Does Intermittent Fasting Really Work?

It’s very clear that there is a crisis with overweight and obesity throughout the world. According to the World Health Organization, nearly 2 billion adults are overweight and more than 600 million patients are obese (1).

High body mass index is associated with an enhanced risk of chronic diseases such as cardiovascular disease, diabetes and cancer (2).

The most common dietary strategy for weight loss is continuous energy restriction which involves restricting energy intake by 15–30% (3).

Recently, intermittent fasting regimens have gained considerable popularity, since many overweight and obese people find the rigidity of continuous energy restriction quite difficult to maintain (4).

What is intermittent fasting?

Intermittent fasting comprises several dietary approaches with alternating phases of fasting (very low or zero energy intake) and normal or high energy intake (alternate day fasting). Intermittent energy restriction is a subclass of intermittent fasting, with limited energy restriction to 20–25% (5).

Intermittent energy restriction may alternate within weeks, e.g. with either two consecutive or nonconsecutive fasting days per week (so-called “5:2 diet”) or fasting every other day (Alternate Day Fasting) (5).

Intermittent energy restriction with periods of more than two consecutive fasting days followed by non-fasting days are subsumed under the term “periodic fasting”.

Fasting periods alternating within the day (e.g. 16 h of complete fasting during 24 h) are called time restricted feeding (5).

So why has intermittent fasting become so popular in the past few years? 

One of the main reasons is simplicity.

There are thousands of diets that involve specific foods or nutrients, but intermittent fasting skirts all those details. It’s much easier to tell someone not to eat than to teach them how to eat correctly!

What does intermittent fasting do?

Essentially, the various forms of intermittent fasting put someone in a caloric deficit. Intermittent fasting works by prolonging the period when your body has metabolized through the calories consumed during your last meal and begins burning fat.

Here are some evidence-based benefits of intermittent fasting:

• Fasting caused the hormone insulin to drop and growth hormone to increase. In addition, cellular function can change by altering which genes they express and then can also start important cellular repair processes (5,6).
• It helps you eat fewer calories and is a very effective tool to lose weight and visceral fat (7).
• Reduce insulin resistance and lower blood sugar levels, at least in men (7).
• Reduce oxidative damage and inflammation in the body. This should have benefits against aging and development of numerous diseases (8).
• It may have essential benefits for brain health, potentially increasing growth of new neurons and protect the brain from damage (9).

Although there are numerous benefits from intermittent fasting, is it the ideal way to lose body fat and get leaner and stronger?

Does intermittent fasting really work?

A recent systematic review investigated the effects of intermittent energy restriction on anthropometric outcomes and intermediate disease markers (10).

The inclusion criteria for studies to be included in this analysis were:

• They needed to be randomized controlled trials with a parallel or cross-over design that compared:
• Intermittent energy restriction as intervention:
• i.e., alternate-day fasting characterized by at least one feast day (24 h) where participants are permitted to eat as much as they wanted, and at least one fast day (24 h) per week, where energy intake is rigorously reduced (to about 25%) or withheld completely (11). For instance, in a (5:2)-alternate day fasting regimen there are 5 feast and 2 fast days.
• versus at least one of the following comparisons:
• continuous energy restriction defined as daily energy restriction by at least 15% (11).
• Usual diet or minimal intervention
• Minimum duration of the intervention for these studies needed to be ≥12 weeks.
• Participants with a mean age ≥18 years.

This systematic review synthesized direct evidence on the effects of intermittent energy restriction vs. continuous energy restriction and usual diet on body weight, waist circumference, fat mass outcomes and intermediate disease markers such as low-density lipoprotein-cholesterol, triacylglycerols, systolic blood pressure, fasting glucose, and glycosylated hemoglobin (HbA1c).

In summary, intermittent energy restriction may reduce body weight, fat mass, triacylglycerols, and probably improves waist circumference and systolic blood pressure as compared to a usual diet.

Moreover, intermittent energy restriction probably slightly reduces body weight, and fat mass compared to continuous energy restriction. No important differences were detected comparing different types of intermittent energy restriction (4:3 vs. 5:2; consecutive vs. nonconsecutive days)

When comparing intermittent energy restriction to continuous energy restriction:

• Intermittent energy restriction loss a little more bodyweight (0.55 kg) and fat mass (0.66 kg), but this was essentially the same with no statistical difference.
• Both continuous energy restriction and intermittent energy restriction have similar effects on low-density lipoprotein-cholesterol, triacylglycerols, systolic blood pressure, fasting glucose, and glycosylated hemoglobin (HbA1c) with no differences between the groups.

