It’s no secret that getting regular exercise is essential for good health. But a lesser known fact is that how you eat beforehand may influence the way your body responds.

The list of benefits associated with exercise is seemingly endless, spanning from lowering blood pressure, cholesterol, and inflammation to helping manage weight and improving insulin sensitivity. Exercise can help stave off obesity, type 2 diabetes, heart disease, and some forms of cancer. And keeping physically active can even brighten mood and deepen sleep.

It turns out there is a pretty complicated relationship that exists between exercise, what we eat, and how we metabolize, or process, food for energy. The food we eat can interact with hormonal and metabolic responses in the body during and after exercise. For this reason, scientists have been studying nutritional strategies to determine how our eating patterns might influence the health and performance outcomes we gain from working out.

With the swift rise in popularity of intermittent fasting as a strategy for weight loss, improving insulin sensitivity, and maintaining overall health, it is a timely question: is it better to eat before exercising, or is exercising on an empty stomach the better way to maximize its therapeutic benefits?

Carbohydrates and fats: the body’s fuel sources

The body relies on the food it consumes to provide it with energy to run on each day. Food is made up of three macronutrients: carbohydrates, fats, and protein. The carbohydrates found in food consist of fiber, sugar, or starch and serve as the primary source of the body’s energy. During digestion, sugar and most starches break down into glucose, swiftly entering the body’s cells to be used for energy.

When the body undergoes a fast either overnight during sleep or sustained during the day, the body breaks down any remaining stored glucose, called glycogen, to be used up for fuel. When the body senses low levels of glucose, the liver breaks down fatty acids to produce an alternate fuel source in the form of  ketone bodies Ketone bodies are compounds that are increased during prolonged starvation and are released into the blood to be transported into tissues to be used for energy. A metabolic state of “ketosis” indicates that the body has transitioned from a state of burning mostly carbohydrates for fuel to burning mostly fat - a process called fat oxidation. Metabolic flexibility describes the body’s ability to switch back and forth between burning carbohydrates and burning fat depending on whichever fuel is most readily available.

How the body uses carbohydrates and fats for fuel during exercise

Fat oxidation, or “fat burn” as it is also known, takes place in the body when energy-giving glucose and glycogen are in short supply, eliciting the body to produce fat-derived ketone bodies for fuel instead. Activities that promote the depletion of glucose and glycogen stores and ramp up fat oxidation include fasting, following a low-carbohydrate diet, and exercising. Using fat for fuel instead of carbohydrates is beneficial for the body in several ways, like boosting cognitive performance and energy, controlling appetite, managing blood glucose, aiding in weight loss, and improving cholesterol.

An active body can burn both carbohydrates and fat for energy, and the amount of each used depends on how intensely an activity is performed. Evidence shows that working out at a lower intensity (up to 45% VO2max) results in the greatest amount of fat oxidation, a mix of fat and carbohydrate oxidation occurs at a moderate intensity (45-65% VO2max), and carbohydrates step in as the primary energy source at higher intensity activity levels (65%+ VO2max).  

Examples of low intensity exercise include walking at a slow pace, cooking activities, or light household chores; moderate intensity exercise includes walking briskly, mopping, or raking the yard; and high intensity activity includes running, shoveling snow, or a strenuous fitness class.

Fasted vs. fed: how the body responds to a workout

How can eating patterns influence the way the body responds to exercise? Here is a roundup of what the latest research says.

Fat oxidation

Evidence demonstrates that exercising while fasting results in a greater rate of fat oxidation when compared to exercising after eating. Recent research shows this increase, reviewing 27 clinical trials that compared the fat oxidation levels of 273 adult participants who performed the same aerobic exercises in under two hours both while fasting and after consuming a 25g carbohydrate meal. The researchers found a significant increase in fat oxidation in the fasted state (3.08g) compared to the fed state (0.79g).

Another study confirmed this boost in fast-related fat oxidation by measuring the fat oxidation rates of ten healthy men who performed four hour-long moderate-intensity exercise sessions before breakfast, after lunch, and after dinner. These rates were also compared to any fat oxidation that took place when at rest. At the end of the study, the researchers saw that over 24 hours, fat oxidation increased only when the subjects fasted before exercise (717 kcal/day). This rate was significantly higher compared to fat oxidation at rest (456 kcal/day), when exercising after lunch (446 kcal/day), and when exercising after dinner (432 kcal/day).

