Gimme Some Sugar

Hunger weakens the ability of regulatory brain areas to put the brakes on reward-oriented ones, a new study has found.

Possession by an insectoid alien, or just hypoglycemia? You decide!

When our brains have enough glucose to go around, a region called the medial prefrontal cortex (mPFC) helps regulate our emotions and hold our attention in a particular spot. But brain scans show that when glucose levels drop, the mPFC loses its control over areas involved in feelings of hunger and reward.

We all know what it’s like to feel cranky from hunger, or to experience a mid-afternoon energy crash – but scientists are only now beginning to fit together the neurological pieces that compose processes like these. As it turns out, hunger and emotion are even more closely interrelated than anyone expected.

As the Journal of Clinical Investigation reports, a team led by Yale’s Rajita Sinha and USC’s Kathleen Page monitored the blood glucose levels of volunteers sitting in fMRI scanners, while showing them photos of high-calorie food, low-calorie food and non-food items. By correlating brain responses to each picture with each subject’s glucose levels at that moment, the researchers found some intriguing changes in the brain’s activity patterns:

Mild hypoglycemia [low glucose] preferentially activates limbic-striatal brain regions in response to food cues to produce a greater desire for high-calorie foods. In contrast, euglycemia [normal glucose] preferentially activated the medial prefrontal cortex and resulted in less interest in food stimuli.

In other words, the more healthy a person’s blood glucose level is, the more likely that person’s mPFC is to take control.

But what’s happening the rest of the time? Well, a brain structure called the hypothalamus – which detects and automatically responds to basic needs like hunger and thirst – triggers activity in areas like the the insula, which helps provide emotional context for our bodily sensations. Another reaction, which seems to be closely related, is an increase of the stress hormone cortisol, which acts on regions like the striatum to contribute to feelings of anticipation and reward.

What this all means is that as you get hungrier, you gradually lose conscious control over your emotions – especially those related to your desire to eat.

The good news is, higher glucose levels reverse these effects, and lead to greater activation of the mPFC.

One interesting twist to this study is that the brains of obese people seem to respond to high-calorie foods as though they never have enough glucose:

These findings … suggest that this glucose-linked restraining influence is lost in obesity. Obese individuals may have a limited ability to inhibit the impulsive drive to eat, especially when glucose levels drop below normal.

The researchers are hopeful that future studies may help us understand where this vicious cycle of obesity and unhealthy eating begins – and how to break it.

For now, though, the takeaway is that our brains function best when we keep our glucose levels on a fairly even keel:

Strategies that temper postprandial reductions in glucose levels might reduce the risk of overeating, particularly in environments inundated with visual cues of high-calorie foods.

In other words, eat snacks that don’t have too many empty calories, but that still provide plenty of energy – especially if ads for junk food tend to give you cravings. When it comes to glucose, a good offense is the best defense.

Take that, Oreos! (That’s a phrase I don’t get to use very often.)

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