There’s no shortage of diet plans out there: Every one from paleo to keto to Whole30 purports to be a life-changing way of eating. But intermittent fasting, another trendy “diet,” has been growing in popularity, and it’s less about what you eat, and more about when you eat it. Maybe you’ve heard buzz about it in your running club, or you know a friend who tried it to lose weight, and you’re curious about it yourself. Here’s a full breakdown on intermittent fasting for weight loss.
What is Intermittent Fasting?
“Intermittent fasting is an eating plan based on times you allow yourself to consume food,” explains Natalie Allen, R.D., an instructor of biomedical sciences at Missouri State University and the team dietitian for all Missouri State athletes. And it’s pretty strict: “Classically, intermittent fasting means you consume only water during fasting periods, but many variants allow herbal and green tea and coffee, but no sugar or sweeteners,” says Jason Fung, M.D., author of The Complete Guide to Fasting.
But there are a lot of different methods for intermittent fasting, and research is still being done on which one is the most effective. Generally, intermittent fasting refers to periods of fasting that last less than 24 hours but are done more frequently, from daily to weekly. “The most popular regimen is 16:8, a 16-hour fast, which means you have an eight-hour eating window—say, 11 a.m. to 7 p.m—and you can do it most days of the week,” says Fung.
“Another popular regimen is to fast just under 24 hours, say from dinner to dinner, so this is sometimes called one meal a day (OMAD). This would be done two or three times per week, but some do it daily.” And then there’s the Fast or 5:2 method, where you eat normally for five days of the week and cut your calories to 25 percent of your normal intake on two nonconsecutive days of the week.
What About Intermittent Fasting for Weight Loss?
Any sort of intermittent fasting is going to affect the way your body works—and that’s why it’s often used as a weight loss technique. “Basically, you’re consuming less calories, so you lose weight,” says Allen. “If you’re following a 16:8 fasting plan, you would likely consume one big meal a day, skipping breakfast and nighttime eating.”
Eating like this can actually help you drop from 3 to 8 percent of your weight over three to 24 weeks, according to a review of studies published in the journal Translational Research; the same study found that people also lost 4 to 7 percent of their waist circumference, which means you can potentially shed the dangerous belly fat that builds up around your middle. Even better, intermittent fasting is less likely to cause muscle loss than continuous calorie restriction, according to research published in the journal Obesity Reviews.
What Are Some Other Results?
On a deeper level, the main effect is on your body’s insulin production. “When you eat, insulin goes up; when you don’t eat (or fast), insulin falls,” explains Fung. “This allows your body to begin to use some of its stored food energy, including glycogen and body fat. And when insulin falls, other hormones increase, including noradrenaline and human growth hormone, which is responsible for the increased energy, well-being, and mental clarity seen with fasting.”
These hormonal changes may increase your metabolic rate by 3.6 to 14 percent, according to research published in the American Journal of Physiology and the American Journal of Clinical Nutrition. “Intermittent fasting can also help with the reversal of type 2 diabetes, prevention of cancer, and Alzheimer’s dementia, and help with anti-aging processes,” says Fung. Studies show, for example, that five consecutive days of fasting can reduce the risk factors for aging and age-related diseases. When you fast, your cells start repair processes including autophagy, which is when cells digest and remove old proteins that build up inside them, according to another study published in the American Journal of Physiology.
What Are the Downsides?
But intermittent fasting only works as well as you stick to the parameters. “Overeating is common,” says Allen. “People know they’re getting ready to fast, so they load up and consume as many calories as they would in a normal day, in just a few hours.” FYI: That completely negates the point of fasting.
Also, limiting your eating to a certain time frame can be tough to sustain. The drop-out rate of people who tried alternate-day fasting was way higher than that of dieters who restricted their calories every day, according to a study published in the journal JAMA Internal Medicine. “Intermittent fasting takes a dedicated person who’s willing to forgo most social eating situations,” says Allen. “It also has a high learning curve, as the carbs need to be low and fat is high, which is opposite of many common meals in the American diet.”
That same study, which lasted a year, found that unhealthy LDL cholesterol had increased significantly after 12 months among the alternate fasting group. “LDL is the ‘bad cholesterol’ associated with heart disease,” says Allen. “We’re concerned about any diet plan that raises LDL.”
Is It Good for Runners?
Just like any other diet plan, intermittent fasting doesn’t work the same for everyone. Pregnant women, children, diabetics, and those with disordered eating tendencies should avoid intermittent fasting, says Allen.
When it comes to elite athletes, “many athletes use ‘training in the fasted state’ to improve performance long-term,” says Fung. Training in a fasted state can allow you to train harder and recover faster, he explains. “This is due to the physiological hormonal changes of fasting. During fasting, noradrenaline (a neurotransmitter involved in your fight or flight response) and sympathetic tone (where muscle tone is maintained predominantly by impulses from the sympathetic nervous system) increase, allowing for more energy and the ability to train harder. Plus, the amount of human growth hormone is increased so recovery is faster.”
That said, there is an adaptation period to fasting, about two to four weeks, when performance may suffer, Fung adds. “When your body isn’t used to fasting, your muscles burn glucose instead of fat. It takes several weeks for them to adapt to fat burning,” he says, during which you may feel sluggish or slow, or even experience stomach issues and diarrhea, adds Allen. It may also affect your mood and motivation.
“People who work out regularly such as runners need carbohydrates for fuel, as carbs are most easily metabolized into energy by the body,” Allen says. “Athletes’ bodies need regular fuel to perform their best. Your blood sugar control, mental clarity (your brain needs glucose), and energy levels can all be negatively affected with intermittent fasting.”
As always, talk to a dietitian or professional about whether it’s right for you if you’re determined to try it while maintaining your run training. At the end of the day, the best diet for you is the one that actually works for your lifestyle, so if intermittent fasting will prevent you from running regularly or feeling strong, then forget the fast, grab a healthy snack, and go crush your next run.