Nature, nurture and the control of food intake

Some weeks ago, my blog was entitled: “Ever seen a fat fox”, the gist of which was that whereas biology can tell us a lot about the control of food intake in animals, in man, with a large pre-frontal cortex, living in a highly social existence, social aspects may be far more important than the biology. I return to this theme today in light of some intriguing research jointly carried out by researchers at the Universities of Washington and Toronto[1]. The research centers around the phenomenon of restrained eating, so let me first explain what this means. As I have previously pointed out, overweight and obesity are not usually the outcome of any conscious decision to get fat (Sumo wrestlers excluded). Restrained eating, on the other hand, is a very conscious decision to do, as the name implies, to consciously count calories in order not to gain weight or to maintain weight loss. Restrained eaters frequently have experienced some weight gain and have then made the lifetime commitment to being a fussy eater.

We know that obesity is highly heritable and based on studies of identical and non-identical twins, the figure for heritability is as high as 90%. So, it seemed a reasonable question to ask if brain function in response to food cues in restrained eating also had a similar level of genetic control. Thus the researchers used the University of Washington’s register of twins. In 2006, the twins took part in a health survey that involved the completion of a restraint-eating questionnaire (10 questions), effectively examining the concerns of the subjects about chronic dieting and weight gain. The researchers then mined the database to identify sets of identical twins that differed radically in their restrained eating patterns. Effectively, each twin pair had one serious restrained eater and one with no restraint as regards eating. By using identical twins, the study immediately eliminated any biological difference, since identical twins are genetic clones. The participants were all female and in their early thirties. The twins were shown photographs of “fattening” and “non-fattening” foods and then underwent a brain scan using a system known a functional Magnetic Resonance Imaging (fMRI). This was then repeated after the volunteers had consumed a milk shake.

The first test showed that the twin with the restrained eating personality showed much higher brain activity following exposure to pictures of high-calorie “fattening” foods than the twin with no tendency to food restraint. Specifically, the areas of brain activity most affected were the amygdala (involved in emotional processing of external cues), the occipital lobe (involved in behaviour modification) and in the right thalamus (involved in visual perception). Interestingly, there was no difference between restrained-eaters and non-restrained eaters in the prefrontal cortex. This is somewhat surprising to a non-neurobiologist such as myself since this is the area of the brain associated with all the higher complex brainpower of humans and since humans are the only species that exhibits restrained eating, one could be forgiven for thinking that the pre-frontal cortex would be involved. Things now get complex after the subjects had consumed the milkshake. In the case of the restrained eaters, all those areas of enhanced activity seen in the first test with pictures of high calorie “fattening” foods were greatly reduced. Strangely however, the sight of the non-fattening foods now switched on several parts of the brain, which was not observed in the non-restrained twins.

The authors point out that the fMRI data are consistent with other data, which show that restrained eaters are highly sensitive to external cues, which, in this study led to enhanced brain activity in the three regions mentioned. They see this as a conflict between food appeal and the desire to restrain eating. After the milkshake, this enhanced activity diminishes which the authors say may have been “cognitively driven”. I take that to mean that the intake of the milkshake “woke them up” in a sense such as to inhibit their natural desires.

For me the big deal in this paper is that an acquired habit regarding the regulation of food intake, restrained eating, is not entirely genetically driven. Moreover, it is not associated with the pre-frontal cortex, which makes man man and mice mice. Previous studies by the same authors in a very large twin study found that restrained eating was partly heritable with a range of 35% to 50% heritability recorded in 95% of the subjects. So some element of restrained eating is inherited and another, learned.  Such is the utter complexity of the regulation of food intake in humans. So, the next time you are at a conference and someone starts to discuss the regulation of food intake in rats and mice, take out the smart phone and do your e-mails!

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[1] Schur AE et al (2012) Acquired difference in brain responses among monozygotic twins discordant for restrained eating. Physiology & Behaviour, 105, 560-567