Intermittent Fasting with a Condensed Eating Window – Part I: Poorer Insulin Sensitivity and Glucose Tolerance?

Breakfast: to skip or not to skip? (Photo by Wonderlane)

I've written about various studies on intermittent fasting on this blog, and so far all of them have noted positive results. All in all, it seems that going through periods of not eating is beneficial for health.

However, many questions are still unclear. How long is an optimal fast? How often should you fast? Should calories intake be zero or just less than normal when fasting? Should you compensate by eating more when you break the fast?

Even with intermittent fasting, there are several different ways of going about it. Some people suggest going without food for 24 hours a few days a week is enough. Others (myself included) eat for 24 hours and then fast for 24 hours. Yet others are on a condensed eating window diet, meaning that they only eat for a few hours each day and fast for the rest of the time.

Two paper, one by Carlson et al. and another by Stote et al., may shed some light on the health aspects of following a condensed-window eating plan. Both papers are about the same study, in which the participants were told to eat either 3 meals a day or 1 meal a day.

People in the first group had breakfast, lunch and dinner as usual, while people in the second group ate one big meal between 4:00 PM and 8:00 PM. Energy and micronutrient intakes were the same in both groups, meaning that the big meal group had to cram a whole day's worth of calories in them in just a couple of hours. Not surprisingly, they reported feeling of hunger when fasting and extreme fullness after the huge meal.

Since these papers have a lot of data to interpret, I'm going to split the analysis into two or three posts. For now, let's concentrate on what happened with insulin and glucose levels in the participants.

Insulin sensitivity and blood glucose tolerance

After an overnight fast, the subjects took an oral glucose tolerance test and had a blood sample taken. Fasting plasma insulin levels were 5.0 in the 1 meal/d group and 5.8 in the 3 meal/d group, but this difference was not statistically significant (P = .4329). However, Oral Glucose Insulin Sensitivity (OGIS) was significantly lower in the 1 meal/d group (403.4) than the 3 meal/d group (458.8). The fasters also had higher morning plasma glucose concentrations: 95.9 mg/dL vs. 85.4 mg/dL.

This means that the fasting group actually had worse results, which is surprising, considering that in most other studies the result is the opposite: fasting improves insulin sensitivity and glucose tolerance. Does this mean that eating huge meals negates all the benefits of fasting?

Well, maybe. On closer inspection these results are not as clear as they seem. One thing that comes to mind is that the test results from the intermittent fasting group were probably still affected by the large meal from the previous evening when they came to the blood test in the morning. Indeed, the authors confirm this possibility:

The latter difference in fasting glucose levels could be explained, in part, by continuing absorption of the greater amount of food consumed in the evening in the subjects on the 1-meal-per-day diet.

Another reason for the unexpected result may be that the subjects in the fasting group were used to consuming all their calories in the evening, while the 3-meals-per-day group was used to eating breakfast each day. Therefore, taking the oral glucose tolerance test (which includes drinking 75 g of glucose) in the morning may not be entirely "fair". As the authors put it:

When not accustomed to a morning meal and then subjected to a morning OGTT, the subjects eating 1 meal per day may therefore exhibit poorer glucose tolerance compared with those adapted to eating breakfast.

Impaired morning glucose tolerance is also usually associated with higher fasting insulin, leptin and glucacon concentrations, meaning that an increase in these markers would've been expected in the fasting group. However, there were no significant differences in the levels of any of the three between the two groups.

All this lends support to the hypothesis that the negative effects seen were not due to impaired glucose tolerance and decreased insulin sensitivity but due to measurement. In other words, blood test results after having eaten 3 small meals throughout the day and after having eaten 1 big meal might not comparable.

Conclusion

In this study, subjects consuming only one meal per day had poorer glucose tolerance and decreased insulin sensitivity compared to subjects consuming breakfast, lunch and dinner as usual. This is in contrast to earlier studies which have reported improvements in fasting subjects.

It is unclear whether the results seen here were due to an actual worsening of glucose tolerance and insulin sensitivity or due to the high-energy meal from the previous night affecting blood test results.

For more information on intermittent fasting, see these posts:

Intermittent Fasting with a Condensed Eating Window – Part II: Blood Pressure, Body Weight & Cholesterol
Intermittent Fasting Reduces Mitochondrial Damage and Lymphoma Incidence in Aged Mice
A Typical Paleolithic High-Fat, Low-Carb Meal of an Intermittent Faster
A Typical Day of Intermittent Fasting