Could Just A Few Days Of Overeating Set You Down The Road To Diabetes?

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Most people would probably agree that it’s OK to get a bit lax with your diet once in a while, so long as you are eating healthily the rest of the time. After all, health is a marathon and not a sprint – if you are healthy, fit and of normal weight, letting yourself go a little over the holidays doesn’t sound like it would do too much harm. However, a recent study suggests that a few days of overeating might cost you more than you would imagine, not by causing weight gain, but rather leading to changes in the brain that set you down the path of obesity and diabetes. Let’s take a look at what researchers did and what they found.

The experiment

Researchers recruited 29 young male volunteers to participate in their study. They were all of healthy BMI (body mass index), between the ages of 19 and 27, and none of them were on any particular diets nor were they especially athletic. 18 participants were then assigned to consume a high calorie diet for 5 days, while the rest were assigned to continue their normal diet. Specifically, those on the high calorie diet were told to increase their caloric intake by 1500kcal per day, and to do so by eating high-calorie ultra-processed snacks. As the average man needs about 2500kcal per day, this represents an approximate 60% increase in daily calorie consumption. After the 5 days were up, participants were asked to return to their normal diet. To assess how well the participants followed this diet, they were asked to keep a food diary.

What happened

On average, participants in the study group increased their calorie intake by about 1200kcal, so a little less than the target. When the researcher assessed participants after the 5 days and then again 7 days later, there were no statistically significant differences in body weight between the high calorie diet and normal diet groups. There was also no difference in blood markers of inflammation, nor did the participants in the high calorie group report any changes in mood or appetite. There was a significant (60%) increase in liver fat in the high calorie group compared to no significant increase in the normal calorie group (this was not remeasured after 7 days). However, the most interesting findings concerned the blood sugar regulating hormone insulin.

The insulin response

Insulin plays a vital role in the body’s metabolism. When blood glucose (sugar) rises after a meal, insulin is released by cells in the pancreas called beta cells. Insulin triggers the uptake of glucose by muscle, liver and fat cells where it is stored either as glycogen or as fat. Insulin also stops the liver from converting other nutrients into glucose – since there’s plenty of glucose to go around, there’s no need to make more.

High calorie diets make cells less responsive to insulin, which means more insulin needs to be produced in order to achieve the same blood sugar reduction. This eventually leads to type II diabetes, in which the body is unable to keep blood sugar under control. So, the researchers first investigated whether the short term high calorie diet in this study led to insulin resistance, and found that it did not – participants who consumed the high calorie diet were just as capable of controlling their blood sugar as those who ate normally. However, what the researchers were really interested in was the effect insulin would have on the brain. This is because insulin not only influences how peripheral tissues like muscle handle glucose, but also affects dietary behaviour by acting on the brain, for example by regulating appetite.

In order to study this, researchers used functional magnetic resonance imaging (fMRI) to observe how blood flow to different regions of the brain changed when participants were given insulin in a nasal spray. They found that the high calorie diet was associated with reduced insulin sensitivity in various brain regions, consistent with a reduction in the reward response to food. This was supported by cognitive tests that also suggesting a blunted reward response that persisted 7 days after the return to normal diet, though it was no longer statistically significant at 7 days. A reduced reward response might lead to more overeating, because more calorie-dense foods are required to achieve the same reward response.

What this means

This research is suggesting that even though upping calorie intake for a week might not have a direct lasting metabolic impact, it may cause persistent changes in the brain that then lead to behavioural changes that could eventually cause insulin resistance and diabetes. The changes in reward responses observed here had already been observed in people who are obese, but this is the first study to provide evidence that a brief period of unhealthy eating can have this effect in young, healthy people, before insulin resistance occurs elsewhere. It lends credence to the idea that insulin resistance and diabetes could actually begin with changes in the brain, which could have important implications for prevention.

However, it is important to note that not all research fully agrees, and there’s a lot still to untangle here. For example, the researchers point to another study that showed that it only took three days of overeating and physical inactivity for subjects to become insulin resistant, suggesting that physical activity may play into this. It’s also uncertain whether the types of food matter – this study asked participants to consume ultra-processed foods. What would have happened if they had increased their calorie intake through other means? And of course, as researchers chose to conduct this study in men due to sex differences in insulin activity, it remains to be seen how generally applicable these results will be.

As for what we can do with this knowledge, there is some good news. While it might be preferable to avoid periods of overeating, this study suggests that the effects on the brain are reversed with a conscious reversion to one’s old diet. Since the participants were being studied, told what to eat and recording their dietary habits, they would have been very aware of what they were eating and probably not at risk of accidentally continuing to overeat after the 5 day intervention ended. People who overeat in a more uncontrolled environment, on the other hand, may not notice that they have slipped into a different dietary pattern until they have gained weight. By being a little more conscious of our dietary habits, we might be able to avoid this pitfall.