by David Greenwalt
Many people who have tried to lose weight are familiar with the dreaded “plateau” of dieting – the point at which they just can’t seem to lose any more weight, despite their continued efforts to eat less and exercise more. Worse yet, some people who have fought hard to lose weight end up regaining it over time, or even ending up at a weight higher than they were when they started.
See my review of what happened to ‘Biggest Loser‘ contestants after they lost a ton of weight and were re-evaluated six years after.
Years ago, scientists looking for an explanation as to why sustained weight loss seems to be an unreachable goal for so many people came up with a theory of weight control called set point.
The Set Point Theory of Weight Control
The set point theory was originally developed in 1982 by scientists looking to explain why repeat-dieting rarely produces long-term weight change (1). According to the theory, each of us has a built-in control system that dictates how much body fat we should have, i.e. our “set point.” If our level of body fat strays too much from this set point, physiological mechanisms go to work to re-establish the set point.
Scientists suggest that the system is controlled by feedback loops that provide our brains with information regarding our current level of body fat. The brain then responds to the feedback it receives by setting into motion responses appropriate to either increase or decrease fat stores to return them to our set point.
The set point can be thought of like the thermostat of a house. When the house temperature strays from the set temperature, the thermostat responds by turning on or off the furnace or air conditioner. In the same way, set point theory says that our bodies respond to changes in fat mass by working to turn up or down our energy intake and energy expenditure. And, just as people set their house thermostats to different temperatures, not all of us have the same set point. This, set point theorists would say, accounts for the variability in body composition and weight between individuals.
The bottom line: The set point theory of weight control says that each of us has a pre-determined “set point” for how much body fat we should have. If our level of body fat goes up or down from this point, then physiological mechanisms work to restore our set point.
The Role of Feedback Loops in Set Point
Feedback loops do in fact exist to regulate aspects of metabolism. Levels of the hormone leptin, for example, are directly proportional to fat mass in individuals with stable body weight (2). That is, the more body fat that a person has, the higher his or her level of leptin will be.
When an individual loses weight, and body fat along with it, levels of leptin go down. The central nervous system detects decreases in level of leptin, and responds by causing increased hunger and decreased energy use (3). These changes seem to support the set point theory – our bodies react against loss of fat mass by trying to increase food intake and preserve energy.
The bottom line: Feedback loops, such as levels of leptin and other hormones, give our brains information about how much fat mass we have. The set point theory says that these feedback loops allow our brains to work to re-establish our set points when our fat mass goes up or down.
Shortfalls of the Set Point Theory
The set point theory does have shortfalls, however. For example, how and when is set point “set”? Since we know there are genetic components that go into determining body mass index, is set point established at birth? If this is the case, then why have researchers observed that as many as one-third of chronic dieters weigh more after dieting than they did previously – has their set point gone up (4)? Why do most of us gain weight as we get older – is our set point changing? If it is, then is it really a “set point”? The theory does not offer answers for these questions.
Perhaps more importantly, set point theory does not explain the environmental and social factors that have been shown to be associated with weight and obesity. For example, we know that lower-income women are more likely to be obese (5). Does income affect set point? It seems unlikely.
The theory also fails to offer an explanation for the obesity epidemic that has occurred over the last 30 years. Did the set point for most of the population increase drastically over the last few decades? Again, it seems unlikely. Instead, it seems that environmental and social changes that have occurred during this time frame are also likely to be factors in the increased levels of obesity that we see.
The bottom line: Set point theory fails to explain the obesity epidemic, as well as social and environmental factors that are associated with weight gain.
Other Theories of Weight Control
A number of other theories have also attempted to explain why most of us gravitate toward a fairly stable weight, and why weight loss efforts can be so difficult.
The Settling Point Model
The settling point model differs from the set point system in that it does account for social and environmental factors that may affect body weight. Rather than being under strict physiological control, this model proposes that body fat is determined more passively by a natural equilibrium that is established between energy taken in from food and energy expended through metabolism (6, 7).
