[From Bruce Abbott (2013.12.27.1050 EST)]
Recently I distinguished between control systems and equilibrium systems. In this post I thought I’d provide an example in which both are involved.
Like many of us, I have a problem keeping my weight down where I want it to be. Over time I gradually put on body fat until my jeans don’t fit and I’m faced with either replacing my wardrobe with larger sizes or getting serious about losing the excess weight. This happens even though I know that being overweight is uncomfortable, unsightly, and a threat to my overall health. I don’t WANT to be overweight. So why do I keep gaining the weight back?
We know that the body includes physiological control systems that initiate feeding from time to time. The food consumed is digested and nutrients enter the blood stream. Some of these are used by the body for repairs or growth, some to fuel metabolism; any excess is stored away as carbohydrates or fat. As the nutrients are depleted in the blood stream, these stores are drawn upon to supply the necessary fuel and materials until the next feeding.
The level of body fat that gets stored away over time depends directly on the rate of caloric intake and inversely on the rate at which those calories are “burned” through metabolic processes. Although we consume thousands of calories in a year, body weight (a measure of stored fat) typically changes only a couple of pounds over that period, this despite variations in the caloric content of our food, amount of exercise, ambient temperature, and so on. How is this stability achieved? Under what conditions is this stability overcome and we gain a significant amount of weight?
The answer, in so far as we know it, is rather complicated. For starters, there is a control system that works over a relatively short time-span (a matter of hours). As glucose (blood sugar) levels fall due to metabolism and the completion of digestion, the liver begins to convert stored carbohydrate (glycogen) into blood sugar and releases it into the bloodstream. This activity also activates sensors that inform the brain of the liver’s activity. We begin to feel hungry (a perception). Our reference for hunger usually is zero; the error drives output that results in a complex set of behaviors that normally result in feeding. Feeding brings nutrient levels up, blood sugar rises and the liver reverses its conversion process, turning excess blood sugar into glycogen and storing it away. Additional nutrients not immediately consumed are stored as fat in the fat cells.
That’s a simplified picture of the process – other factors can also lead to a perception of hunger, and we may eat for reasons other than hunger – but this picture is I think adequate for our purpose here.
There appears to be another control system that works on a longer time-scale and involves the level of stored fat. Fat cells release a hormone whose concentration is proportional to the level of stored fat. Receptors in the brain perceive this hormonal signal. This signal also contributes to the perception of hunger, but how it interacts with the short-term, meal-initiating hunger control system is a detail I’m not clear on. It might turn the gain up on the hunger-control system, so that smaller levels of hunger would lead to more frequent meals and/or larger consumption during meals. Or it might amplify the hunger perception itself. (There are other possibilities as well.) We do know that in some individuals a genetic error renders the hormone’s receptors nonfunctional; such individuals report being in a continuous state of hunger and have to be prevented from eating too much if they are not to become morbidly obese. But for most of us the receptors work as they should.
This fat-regulation system seems to have the ability to affect stored fat levels in two ways: by altering how much we consume and by changing our metabolic rate. Metabolic rate increases when body weight gain increases by around 10%, thus tending to burn off more of the calories we consume. Similarly, metabolic rate decreases when body weight falls by about 10%, thus helping to conserve our stores of energy.
With these control systems at work, why do so many of us end up carrying around more body fat than we would like? According to “set-point” theory, the body’s fat-regulation system has a set-point (reference level) that differs from person to person. Some of us have a higher set-point than others. One piece of evidence for this view comes from a study involving women who normally were quite obese but who had succeeded in losing the excess weight through diet and exercise. These women were haunted by thoughts of food and had to continually fight the impulse to give in and indulge. But more tellingly, these women no longer had their monthly periods. Cessation of the feminine cycle is an adaptive response of the female body to starvation. Although now weighing about normal for women their age, these women’s systems were acting as though the women were starving, or in other words, at a level of fat storage well below set-point. As for those individuals with defective receptors, their set-points may be at normal values, but their systems perceive that the body-fat level is zero and constantly act in an attempt to bring it up.
Our regulatory systems appear to respond strongly to deviations that fall below the reference level. We don’t appear to respond as strongly to deviations in the other direction. When overweight, we may not feel as hungry, on average, as we might otherwise feel, which normally would lead to less calorie intake and weight-loss. But we eat for reasons other than hunger. We eat because we like the taste. We eat to combat boredom. We eat when we feel anxious. (Weight gain is a side-effect of our attempts to control these other perceptions.) We eat out of habit, because it’s time to eat. And as we gain weight, it becomes more difficult and less pleasurable to exercise, so we do less of it.
With more calories coming in and fewer being burned off, more and more fat gets stored away. Here’s where equilibrium comes into play: Our weight continues to rise until the extra calories burned in hauling around all that extra weight equals the number of excess calories being consumed.
Several years ago there was a debate among researchers in this field about whether body weight reflects a set-point (reference level) or a “settling point,” by which they meant settling to an equilibrium value. It was an ill-framed debate, because, as I hope I’ve made clear, we don’t have to choose between these two explanations. Both are involved in the determination of body weight (or more accurately, level of stored body fat).
Bruce