Fasting Versus Standard Calorie Restriction

Fasting has become a very popular weight loss strategy. Proponents of fasting believe that it results in increased weight loss and improved health in comparison to standard calorie restriction. Does the research literature back this up? A new study has recently been published that I believe allows us to answer this question definitively.

The Study

This investigation is a systematic review and meta-analysis out of Harvard University and was published in the British Medical Journal (Reference #1). This type of study accumulates all available evidence on a given topic and comes up with a statistical summary estimate. This study included 99 randomized trials that compared different types of fasting to standard calorie restriction:

Here are the results for the 2 most commonly practiced forms of fasting:

Whole day fasting: 24 hour food restriction a few days per week.

Time restricted fasting: Limiting eating to a certain window of time each day.

-In both short and long term studies, there was no additional weight loss when comparing these two types of fasting to standard calorie restriction.

-The study looked at the health effects of these diets on risk factors that included blood pressure, total cholesterol, HDL, LDL triglycerides, HbA1c and fasting glucose. The only significant difference was that the standard calorie restriction diet showed a more favorable effect on total cholesterol than time restricted fasting.

-More than half of the studies that reported side effects recorded adverse reactions to fasting. This included constipation, dizziness, diarrhea, headache, insomnia, low energy, strong feelings of hunger, difficulty concentrating, irritability, fatigue, bad breath, nausea, anxiety, hypoglycemia, cramps and palpitations.

Conclusions And Recommendations

This is a really well-done study by a top research university in a very well-regarded journal. For me the conclusions of this investigation are definitive. Looking at 99 randomized trials, it is clear that whole day fasting and time restricted eating are not better options than standard 3 meal a day calorie restriction.

These fasting methods do not result in additional weight loss, are not associated with improved health and seem to come with a whole host of side effects that don’t look like a lot of fun to me.

If you are trying to lose weight and improve your health, there is really no reason to starve yourself.

References

1) Semnani-Azad Z, et al. Intermittent fasting strategies and their effects of body weight and other cardiometab0lic risk factors. British Medical Journal 2025;389:e082007