Much of the interest in intermittent fasting is about weight loss and fat loss. However, results of the most recent study suggest it might not be helpful for either. I'll tell you what you need to know about intermittent fasting for fat loss and weight loss.
What is intermittent fasting
In a nutshell, intermittent fasting involves limiting the time frame during which you eat. So for example, instead of eating from 8 AM to 9PM, you might limit that window to five hours on some days or to eight hours every day. You can read about the types of intermittent fasting on my blog to get a breakdown of the common types of intermittent fasting diets.
The hype around intermittent fasting
There’s a lot of excitement around benefits of intermittent fasting, such as lower levels of inflammation, improvement in cellular repair, which is thought to protect your brain from aging, and potentially lowering your risk of type 2 diabetes due to improvements in how your cells respond to insulin. Intermittent fasting is also being studied for enabling your body to resist the damage from oxidative stress. This form of cellular damage, along with inflammation, are thought to be driving forces of all sorts of diseases, including type 2 diabetes, heart disease, Alzheimer’s disease, and mood disorders, like depression. This is all very exciting, yet many of these findings come from animal research, which is suggestive, but not the type of human research that’s needed to draw more definitive conclusions. Of course, there’s also a lot of interest in intermittent fasting for fat loss and weight loss.
The downside of intermittent fasting for fat loss
A new study that dropped this week examined the effects of intermittent fasting compared to a typical eating pattern that included three meals a day. It was a 12-week study involving more than 100 people and it specifically looked at the time restricted eating window of 12:00 PM to 8:00 PM—a window that I’ve found very typical of people who practice intermittent fasting. Study participants were allowed to eat what they wanted during that period, but had to abstain from eating after 8:00 PM. The three-square meal group was instructed to have three structured meals, and snacking was allowed.
There was pretty good study compliance, although the drop out rate was a little higher in the intermittent fasting group, indicating it’s a little harder to follow. (It’s hard to put up with hunger, don’t you think?)
Here’s the kicker: Between the groups, there was no significant difference in the amount of weight lost over the 12-week period. But the intermittent fasting folks had a significant decline in muscle tissue. The other group didn’t experience this undesirable effect.
When you want to lose weight, the goal is to lose fat, not muscle, because muscle is a metabolically active tissue, which means that it’s your calorie-burning tissue. At the same time, muscle helps you stay active and healthy so you can continue to enjoy things as you age. Muscle is your ultimate anti-aging tissue: It helps support your bones and protects against falls, so it reduces the risk of serious disability as you get older.
What should you do?
I think there is one clear benefit of intermittent fasting that can also help with fat loss. The structure of limiting the window during which you eat will minimize the late-night grazing and unintentional overeating that can make it difficult to reach a comfortable weight or manage your weight. You can try a less extreme form of intermittent fasting, such as eating within a 12-hour window—say, from 8 AM to 8 PM—which I recommend for most people. Your body needs time to digest your last meal before it goes into its nightly maintenance mode. As part of its routine maintenance, your body breaks down muscle tissue, but as this study highlights, eating breakfast is an important opportunity to feed (and therefore, preserve) your muscles. Most adults should aim for about 20 grams of protein at breakfast. You can find several high protein breakfast foods that hit this target and some ideas for how to prepare them on my site.
If you still want to experiment with intermittent fasting for fat loss, try moving your window up to include breakfast and finish eating earlier in the day. If you exercise (and I hope you do!), you should plan your workouts during the eating window so your body gets the fuel it needs to support you when you’re active as well as the nutrients you need to recover from exercise.
On the other hand, if you’re looking to manage your weight and your health over time, it can be as simple as:
Eat a plant-focused diet (even if you’re including animal foods) that emphasizes fruits, veggies, whole grains, pulses, nuts, and seeds.
Eat heaps of non-starchy veggies at lunch and dinner and have two fruits, perhaps at breakfast and as a snack. At lunch and dinner, try to eat more veggies than anything else.
Think holistically about your health and work on ways to fit in activity on most days, cultivate healthy relationships with friends and loved ones, and cope with stress. And make it a priority to get sufficient sleep.
Want to geek out on the science with me? References are below.  https://pubmed.ncbi.nlm.nih.gov/23244540/  https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3106288/  https://www.sciencedirect.com/science/article/abs/pii/S193152441400200X