It is well known that energy expenditure through metabolism decreases as body weight decreases. Essentially, it takes less energy to “run” a smaller body. Therefore, if an individual eats less food and loses weight and body fat, they will also require and use less energy. This decreased energy expenditure will offset the changes in food intake, essentially bringing the person back to where they started from and back to an equilibrium between energy taken in and energy used.
For example, if an individual lowers calorie intake from 2000 to 1500 calories a day, they would be expected to lose weight and fat mass. However, once weight and fat mass have decreased to the point where energy expenditure has also dropped from 2000 to 1500 calories a day, then weight loss would stop because the individual would again be in energy balance. If the person, having lost the desired weight, stops “dieting” and eventually returns to eating 2000 calories a day, their body and fat mass will increase, causing increased energy expenditure, and the individual will re-settle at his or her original weight.
In the settling point model, any factor that causes a change in energy input can cause a change in body fat mass. For example, the obesity epidemic could be explained by the increased availability of food, increased portion sizes, increased prevalence of energy-dense foods, and increased proportion of meals eaten away from home (where portion sizes are typically larger).
The bottom line: The settling point theory of weight control proposes that body fat level is affected by social and environmental factors, such as food intake and availability.
Shortfalls of the Settling Point Theory
This model does have shortfalls, however. For example, it does not explain why, even in the same environment, some individuals are more likely to gain weight than others (8). Research has shown that genetic makeup affects likelihood to gain weight, because there is a genetic contribution to body mass index (9; 10; 11). In addition, there is evidence that energy expenditure does not simply fall in response to decreased body weight. Instead, it decreases at a greater rate than does body weight, to try to oppose loss of body weight and fat (12; 13).
The bottom line: Settling point theory takes outside factors into account, but does not consider physiological or genetic influences on weight and body composition.
So…is trying to lose weight futile?
It seems now that we are back where we started from – that is, our bodies appear to have their own ideas about how much we should weigh, and they will work against us if we try to weigh less than that.
So…now what? Should we just throw in the towel, so to speak, and accept that our body weight is largely out of our control?
First of all, I would suggest that the answer to that is a definite “no.” While some of us are surely genetically set up to weigh more than others, people can be healthy at a variety of weights. Healthy eating and activity are still crucial for good health.
Furthermore, while common lore has suggested that the vast majority of people who lose weight eventually gain it back, research has shown that is far from always the case. A meta-analysis of 29 studies found that among obese individuals who lost weight in structured weight-loss programs, most had not regained all of the weight 5 years later, but in fact weighed on average 3.2% less than they had 5 years earlier (14). Considering that average weight gain among adults is 1 to 2 pounds per year, this sustained weight loss over time is remarkable (15).
The bottom line: Health, not body weight, is the most important consideration. However, people whose weight is putting them at risk for chronic disease (including diabetes, heart disease, metabolic syndrome, some types of cancer…) should definitely not give up trying to achieve a healthier weight!
Another Explanation – The General Intake Model
A more recent theory of weight control, the general intake model, suggests that food intake is affected by physiological, environmental, social, psychological and dietary factors (16). Physiological factors (such as levels of leptin and other hormones) have feedback loops, such as those discussed for set point theory. That is, increased levels of leptin caused by increased fat mass would tell our brains that we should cut down on our food intake.
The general intake model says that other factors, however, including environmental or social factors such as food availability or portion size, do not have feedback loops. They do not send similar messages to our brains to regulate food intake. For example, when more food is available, or more varieties of food are available, we tend to eat more, not less (17, 18).
This model, then, proposes that while our brains and bodies do play a role in trying to regulate our food intake and body weight, these physiological mechanisms could be overpowered by aspects of our modern environment that make overeating and under-exercising so easy.
While some risk factors for obesity are out of our control (genetic factors or mother’s diet during pregnancy (19), for example), MOST others surely are within our control. We can choose to eat healthfully, control our portions, and exercise regularly. We can choose to create healthy food environments at home for ourselves and our families. And we can choose to eat when we are hungry, stop when we are full, and enjoy meals in the company of friends and family. And, even when fake-food addicted, we can choose to engage in the level of recovery necessary to be successful.
While we may not be as thin as we would like (or perhaps as thin as society has convinced us is desirable), we can definitely increase our odds of living a healthy and long life.