Sunday, March 8, 2009

Intermittent Fasting with a Condensed Eating Window – Part III: Fasting Blood Glucose, Cortisol & Conclusion

Which is better, several small meals or one big meal?
Which is better, several small meals or one big meal? (Photo by morbuto)

This is the third and last post in a series discussing the effects of a reduced meal frequency on various health markers. To read the previous posts, see Part I: Poorer Insulin Sensitivity and Glucose Tolerance? and Part II: Blood Pressure, Body Weight & Cholesterol.

As mentioned before, reducing the frequency of meals to just one per day effectively means the participants were on a condensed eating window version of intermittent fasting. They consumed their entire energy intake during four hours in the evening. Macronutrient composition was not significantly different from the control group, but the fasting group ate 65 kcal less than the control group.

Fasting blood glucose levels

As mentioned in Part I, glucose tolerance as measured by an oral glucose tolerance test actually seemed to worsen as a result of consuming only one meal per day. However, there is some doubt over the validity of these results, because the morning glucose tolerance test of the fasters may have been affected by the large meal from the previous day.

Fasting blood glucose levels, on the other hand, were lower in the 1-meal-per-day group (85.9 mg/dL) than in the 3-meals-per-day group (89.4 mg/dL), though this difference was not statistically significant. Again, there is the difficulty of comparing these results, because the blood results were taken in the evening (after an 18-hour fast) in the intermittent fasting group and in the morning (after a 12-hour fast) in the control group.

Cortisol levels

Cortisol, which is also known as the "stress hormone", was significantly lower in the fasting group (7.2 micrograms/dL) than in the control group (14.1 micrograms/dL). In other words, eating one big dinner instead of the usual three meals cut cortisol levels almost by half.

This result is somewhat peculiar considering that higher cortisol levels are usually associated with higher blood pressure. In this study, the intermittent fasting group had much lower cortisol levels but slightly higher blood pressure. Since cortisol levels are typically higher in the morning and lower in the evening, the difference may again be explained by the fact that blood samples were taken at different times between the groups.


Eating one large evening meal and fasting the rest of the day instead of eating breakfast, lunch and dinner did not result in major health benefits in healthy, non-obese subjects. In fact, several health markers appeared to be negatively affected: insulin sensitivity and blood glucose tolerance worsened and blood pressure increased. Since measurements were taken at different times, the results may not be comparable, however.

Body weight and body weight were lower in the intermittent fasting group. This may be partly explained by the fact that energy intake was also lower by 65 kcal in subjects consuming only one meal per day.

Both HDL and LDL levels increased in the intermittent fasting group compared to the control group. Triglycerides appeared to be lower, but the difference did not reach statistical significance.

Cortisol levels, which are indicative of stress and immune function, were significantly lower in the intermittent fasting group. As measurements were taken early in the morning in the control group vs. later in the afternoon in the fasting group, this difference may also be due to diurnal changes in cortisol levels.

Following a condensed eating window version of intermittent fasting, where all of the daily calories are consumed during a few hours, may not improve health markers when calorie intake is not reduced. Whether the effects are in fact negative is unclear based on these results, but for those trying this version of IF, regular monitoring of health markers is recommended.
For more information on intermittent fasting, see these posts:

Intermittent Fasting: Understanding the Hunger Cycle
A Typical Paleolithic High-Fat, Low-Carb Meal of an Intermittent Faster
A Typical Day of Intermittent Fasting
Intermittent Fasting Improves Insulin Sensitivity Even without Weight Loss

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