Humans, like other animals, can eat for a number of different reasons. In a state of acute hunger, we may not consider many of these reasons, as we generally feel as though we are eating in order to fulfill some kind of immediate nutritional need. When speaking of appetite, we generally want to keep things simple and distinguish between eating for homeostatic and hedonic reasons. Keeping one’s energy balance on an even keel is a homeostatic necessity, but gorging on doughnuts and Kraft Dinner or dunking your strawberries in melted chocolate is pure hedonism.
As a general rule, foods that are rewarding are high in calories, salt, sugar, fat, and other ingredients that tend to be unhealthy in high quantities. Indeed, it is only in our modern society that we can even refine these ingredients and put them into our food supply in massive quantities. Not surprisingly, the issue of obesity is similarly contemporary.
In today’s paper, Olszewski et al. ask whether the relationship between FTO and obesity has more to do with impaired energy regulation, or increased sensitivity to food reward.
The relationship between FTO and obesity was discussed in my last entry, so I will not re-hash the human data in any great detail. Many different parts of the body appear to express FTO, but the area with the most abundant expression is the hypothalamus. The hypothalamus is a region of the brain that is critical for controlling hunger, motivation, body temperature, thirst, circadian cycles, and many of the body’s main hormonal systems.
In the first series of experiments, Olszewski et al. showed that exposure to palatable food (sugar water, etc.) for various lengths of time (48h or three weeks) did not alter FTO levels in the hypothalamus. In another experiment, mice were classified based on their tendency to consume either sugar or fat when given the choice between either. Here, a subset of mice showed clear preference for one or the other, but interestingly there was no difference in FTO expression between the two groups.
The next block of experiments looked at changes in FTO expression under conditions of food deprivation. FTO was increased in the hypothalamus of mice that had been food deprived for 16 hours. In another experiment, mice were given a diet consisting of regular chow, sugar water, and a delicious fatty syrup called Intralipid. Mice were classified into “big eaters” and “small eaters” depending on how much of these foods they voluntarily consumed. Small eaters had markedly elevated FTO expression in the hypothalamus.
So what do these results tell us? Well it seems that FTO expression has no relationship to the animal’s tendency to choose one or another type of palatable food. Nor does FTO expression change when the animal is given a steady diet of tasty treats. In human terms, it seems that FTO may not be especially related to hedonistic eating – choosing doughnuts over almonds may not be related to FTO either. On the other hand, when animals ate because they were hungry and food deprived, FTO did seem to be involved. This seems to suggest that FTO may have more to do with the control of energy balance than it does food choice per se. Now it is clear that people of all different sizes make unhealthy food choices and favor taste over nutrition. Quantity consumed is likely a more important contributor to obesity than food choice alone (any food that has calories could make you gain weight). So in a way these results make sense in the context of what we know about human feeding patterns.
Olszewski, P., Fredriksson, R., Olszewska, A., Stephansson, O., Alsiö, J., Radomska, K., Levine, A., & Schiöth, H. (2009). Hypothalamic FTO is associated with the regulation of energy intake not feeding reward BMC Neuroscience, 10 (1) DOI: 10.1186/1471-2202-10-129