Why Are We Overweight? Part 7

An overweight epidemic is in full swing and for the first time in history, more people are overweight than malnourished. Indeed, at present, overweight people make up no less than one-sixth of the world’s population and 30 percent of that number to an extent such that they are considered obese. Obviously, there is no simple answer to the complex question of why this is the case, but in previous installments, I reviewed research that implicates changes in what eating involves as a strong determinant.

A homeostatic control mechanism within our bodies should regulate the amount of fat we store. Simply stated, much like we sweat when hot or shiver when cold to maintain the ideal body temperature, we should be hungry when there’s an energy deficit (i.e., when expenditure exceeds intake) and not hungry when there’s a surplus such that our actions will provide for body fat accumulation that stays at least close to the appropriate amount. The hormones insulin and leptin are major players in this regard and they do, indeed, respond in the correct manner when energy balance is skewed. For example, when a deficit is in effect, circulating insulin and leptin are reduced, which enhances a reward circuit in our brain. Satisfying this circuit requires eating. This makes sense being that the absence of a desire to eat under these circumstances would be a serious threat to perpetuation of the species. Interestingly, this hormonal influence also rewards activity, which is contrary to what would be predicted: After all, if you’re running on “E,” the last thing you should want is to go out for a spin. But this drive is based on the assumption that food-seeking behavior is required. In other words, energy had to be expended in order for energy to be ingested when we had to hunt.   

It should be apparent that this homeostatic system works quite well when a negative energy balance prevails. But what about a positive energy balance? Well, once such a condition is encountered (for example, let’s say it only took 1000 calories to hunt down and eat 1500 calories of energy), insulin and leptin will rise and the reward associated with eating will be removed. This means the reward associated with physical activity will also be blunted, but that’s okay because the natural sequence would be to fall asleep until the whole cycle begins again.

Homeostatic control of energy balance via insulin and leptin should ensure a set point to which body fat levels gravitate. Barring what would likely be minor genetic differences, this means that much like body temperature, everyone should carry approximately the same amount of fat. Obviously, this isn’t the case. Within this schema, the reason is that another reward circuit exists that is based upon food for pleasure as opposed to necessity. This ‘hedonic’ drive overrides the homeostatic one and creates a positive energy balance because we eat for reasons other than simply balancing the books. And with the resultant excess present, there will be no counterbalancing drive toward expending more energy, which is what would be required to keep things in check.

Back to how things should be. If you’ve ever seen a tiger devouring a deer on a nature show, a couple things should be apparent. First off, he must have awakened with a negative energy balance and a natural urge to be active and eat. It is also a safe bet that he added to that deficit by using a lot of energy to find his dinner. Consequently, the tiger’s account was, no doubt, considerably in the red when the deer met its fate and he probably wasn’t concerned with white meat vs. dark meat or well-done vs. rare. But once he had eaten enough, he also wouldn’t overeat because there was no urge compelling him to do so. In essence, he’s operating completely due to homeostatic drive with no hedonic influence.

As you can see, how things are for humans today is far different from how things are for the tiger. We might wake up with a negative energy balance and be driven toward both activity and eating; however, instead of ingesting energy just for necessity, many other things will also influence what and how much we eat. For example, we might get in the habit of eating at a particular time of day such that even if a positive energy balance is still present (e.g., because of having eaten more than usual at the last meal or being less physically active than normal in between), our circadian clock will override our homeostatic inclination and we’ll eat anyway. The same might occur because of various food selections that are available; for example, even if the homeostatic circuit tells us that we’ve satisfied an energy deficit after eating a salad, the hedonic one might tell us that we’re still hungry . . . at least for dessert! We also eat for social reasons, to deal with stress and, if raised as such, to be a good boy and finish all of the food on our plate. The end result is that many factors make us eat in a non-homeostatic manner with a positive energy balance while homeostatic signals regarding physical activity still suggest that we stay on the couch.

This article was originally published in New Living Magazine, which can be accessed on-line at www.newliving.com.





Go back