Intermittent Fasting: 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?

What is intermittent fasting?

Also known as "alternate day fasting," intermittent fasting can take many forms. Some 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 food as they want. However, the majority of programs recommend healthy eating on non-fasting days.

Is intermittent fasting a good way to lose weight?

A study published in 2017 followed 100 people for one year and found that those who used an alternate-day fasting program lost about the same weight as those who ate 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% on feast days.

The calorie-restricted group would consume 75% of their estimated daily calorie needs. Perhaps the reason for the similar 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. In contrast, 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 a 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 an extended period 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 six months. Still, more extended studies with more participants are needed.

Is intermittent fasting safe?

One small study of 59 subjects using alternate-day fasting for eight 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, so they need to pay careful attention to their blood sugar levels and consult with their physician before attempting an alternate-day fasting program.

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

Our take on intermittent fasting:

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 the 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 two 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.

By Lynn Grieger, RDN, CDE, CHW, CPT

References:

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.