Is this chemical making you fat?

Hormones are necessary chemicals that circulate in the bloodstream to keep our bodies running smoothly.  They regulate all bodily functions including sleep, sex-drive, moods and appetite although the precise mechanisms of their actions is imprecisely understood, and becomes perhaps more so as we gain more knowledge of all the moving parts.  In recent years,  the hormone leptin acquired a certain mystique for its potential role in the regulation of appetite.  Leptin reduces appetite but its effects are complex and people who are overweight or obese may become resistance to its actions.  The hopes of drugs to modify leptin have been somewhat confounded by the complexity of the system but new hope has arisen since the discovery of ghrelin, a hormone that acts as leptin's counterpart by stimulating appetite under certain conditions.  Ghrelin is considered to be the first circulating hormone that stimulates hunger.  It is increased before a meal and decreased during the meal so that as a person becomes full, the hormone levels drop and appetite is suppressed. 

Ghrelin's action is also very complex but a study released today by the Journal of Neuroscience (April 13th issue) provides an interesting new piece of data. It seems that ghrelin enhances the sense of smell, causing rats to sniff more often and smell more intensely. The investigators suggest that this may be an important mechanism to help animals find food when they are hungry.  When humans were given ghrelin in the same study, they inhaled air tainted with various 'flavors' more deeply than without ghrelin.  There was no difference, however, in how much they like the smells after ghrelin.  It seems ghrelin may make us more aware of potential foods around us but the effects of its evolutionary advantage in over-fed humans is not well understood. Evidence does suggest that ghrelin plays a key role in appetite regulation along with the other hormones, insulin and leptin. The big questions that may help us understand how to use these data to address the current obesity epidemic revolve around how to seperate our physiological drives to eat from the psychological ones.  David Kessler, former FDA commissioner, has suggested that the Western diet itself alters our body's ability to regulate our appetite and our food intake and these hormone likely have a critical role.  Once we get into the vicious cycle, it takes heroic efforts to break out of it.  With high calorie, processed food being cheaper and more accessible than more nutritious fare, it seems the odds are stacked against us.
As we understand more about the effects of particular foods on our responses to these 'hunger' hormones, perhaps we will find the evidence we need to regulate food production more appropriately so nutritious foods are more widely available at reasonable cost.  In the meantime, yet another study has shown that sniffing a certain food can actually decrease our appetite.  In women, just the smell of dark chocolate suppresses appetite and interestingly, it appears to be accompanied (or preceded?) by a change in circulating ghrelin in these subjects. 

Even with all the uncertainties of the system, it does seem that exercise and a reduction in calorie intake both work together to move the regulatory effects of leptin, insulin and ghrelin in a favorable direction.  So in other words, we can't go wrong if we walk a little more, eat a little less, and take a whiff of dark chocolate now and again.