What About Intermittent Fasting?

I’ve been following the intermittent fasting (IF) trend for several years but the new April study in the New England Journal of Medicine brought it the forefront again. I worked on an article about IF for the Nutrition Source in 2017, so it was reassuring that this new study was entirely consistent with our content! When you hear that fasting or calorie restriction helps you to live longer, boost immunity and improve metabolism, the evidence is mainly with animal studies…using fat rodents to be exact. Human studies also show benefits but not more so than other calorie-restricted plans.

What brought the NEJM study into the limelight was New York Times coverage and their startling title: “Scientists Find No Benefit to Time-Restricted Eating.” It’s pretty clear that IF has been one of the most popular methods for weight control and general health benefits in recent years, so that headline could be jarring to some. But as usual, when you take time to read beyond headlines, the findings weren’t that shocking. Basically they followed a group of people with obesity for one year. All ate about the same range of daily calories, with slightly more for men (1500-1800 cals) than women (1200-1500 cals). They were randomized to follow intermittent fasting (the 16:8 method—16 hour fast, 8 hour eating window—in which they ate from 8am-4pm) or to eat without time restrictions. After one year the IF group lost 18 pounds and the time-unrestricted group lost 14 pounds; statistically there was no difference. Both groups also showed benefits of decreased blood pressure, cholesterol, and blood glucose, but again not statistically different.

So first of all, the NYT title was off because IF did produce benefits of weight loss and improved metabolic measurements. What the study found—which was nothing new because it mirrored findings of previous studies (refer to the Nutrition Source article above)—was that IF isn’t necessarily more effective for weight loss than other types of restricted eating. However that doesn’t mean it can’t be explored as an eating plan.

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When it comes to weight control, there are endless methods and not one works for everyone. IF is definitely the most asked-about method I receive in the clinic, but I always review a patient’s history and discuss pros/cons.

Three general IF methods:

1) Alternate-day fasting in which you reduce your calorie intake by up to 75% every other day or 3 days/week.

2) Whole-day fasting in which 1-2 days of the week you eat up to 25% of your usual calorie intake.

3) Time-restricted feeding in which you fast during designated hours and eat the remaining hours. This is the most popular method I see.

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  • Structure. If you’re an all-day grazer, which could mean 3 meals and 3+ snacks a day, I believe there’s a risk for weight gain because you’re constantly introducing food and stimulating the biochemical actions occuring with digestion. We’re not designed to be eating all day long. Using the time-restricted method of IF helps define a specific timeframe to eat. This structure can help people better plan out meals and create a routine. When it comes to eating, sleeping, and exercising, the body generally appreciates routines!
  • End late night noshing. A 16:8 or 14:10 method might help people who want to change longstanding habits of late night eating and its negative health effects. Many of us are distracted during the day from busy jobs or families, which might cause us to skip meals and then become ravenous at night eating large volumes of food. Late eating is a major trigger for gastroesophageal reflux disease (GERD), stomach cramps, and disrupted sleep (either from the stomach upset or continued digestion that doesn’t allow the body to enter deeper sleep stages). Night eating is also strongly associated with weight gain and increased risk of type 2 diabetes. The structure of a cut-off time reminds people to wind down the eating after dark and focus on other things.
  • Potential metabolic benefits. An extended fast of more than 12 hours causes the body to use glucose or sugar in the blood as well as stored glucose in the liver, which lowers insulin levels. When glucose is depleted and insulin levels are low, the body seeks energy from fat stores by creating ketones. Blood levels of triglycerides and VLDL cholesterol (a type of “bad” LDL cholesterol) decrease. Feel-good endorphins may be released and mental clarity improves. Although animal studies show that ketosis can also reduce inflammation and stimulate the disposal of damaged cells while building new cells (called autophagy), these effects aren’t as clear in humans.


  • Not for everyone. IF can be risky for some: those with a history of disordered eating that involves food restriction for extended hours, those taking blood sugar lowering medications such as insulins and sulfonylureas (glipizide, glimepiride), those taking medications for blood pressure that can make them more sensitive to dips in electrolytes (sodium, potassium) that could harm the heart, children and adolescents who are still actively growing, and those who are pregnant or lactating.
  • Especially not for certain people. If someone has a history of unhealthy eating patterns specifically targeting extreme weight loss, they may use IF as a guise to not eat. This includes: anorexia nervosa, young tweens and teens who are trying to lose weight quickly, and even older adults who have a history of yo-yo dieting. Young adults who grow up with social media are especially vulnerable, as they may see celebs and health influencers doing IF, which is perceived as a very healthy practice. When I speak to people like this in the clinic, I try to delve deeper into their reasons for wanting to do IF. Because even if the person does not currently have disordered eating patterns, starting IF can trigger a negative relationship with food. If I sense there is a deeper unhealthy psychological drive, we discuss other options.
  • Overeating. This doesn’t happen with everyone because a moderate fast can actually help regulate or depress appetite hormones, but there is a psychological component when undergoing an extended fast that you don’t have to worry so much about portions and types of foods eaten during your eating window. But you do. It’s extremely important to continue eating a high-quality nutrient-dense diet with reasonable portions.
  • Fatigue and lower quality of exercise. Again this may not happen with everyone; in fact some notice an energy boost. But it may take time for your body to adjust to a new eating schedule. You may also need to adjust timing of exercise. For example if you are fasting from 6pm-10am, you may not do well exercising vigorously at 6am, as you haven’t eaten for a long stretch and then cannot eat for several hours after exercising.

Have you tried intermittent fasting? If so, why did you do it and what are your experiences with it?

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