Intermittent Fasting for Weight Loss: A Scientific Look

There’s been a lot of talk lately about intermittent fasting for weight loss... but is this all just talk or is there scientific information to back it up? In this article, you will learn what intermittent fasting is, whether it is a good way to lose weight, and if it is healthy and safe.

What is intermittent fasting?

Also known as "alternate day fasting" intermittent fasting can take many forms. Some of the more common forms include reducing calories by 50-70% every other day, or perhaps two days in a row, followed by a day or two of regular eating. Some protocols recommend a fasting day followed by a “feast” day, where participants eat as much of any food that they want. The majority of programs, however, recommend healthful eating on the non-fasting days.

Weight Loss and Intermittent Fasting

A research study published in 2017 followed 100 people for one year, and found that the people who used an alternate-day fasting program lost about the same amount of weight as people eating an overall lower-calorie diet. In this study, the protocol recommended that the alternate-day fasting group consume 25% of their calorie needs on fast days and 125% of their calorie needs on feast days while the calorie- restricted group would consume 75% of their estimated calorie needs every day.

Perhaps the reason for the similar amount of weight loss is that participants in the alternate-day fasting group ate more than prescribed on fast days and less than prescribed on feast days, while those in the daily calorie restriction group generally met their prescribed energy goals.

Is Intermittent Fasting Healthy?

A few research studies investigated possible beneficial health effects of intermittent fasting and discovered overall good results including reduction in visceral body fat (the type of fat strongly associated with
increased risk of heart disease and type 2 diabetes), fasting insulin levels, and insulin resistance.

One negative is that it’s possible that using an intermittent fasting approach for a long period of time could potentially lead to decreases in muscle mass unless strength-training exercise is also included.

It is unknown whether intermittent fasting can lead to changes in the frequency or length of a woman's menstrual cycle. One study reported a longer than average menstrual cycle in women who followed a program of two consecutive days of reducing calorie intake by 70% each week for 6 months, but longer studies with more participants are needed.

Safety of Intermittent Fasting

One small study of 59 subjects using alternate day fasting for 8 weeks showed that, while a few people reported constipation, weakness, dizziness, and bad breath, these were not reported as serious side effects. There were no increases in eating disorder symptoms such as binging or purging, and overall participants reported improved mood.

It’s possible that people with diabetes on medication or insulin could experience hypoglycemia on fasting days, and so they need to pay careful attention to their blood sugar levels as well as consult with their physician before attempting an alternate day fasting program.

While it’s possible to exercise while using an intermittent fasting approach, there is no research on the effects on long-distance endurance exercise or very intense exercise.

More Research is Needed

Preliminary research shows that intermittent fasting may be a helpful program for people interested in losing weight and improving their health. However, more research on long-term outcomes of intermittent fasting with large groups of people is necessary.

If you decide to try intermittent fasting, check with your physician first and keep these tips in mind:

Reduce usual calorie intake by 50-70% every other day, or perhaps for 2 consecutive days each week.

Choose a healthful, balanced diet that includes lean sources of protein, vegetables, fruit, whole grains, and healthful sources of fat every day, even on so-called “feast” days.

“Feast” days are not a license to overeat or consume large amounts of sweetened, processed foods.

This information is for educational purposes only. The statements have not been evaluated by the Food and Drug Administration and are not intended to diagnose, treat, cure, or prevent any disease. Consult your physician if you have any question regarding a medical condition.

1. Trepanowski JF, Kroeger CM, Barnosky A, Klempel MC, Bhutani S, Hoddy KK, Gabel K, Freels S, Rigdon J, Rood J, Ravussin E, Varady KA. Effect of Alternate-Day Fasting on Weight Loss, Weight Maintenance, and Cardioprotection Among Metabolically Healthy Obese AdultsA Randomized Clinical Trial. JAMA Intern Med. Published online May 01, 2017. doi:10.1001/jamainternmed.2017.0936

2. Harvie M, Howell A. Potential Benefits and Harms of Intermittent Energy Restriction and Intermittent Fasting Amongst Obese, Overweight and Normal Weight Subjects—A Narrative Review of Human and Animal Evidence. Sainsbury A, Luz F, eds. Behavioral Sciences. 2017;7(1):4. doi:10.3390/bs7010004.

3. Hoddy KK, Kroeger CM, Trepanowski JF, Barnosky AR, Bhutani S, Varady KA. Safety of alternate day fasting and effect on disordered eating behaviors. Nutrition Journal. 2015;14:44. doi:10.1186/s12937-015-0029-9.

4. Barnosky AR, Hoddy KK, Unterman TG, Varady KA. Intermittent fasting vs daily calorie restriction for type 2 diabetes prevention: a review of human findings. Transl Res. 2014 Oct;164(4):302-11. doi: 10.1016/j.trsl.2014.05.013. Epub 2014 Jun 12.

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