Why Are We Overweight? Part 4
In previous installments, I reviewed research from the laboratory of Professor Katarina T. Borer of the University of Michigan. In a nutshell, Professor Borer found that human appetite is not sensitive to short-term fluctuations in energy availability. This means that appetite is not regulated by the prevailing energy balance, which is contrary to what would be predicted if homeostatic control was present (for example, regulation that conforms to the ‘set-point’ theory). Interestingly, the researchers also found that the hormones that are believed to be involved in such homeostatic regulation (i.e., ghrelin and leptin) do, indeed, function as would be predicted when a negative energy balance is present and both also respond appropriately when the imbalance is corrected via nutrient infusion. The take-home message is that despite the proper hormonal responses, human behavior does not react in what would be considered the appropriate homeostatic manner.
There are many physiological responses that occur within the human body to correct deviations from normalcy and these homeostatic mechanisms are responsible for the body’s ability to maintain a stable internal environment in the face of myriad stressors. Indeed, were it not for this amazing adaptive capacity of the living organism, the very existence of life as we know it would be impossible. In the case of the overweight state, homeostatic control should prevent this threat to survival initially when the positive energy balance that perpetuates weight gain goes into effect. This research suggests that it does not and the present findings are in line with what is apparent given the current obesity epidemic. But before we conclude that our bodies are doing us a disservice, it’s important to remember that life on this planet was not always life as we know it. For example, while a homeostatic system where excess weight gain would reduce appetite and stimulate metabolism and the desire to be physically active would work well given the current state of affairs, it would not have been conducive to survival for our early ancestors who had to hunt for their food.
If an energy excess stimulated activity, an energy deficit would promote inactivity and that is the exact opposite of what is necessary when a species must perform considerable amounts of physical work in order to procure food. This is apparent when you consider reward systems that operate within the human brain. For example, Professor Borer explains how the hormones insulin and leptin, which circulate in proportion to the amount of body fat that is stored, act upon two distinct hypothalamic circuits that serve both motivation and reward. One of these circuits, which she refers to as homeostatic, does, indeed, respond to both short-term energy availability and body fat levels such that the reward associated with its stimulation is enhanced when low levels of insulin and leptin are circulating (e.g., when an energy deficit is present). Importantly, the reward derived from this circuit is satisfied by food seeking, which means that if a lot of physical work must be performed to acquire food of relatively low energy content, this circuit naturally promotes maintenance of a lean body weight. However, if energy-dense food can be acquired without expending much energy, high levels of insulin and leptin will dampen the reward derived from stimulating this circuit and the desire to be physically active will be suppressed.
Unlike the homeostatic circuit, which promotes activity when energy intake is insufficient, the hedonic reward circuit does not typically respond to declines in energy balance. Instead, this system is engaged at stable body weights and promotes fat gain because the reward derived herein is a function of the palatability of food and not its energy content per se. Consequently, this circuit encourages a finicky approach to eating; for example, overeating palatable food, but undereating and losing weight if only bland food options are available. So, while both circuits comprise brain substrates of reward that promote the acquisition, ingestion and storage of energy, the homeostatic one does so by increasing appetite and the desire to be physically active when an energy deficit prevails whereas the hedonic one promotes seeking palatable food with no regard for its energy content or prevailing energy balance (i.e., surplus or deficit).
In summary, this research suggests non-homeostatic regulation of spontaneous physical activity that is likely attributed to inhibitory actions of insulin and leptin on brain substrates of reward. Furthermore, these findings confirm that human appetite is influenced by hedonic factors (e.g., signals generated by food passage through the mouth and GI tract) and not by homeostatic signals that reflect energy availability. In this regard, it is important to recognize that what, when and how much we eat involves a complex set of variables that depend upon social, environmental and physiological cues. For example, Professor Borer explains that the presence of a non-homeostatic coordinating central nervous influence that activates hunger at habitual times throughout the day might be a major contributor to the fact that meal-to-meal eating does not obey homeostatic control. Specifically, within this schema, a circadian clock would override the influence of the prevailing energy balance and convince us that we should eat simply because it is typical for us to do so at that time of day.
This article was originally published in New Living Magazine, which can be accessed on-line at www.newliving.com.
