A randomized controlled trial published in The Journal of Clinical Endocrinology and Metabolism shows two days of intermittent fasting per week improves fat loss and markers of inflammation.
This study contrasts with a recent study we covered last week, which concluded intermittent fasting was not useful for weight loss.
We can’t blame you if you feel like you have whiplash trying to keep up with all the seemingly contradictory conclusions. But this demonstrates how a study’s details make a big difference and impact the results and conclusions we walk away with.
In the current study, 46 participants with obesity and at least two components of metabolic syndrome were randomized to an intermittent fasting group or control group. In the control group, each participant continued their typical eating pattern. Researchers instructed the intermittent fasting group to reduce their calories by 75% for two nonconsecutive days each week and then eat normally the other five days.
After eight weeks, the fasting group lost 3.5 kg compared to 1.2 kg in the control group. Fat mass decreased by 2.4 kg compared to 0.9 kg, and the fat-free mass (either water weight or lean body mass) decreased by 1 kg compared to 0.3 kg. (Fat-free mass was measured by bioimpedance scales, not the gold standard DEXA scan.)
Markers of metabolic health such as blood glucose and insulin levels appeared to improve more in the fasting group but the improvements were not statistically significant compared to the control group. The nonsignificant difference could be due to the small sample size or may be an indication of no meaningful effect.
The study authors also found improvements in markers of inflammation and oxidative stress.
We can stop right there and say, “Intermittent fasting works!”
But that would be an overstatement, just as taking one study with negative results and saying, “Intermittent fasting doesn’t work” is an overstatement.
That leaves us asking, why would this study on intermittent fasting show a beneficial effect on weight loss, whereas the recent one, published in JAMA Internal Medicine, did not?
Right away, we see a marked difference in the study design. In the current trial, intermittent fasting participants dramatically reduced their caloric intake for two days, eating 31% of their usual intake. They did not increase their calories on the non-fasting days.
In the study we discussed last week, there was no difference in caloric intake between the fasting and control groups.
Therefore, we can likely conclude that intermittent fasting works well for weight loss and fat mass loss when it helps achieve intermittent caloric reduction.
We could stop with those conclusions.
However, the current trial also makes claims about the impact of intermittent fasting on the gut microbiome and multiple secondary outcome measures that are poorly defined and may not be meaningfully related to health outcomes.
Despite this study’s weaknesses, its timing and juxtaposition with the recent negative results of another recently published study highlight how the details of studies matter. It also shows how intermittent caloric reduction can be an essential part of intermittent fasting.
What about the concept of periods of fasting to lower insulin levels, improve metabolic health, and improve autophagy and longevity?
These are more challenging to measure, especially the longevity question. We still don’t have strong randomized trial data to support these benefits. However, as we cover in our intermittent fasting and time-restricted eating guides, we do have enough data and theory to support people without contraindications giving it a try.
Thanks for reading,
Bret Scher, MD FACC
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