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What We Know About Intermittent Fasting

It’s that time of year when Google sees an uptick in searches for ‘healthy diets’ and ‘diets for weight loss’. While intermittent fasting (IF) has been around for decades, it has been popularized more recently and is being used as a strategy for losing weight and improving health. While studies in animals are promising, the long-term effects of IF in humans is still largely unknown. 

What’s known: Intermittent fasting could help with weight and overall health management.

There are several different ways to do intermittent fasting, but they are all based on choosing regular time periods to eat or not eat, i.e., “fast”. When we eat, we pump nutrients into our bloodstream that can be used for immediate energy and/or stored for later use, generally as glycogen and fat. In contrast, when we fast, our body eventually exhausts the nutrients available in the bloodstream and is forced to start metabolizing its energy stores. When our body runs out of sugars available for use from the bloodstream or storage (i.e., glycogen), it starts burning fat for energy instead.

The process of moving from burning primarily sugars to fat is called metabolic switching. Metabolic switching is thought to be the critical biological factor contributing to the touted health benefits of fasting. Such health benefits include improved metabolic and cardiovascular health, blood sugar control, weight management, and improved cognition. However, importantly, much of the research showing biological benefits of intermittent fasting are done in animals. The fewer studies done with humans have shown that intermittent fasting does not always promote greater weight loss or health benefits than typical calorie restricting or consistent meal (e.g., three meals per day) diets. Moreover, when fasting we need to make sure we are not eating so few calories that our body starts to preserve energy stores (i.e., hold onto fat) because it thinks we’re starving!

Common intermittent fasting regimens:

  • Alternate-day fasting
    • Alternating fasting days, where no calorie-containing foods are consumed, with non-fasting days
  • Modified fasting, a.k.a. 5:2 diet
    • Severe energy restriction (20-25% of estimated energy needs) for 2 days of the week and regular/non-restricted eating the remaining 5 days
  • Time-restricted feeding
    • Fast and eating within restricted time windows each day. Daily fasting intervals vary widely, with one of the most popular regimens today being 16 hours of fasting and 8 hours of regular eating.

*Non-caloric beverages, including black coffee and tea are allowed on fasting days.

**It is recommended to follow a balanced diet rich in high-quality whole grains, lean proteins, and a variety of fruits and vegetables when not fasting.

What’s not known: Is intermittent fasting right for you?

Intermittent fasting promises a lot, however, knowing whether it is the right dietary pattern for you is important. Science tells us that intermittent fasting is not appropriate for children and teens under 18 years old, women who are pregnant or breastfeeding, people with diabetes or other blood sugar problems, or those with a history of eating disorders. Also, while there is a substantial amount of evidence supporting the health benefits of intermittent fasting in animals, there is still limited data in humans, including limited knowledge about the long-term effects on health and weight management.

The Bottom Line: If you’re interested in trying intermittent fasting, talk with your dietitian and doctor before starting to ensure you have the best personalized plan in place.

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