Fans of intermittent fasting say consuming fewer calories by skipping meals helps with weight loss and leads to other health benefits.
But what happens to your body when you add exercise to the mix?
matthew lee Y ericwilliamson, both from the University of Toronto School of Kinesiology and Physical Education, have studied the effects of intermittent fasting on muscle health in the general population and older adults. Lees, a postdoctoral researcher, and Williamson, a doctoral student and registered dietitian, conducted the research with associate professor Daniel Moore.
“Finding ways to lose weight that are as simple as skipping a meal is very difficult because many people find it difficult to control their hunger while in a caloric deficit,” says Williamson. “But, if they find that their hunger is well controlled with intermittent fasting and they plan to exercise at the same time, then it can be an effective tool for fat loss.”
Lees and Willamson spoke with the writer Jelena Damjanovic about the benefits of supplementing intermittent fasting with exercise.
What is intermittent fasting?
Eric Williamson: Intermittent fasting means not eating for an intentional period of time. There’s no real strict definition of how long that time should be, but for the most part, it’s at least 12 hours. Most people will practice intermittent fasting with the intention of losing weight. The thinking behind this is that intermittent fasting will keep your insulin levels lower and by lowering insulin, which is known as the fat-storing hormone, you will lose body fat.
What does science say about it?
EW: Research in this area provides ample evidence that most people who practice intermittent fasting will generally lose weight, but not for the purported reason of lowering insulin levels. Rather, the evidence shows that skipping meals does not usually lead to compensatory eating. So if you skip breakfast, you’ll usually eat a little more at lunch, but you won’t eat as much as if you had breakfast. That ends up putting you in a caloric deficit and, according to the laws of physics, is what leads to mass loss over time.
What is the appeal of intermittent fasting?
EW: Finding ways to lose weight that are as simple as skipping a meal is very difficult because many people find it difficult to control their hunger when they are in a caloric deficit. But, if you find that your hunger is well controlled with intermittent fasting and you plan to exercise at the same time, then it can be an effective tool for fat loss.
What is the benefit of combining intermittent fasting and exercise?
EW: Research indicates that if you’re going to do intermittent fasting, you should combine it with exercise, particularly resistance training. The reason for this has to do primarily with protein metabolism. We know that having our protein intake in multiple servings throughout the day is better for our lean mass and lean mass retention over time. Muscle is a major component of lean mass, so if you’re fasting, you’re missing out on these opportunities because of the effects of protein intake on muscle metabolism and risk muscle atrophy.
Simply put, when people intermittently fast without exercising, they are losing weight, but much of it often comes from lean muscle mass. If they’re exercising, then that changes from a loss of muscle mass to a loss of fat mass, so that’s a huge benefit.
Are certain types of exercise better combined with intermittent fasting than others?
EW: Yes, the effects of resistance exercise in particular are so powerful that you can probably still keep your muscle, or at least not lose it as quickly, if you do intermittent fasting at the same time. On the other hand, if someone is looking to gain as much muscle and strength as possible, such as a weightlifter or bodybuilder, they may want to avoid intermittent fasting, as they will need both the stimulus of resistance training and eating protein throughout the day. . Some may find that they still gain muscle while intermittent fasting, but it probably won’t be as fast as with regular meals and snacks.
Athletes who are trying to maximize performance should also be warned, as they have very high energy needs; that is, they will need a lot of food. If you’re intermittent fasting, you’re limiting your opportunities to get that energy. By missing out on protein feeding, they may not be at as high risk of atrophy, but they are at risk of maximizing their potential. Athletes who eat more regularly throughout the day will be more likely to meet their energy needs, recover better and adapt better to their training.
Does it matter what time of day you choose to fast?
EW: There are potential benefits to practicing intermittent fasting later in the day. Most people skip breakfast because it’s easy: you’re in a hurry, you’re not that hungry because our appetite decreases while we sleep. But from a metabolic perspective, you’re better off skipping lunch or dinner than breakfast.
There are a couple of reasons for that. One is that we are more metabolically prepared in the morning, not necessarily for protein intake, but for other nutrients. We tend to metabolize nutrients better in the morning. Second, nighttime is usually when the junk food comes out. So if someone says I’m not going to eat after eight, for example, they’re probably cutting out some of these high-calorie, low-nutrient foods.
How does age influence?
Matthew Lees: As we age, we are at even greater risk of muscle atrophy. The body becomes less efficient in the use of the protein that we consume in the diet and this process, known as anabolic resistance, worsens with a sedentary lifestyle.
You see what happens with people after a hip fracture, for example. Even brief periods of bed rest tend to generate anabolic resistance. If you’re also truncating all your meals within, say, an eight-hour window of time within the day, you have long stretches where you’re not eating. No anabolism occurs during that period, because there is no eating and no exercise which makes it even worse.
So would you advise against intermittent fasting in older adults?
ML: From a muscle health perspective, it’s just not ideal for an older person to engage in intermittent fasting, because it goes against what’s best for muscles in an aging population. They need regular stimulation of the processes that build muscle, and that’s what diet and exercise do. Having a long period of not consuming any dietary protein is simply not conducive to skeletal muscle health in an older population.
If your primary goal is weight loss, then it can be a useful tool in younger populations if it fits into your lifestyle. But for older people, weight loss isn’t always the primary goal and can often backfire. There are studies that show that a little extra weight in older people is actually beneficial.
Is there anything else people should know about intermittent fasting?
EW: Intermittent fasting may be a useful tool for younger populations, who are at lower risk of stunting. But no matter the tool, I always recommend talking to a professional before trying to lose weight. There are some minor physical risks associated with weight loss, but the mental health issues can be big. There is increasing evidence to suggest that food-related problems can occur for some people who intentionally skip meals. For the sake of maintaining a positive relationship with food and the body, as well as to preserve or improve physical health, I suggest that no one attempt to lose weight without talking to a professional.
