 600 pairs of identical and fraternal twins reared together or apart. This study found that identical twins were twice as likely to have similar weights as fraternal twins - even when reared apart. The researchers concluded that heredity was the most significant determinant of each person's weight.
According to another study of 12 pairs of identical twins, some people have a genetic tendency to gain more weight than others, even when energy intakes are comparable. One possible explanation is that genetic factors influence the body's tendency to store excess energy as fat or use energy to build lean tissue. Genetics may also influence the body's regulation of energy expenditure. For example, the differences in BMR between individuals are greater than can be explained by age, sex, and body composition alone. Similarities within families suggest a genetic influence on BMR, and a low metabolic rate is a major risk factor for weight gain.
Fat cell metabolism
Some of the research investigating the genetic influence on obesity focuses on the enzyme lipoprotein lipase (LPL). Fat cells and muscle cells produce LPL to help them store energy as fat. People with high levels of LPL are especially efficient at storing fat. As you might expect, obese people have much more LPL activity in their fat cells than lean people; LPL activity in muscle cells is comparable.
The activity of LPL is partially regulated by gender-specific hormones - estrogen in women and testosterone in men. In women, fat cells in the breasts, hips, and thighs produce abundant LPL, putting fat away in those body sites; in men, fat cells in the abdomen produce abundant LPL. This explains why men tend to develop "pot bellies" whereas women more readily develop "saddlebags".
The activity of LPL is also influenced by weight loss and may explain why lost weight is so easily regained. One group of researchers measured the LPL in nine obese people before they followed a very-low-kcalorie diet and again after they had lost an average of 90 pounds. The researchers found that LPL activity rose after weight loss, and that it rose highest in the people who had been fattest prior to weight loss. Researchers speculate that weight loss serves as a signal to the gene that produces the LPL enzyme, saying "Make more enzyme to store fat". This response to weight loss helps explain why obese people easily regain weight after having successfully lost it, and why their repeated efforts at weight loss are so difficult - they are battling against enzymes that want to store fat.