The evidence indicates that intermittent energy restriction may improve body weight, fat mass, triacylglycerols, systolic blood pressure as compared to a usual diet, but likely makes little differences compared to continuous energy restriction.

Although there is a little more weight and fat loss in the intermittent energy restriction group, these studies didn’t equate for isoenergetic conditions.

These randomized controlled trials weren’t equating calories, which is common in these studies.

Almost all studies where calories are equated show absolutely no difference in weight or fat mass loss between these groups.

My take on the research on intermittent fasting

Intermittent fasting is a reasonable way to lose body weight, fat mass and improve health markers, however the research indicates that this is due to a calorie effect which is completely tied to the energy restriction of these diets.

This appears to be just as effective as continuous energy restriction when it comes to weight loss, fat mass loss and health markers.

If you prefer intermittent energy restriction and are just looking for some weight loss, I believe this is valid approach, but continuous energy restriction will get you the same results.

But if your goals are to optimize lean muscle tissue, strength and athletic performance, I don’t think this is the best method for that.

Some of the claims that the intermittent fasting community are making regarding insulin sensitivity, health makers, longevity, etc. isn’t supported by data in relation to what we are seeing in terms of intermittent fasting compared to continuous energy restriction. 

Essentially, you can be healthy eating 6 meals a day that include carbs in every meal or you can be healthy doing intermittent fasting. 

I think the major point is to consider what your ultimate goals are and to pick something that feels sustainable and feels like the least amount of restriction for you.

References:

1. 2017, W. (2019) 10 facts on obesity.

2. Guh, D. P., Zhang, W., Bansback, N., Amarsi, Z., Birmingham, C. L., and Anis, A. H. (2009) The incidence of co-morbidities related to obesity and overweight: a systematic review and meta-analysis. BMC Public Health 9, 88

3. Yumuk, V., Tsigos, C., Fried, M., Schindler, K., Busetto, L., Micic, D., Toplak, H., and Obesity Management Task Force of the European Association for the Study of, O. (2015) European Guidelines for Obesity Management in Adults. Obes Facts 8, 402-424

4. Varady, K. A. (2011) Intermittent versus daily calorie restriction: which diet regimen is more effective for weight loss? Obes Rev 12, e593-601

5. Patterson, R. E., and Sears, D. D. (2017) Metabolic Effects of Intermittent Fasting. Annu Rev Nutr 37, 371-393

6. Bagherniya, M., Butler, A. E., Barreto, G. E., and Sahebkar, A. (2018) The effect of fasting or calorie restriction on autophagy induction: A review of the literature. Ageing Res Rev 47, 183-197

7. Barnosky, A. R., Hoddy, K. K., Unterman, T. G., and Varady, K. A. (2014) Intermittent fasting vs daily calorie restriction for type 2 diabetes prevention: a review of human findings. Transl Res 164, 302-311

8. Jordan, S., Tung, N., Casanova-Acebes, M., Chang, C., Cantoni, C., Zhang, D., Wirtz, T. H., Naik, S., Rose, S. A., Brocker, C. N., Gainullina, A., Hornburg, D., Horng, S., Maier, B. B., Cravedi, P., LeRoith, D., Gonzalez, F. J., Meissner, F., Ochando, J., Rahman, A., Chipuk, J. E., Artyomov, M. N., Frenette, P. S., Piccio, L., Berres, M. L., Gallagher, E. J., and Merad, M. (2019) Dietary Intake Regulates the Circulating Inflammatory Monocyte Pool. Cell 178, 1102-1114 e1117

9. Bjorkholm, C., and Monteggia, L. M. (2016) BDNF - a key transducer of antidepressant effects. Neuropharmacology 102, 72-79

10. Schwingshackl, L., Zahringer, J., Nitschke, K., Torbahn, G., Lohner, S., Kuhn, T., Fontana, L., Veronese, N., Schmucker, C., and Meerpohl, J. J. (2021) Impact of intermittent energy restriction on anthropometric outcomes and intermediate disease markers in patients with overweight and obesity: systematic review and meta-analyses. Crit Rev Food Sci Nutr 61, 1293-1304

11. Varady, K. A., and Hellerstein, M. K. (2007) Alternate-day fasting and chronic disease prevention: a review of human and animal trials. Am J Clin Nutr 86, 7-13