By now you haven’t just heard about one of the latest weight-loss trends, you know at least a handful people who are devoted to intermittent fasting (IF). Maybe you’ve even tried it yourself.
There are a lot of opinions surrounding IF, and there are different ways to do it. So you no doubt have a few questions. How effective is it? What’s the best method? What factors into the timing? How do you fit workouts around fasting? Here’s what the science actually says about it.
Jordan Mazur, M.S., R.D., is the coordinator of nutrition and team sports dietitian for the San Francisco 49ers.
What is Intermittent Fasting?
Intermittent fasting has been around for centuries, since ancient hunter-gatherer times, but it’s risen to popularity in the past five years or so. There have been a few catalysts: a 2012 documentary called Eat Fast, Live Longer; as well as a slew of books including The Fast Diet, The 5:2 Diet Book, and The Obesity Code. All of this media, combined with anecdotal success, have created a positive buzz around the trend.
IF is an eating pattern that cycles between periods of fasting and eating. It doesn’t specifically say which foods to eat or avoid, but rather when you should be eating. It’s actually more of an eating pattern than an actual diet per se. Modern IF methods can be summed up by these four types:
- Eat-Stop-Eat: This involves fasting for 24 hours once or twice per week.
- The 5:2 Method: You consume only 500 calories on two non-consecutive days of the week, but eat a normal pattern the other five days of the week.
- Alternate-Day Fasting: You fast every other day. This can be adjusted, but some methods allow 500 calories or fewer on the fasting days.
- The 16:8 method: This involves skipping breakfast and restricting your daily eating period to eight hours, then you fast for 16 hours before eating again. For example, your feeding window might be 12-8 p.m. but within that window, you can fit two or three meals, then fast until 12 p.m. the following day.
The 16:8 method is the most popular and easiest for people to stick to. Overall, no matter what method you choose, as long as you’re reducing your caloric intake, any method should cause weight loss as long as you aren’t over-consuming during the feeding timeframes.
How Does It Affect Your Body and Help You Lose Weight?
Fasting causes a number of reactions in the body that affect your cells on a molecular level. The main idea behind IF is that it helps your body access and mobilize fat stores by adjusting the hormones that influence lipolysis (the breakdown of fats). Without diving too much into the science and mechanisms of how this works, here are a couple of the changes that occur in the body during periods of fasting:
- Increase in human growth hormone: Growth hormone, or somatotropin, is a peptide hormone that stimulates cell growth, cell reproduction, and regeneration in humans. More importantly, it stimulates the production of IGF-1, which positively impacts insulin levels and levels of body fat. Research shows that fasting can cause growth hormone levels to skyrocket, which has benefits for fat loss and muscle gain, among other benefits.
- Insulin sensitivity can improve: Fasting can cause levels of insulin to drop dramatically, which makes your ability to burn body fat more accessible. The idea behind fasting is to allow insulin levels to drop far enough that we burn fat because we have to tap into those energy stores in times of fasting or starving.
Among those benefits mentioned, a recent review of the science of IF in the New England Journal of Medicine does a deep dive on current research, explaining how IF can improve metabolism, lower blood sugar, decrease inflammation, and improve chronic conditions like asthma and arthritis. There’s even evidence to show it can reduce damaged cells and enhance brain function.
How Will It Affect My Training? How to Time IF Around Workouts?
If you’re considering IF and like to work out or train, there are some things to consider. There’s some research that shows exercising during a fasted state can help muscle biochemistry linked to insulin sensitivity. In layman’s terms: It can help regulate blood sugar levels. On the other hand, there’s research to show the benefit of eating, especially carbs, before exercising.
If you’ve never tried it before, here’s what you could experience if you fast before you train:
- You may burn more body fat. This depends on other factors, too, but for the most part, your body will need to get its energy from stored fat since glucose isn’t readily available.
- Your performance might suffer. This is especially true of high-intensity training where oxygen isn’t readily available. Low-intensity, long-duration workouts are best while fasted, as the body is more efficient at mobilizing fat stores when oxygen is more available.
- You might lose or maintain muscle mass. Your strength training sessions will likely suffer without nutrition, so don’t rely on fasted workouts to help you build muscle.
Our take: If you’re an elite or professional athlete, don’t train or compete while fasted. If you’re a weekend warrior looking to improve body composition, fasted cardio might be worth experimenting with. Make sure you think through the timing of your workouts when fasting based on your goals and individual performance. Working out before the eating window while fasted is preferred for those who want to improve body composition and know they can personally perform well on an empty stomach. Working out after the eating window is ideal for those who perform better after fueling, or those who don’t have the time in their schedules to work out before the window.
What you eat during your feeding window also matters. Although the concept of IF is more of an eating pattern than a diet, the food and macros you eat during the window should reflect your goals as well. If you’re strength training, higher carbs and protein are needed. If you’re endurance training, carbs and protein are also important—but in a different ratio (more carbs to protein). If your goal is to improve body composition and lower body fat, then following a low-carb diet in your eating window might be ideal for you.
No matter what type of IF you decide to try, make sure to listen to your body. If you feel weak or dizzy, you might need to switch it up. Your blood sugar may be too low, or you may be dehydrated. Make sure you eat well-balanced, high-quality foods during your feeding window, and always remember to stay hydrated and include electrolytes to achieve optimal performance.
The bottom line is there are many different types of IF. There’s positive research to back it up and improve overall health. However, there are a few considerations you need to consider to pick the ideal type and timing of IF that works for you and your goals.
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