This is an excerpt from Practical Guide to Exercise Physiology by Robert Murray & W. Larry Kenney.
In one sense, losing weight can be as easy as moving more and eating less. For some people it is that easy. After all, the laws of thermodynamics can't be broken: If you ingest fewer Calories than you expend, you will lose weight over time. Yet for other people, moving more and eating less do not have the same satisfying results. In fact, some people who are on an identical regimen of exercise and diet don't lose much weight at all. Why is that? Some of the factors that affect weight loss are summarized in figure 7.5, and the following sections discuss in more detail some of the variation among individuals.
Many factors influence and complicate the energy balance equation regarding weight loss.
You just can't seem to get away from your pesky genes. Some scientists think that genes determine body weight and make futile any attempt to establish a new body weight outside of a fairly narrow gene-determined range. Other scientists agree that genetics play a role in body weight but not as the sole determining factor. Otherwise, how do you explain people who lose hundreds of pounds and maintain their new lower weight for decades?
Undereating and overeating produce wide variations in weight loss and gain. For example, overeating 1,000 Calories per day results in storage of 100 to 700 Calories per day. Resistance to fat gain is explained by increases in NEAT, TEF, and energy expenditure.
This sounds more complicated than it is. The thinking is that the body has a set point for body weight, and attempts to alter weight provoke compensations to counter those attempts.
Here's a simple example: People who introduce exercise into their daily routine in an attempt to lose weight are often frustrated by the slow pace of weight loss. Research shows that when people begin to exercise regularly, they often increase their food intake and decrease their physical activity during the rest of the day. This also helps explain why people on very low-calorie diets often struggle to lose weight as the body tries to compensate for low energy intake by reducing RMR.
Consistently undereating decreases resting metabolic rate (RMR), the thermic effect of food (TEF), the energy cost of movement due to weight loss, nonexercise activity thermogenesis (NEAT; posture, fidgeting), and satiety hormones (e.g., leptin, insulin, cholecystokinin), while hunger hormones (e.g., neuropeptide Y, ghrelin) are increased. In other words, undereating results in compensatory metabolic and behavioral responses that combine to work against weight loss.
Overeating can lead to increased thermogenesis (heat production) because muscle and other cells increase fat oxidation and energy expenditure. Unfortunately, that thermogenesis cannot increase sufficiently to offset overeating. The body is better at protecting against starvation than against gluttony.
The neuroendocrine system (brain, central nervous system, hormones) plays an important role in the regulation of hunger and satiety. Eating provokes the release of dozens if not hundreds of hormones from the brain, gut, pancreas, liver, and other organs, creating a rich mix of signals that influence feelings of hunger and satiety. Some people may be more sensitive to those signals than others, and that higher sensitivity means that hunger is turned off sooner, so they eat less and don't gain weight.
Although some people believe that high-fat diets are good for appetite control, fat is the weakest macronutrient at producing satiety and increasing its own oxidation. True high-fat diets deplete muscle and liver glycogen, leading to the loss of water molecules that are normally stored with glycogen. Water loss, not fat loss, explains most of the rapid weight loss that occurs in the early stages of such dieting.
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