Exercise performance

Researchers have also identified different effects that eating versus fasting before exercise may have on performance during a workout. A comprehensive review of 46 studies that assessed how eating habits influenced cardio exercise performance found no difference in performance between those who fasted and those who ate pre-workout when workouts were less than 60 minutes. For longer workouts, the evidence was mixed, with 54% of studies finding that cycling time to exhaustion and running time to exhaustion improved when a meal was eaten before.

Blood glucose control

Proponents of fasting claim that the greater fat oxidation that occurs during exercise improves insulin sensitivity by keeping blood glucose and insulin levels low while also reducing levels of intramuscular fat by up to 60%. And it appears that the fasting fat-burn advantage persists long after the workout session ends, maintaining fat oxidation at rest even 9 to 24 hours after exercise is completed.

However, a blood glucose-lowering effect may also be seen after exercising following a meal. Eating a meal will increase blood glucose and insulin concentrations, but when exercise is performed afterward, the glucose is rapidly absorbed by the skeletal muscle, leading to a drop in blood glucose. This may explain the recent finding that exercising moderately for 30 to 60 minutes starting a half hour after eating can help lower blood glucose levels, particularly in people with type 2 diabetes.

Daily calorie consumption

Might working out on an empty stomach lead to overeating later in the day? Several studies examined whether fasting before exercise induced an increase in calorie consumption over 24 hours.

One study followed the exercise and eating patterns of 12 active men who ran for 60 minutes while either fasting or having eaten breakfast, and then eating as much as they wanted for lunch and dinner. On the days the subjects ate breakfast, 11 of the 12 men consumed an average 923 more calories over the 24 hour period than when they fasted. Interestingly, they ate the same amount at lunch whether they fasted or not, but consumed 342 more calories at dinner on the days they ate breakfast. So, fasting before their morning exercise appeared to decrease their full-day calorie intake.

A second study analyzed the effects of brisk walking among 12 healthy men to determine appetite hormone responses and gastric emptying rates. The rate at which food moves through the digestive system, known as gastrointestinal motility, can impact appetite because it influences how quickly the stomach enlarges to signal to the brain that it is full. In this study, the researchers aimed to figure out how fasting and eating before exercising each affected these factors to determine whether either led to overeating later in the day. The subjects completed a brisk 45-minute morning treadmill walk either while fasting or following a breakfast meal. Afterward, both groups consumed lunch. While the fasting group showed a higher level of fat oxidation prior to eating lunch, the two groups did not show any difference in gastric emptying rates after lunch, and both groups also reported similar levels of appetite post-exercise. This suggests that fasting may not induce overeating in the hours after exercise.

Another 2018 study confirmed these findings. In this study, 12 physically active men ran on the treadmill for one hour either while fasting or two hours after breakfast. After the workout, both groups ingested a mixed-macronutrient drink and then were given the opportunity to eat as much as they wanted during lunch 90 minutes later. The researchers found that energy intake - that is, the number of calories consumed - during lunch was similar between the two groups, even though one of the groups exercised on an empty stomach. In fact, energy intake for the fasting group was less than the breakfast-eating group overall.

How does eating behavior affect resistance training?

Most research that studies the effects of fasting versus eating prior to a workout involve aerobic training, but what about resistance training? Does fasting before a workout lead to greater fat burning? It appears so. A 2018 study looked at the fat and carbohydrate utilization effects of resistance training sessions among 12 female adults. Each subject completed one session consisting of bench presses, back squats, and military presses after completing a 10-hour fast and another session following a meal, where their respiratory exchange ratios (RER) were measured. The RER is the proportion of carbon dioxide the body produces to oxygen that is taken in by the body, and can be used to indicate whether carbohydrates or fats are being metabolized to provide its fuel. Decreased RER values specify that fat is mainly used for energy, whereas higher values signify that carbohydrates are the primary source. The researchers found that during the back squat and military press exercises, the average RER was much lower when the subjects fasted compared to when they ate. This suggests that fasted resistance workouts burn more fat than carbohydrates.

The bottom line

Considering whether it is better to eat or fast during exercise to boost your performance? The evidence appears mixed on that. But when it comes to managing blood glucose, fasting while exercising has been shown to induce the body to burn more fat than carbohydrates, which can provide extra support in maintaining a steady blood glucose level. Exercising while fasting might also help to keep your calorie intake in check throughout the day. Whether you choose to exercise on an empty stomach or fuel up on food first, one thing remains true: making regular exercise a part of your lifestyle is a healthy choice that is vital to keeping your body and mind strong and in tiptop shape.