Fasting is a popular dietary strategy where food intake is restricted for set periods of time. Pairing fasting with exercise has been a popular strategy for different body composition and fitness goals. Research on fasted exercise can be confusing because there are a variety of approaches to fasting and different styles of exercise that have been examined. We’re going to cover the basics of fasting, what happens to our bodies when we exercise while fasting and discuss practical applications to combining fasting and exercise.
Intermittent fasting is a popular dietary strategy where people consume little food during set periods. Some of the most popular fasting diets include:
Time-Restricted Fasting: This is a daily dietary strategy where food is restricted to specific times of the day. The most common way this diet is done is following a 16:8 split, where people will fast for 16 consecutive hours separated by an 8-hour period where they can eat food. Another popular format follows a 20:4 split, where people will fast for 20 consecutive hours and have an eating period lasting 4 hours.
Periodic Fasting: These styles of fasts are longer intervals than time-restricted fasts. A common periodic fast is the 5:2 split, this involves eating normally for 5 days and fasting for 2 entire days out of the week.
Alternate Day Fasting: This involves fasting every other day. In some protocols, individuals will consume zero calories on their fasting days while in other protocols like a Modified Alternate Day Fast, they can consume up to 25% of their daily calorie needs.
The strictness of fasting periods will vary with some protocols prohibiting the consumption of any calories while others may allow the consumption of up to 25% of one’s daily caloric needs. [1,2]
In a systematic review of 27 clinical trials over a 20-year period, the researchers found intermittent fasting to produce weight loss results ranging from 0.8 to 13% of the subject’s baseline weight, with most of the weight loss coming from fat tissue.
A meta-analysis that reviewed the effects of obese adults following intermittent fasting diets found the dietary strategy to be effective for supporting weight loss, with the greatest effects being seen in those following 5:2 and Alternate Day Fasting diets.
When compared to standard continuous energy restriction diets, research has shown intermittent fasting to be fairly equivalent for the purposes of weight loss. [5,6,7,8]
When we exercise, the two main fuel sources we use are fats and carbohydrates. Our utilization of these fuel sources will vary based on factors like exercise intensity, training status, and dietary composition. In general, lower intensity exercise tends to utilize fats as the main energy source while higher intensity exercise uses carbohydrates.
Fasting is commonly paired with exercise for the purpose of trying to improve body composition and physical fitness. Fasted exercise is associated with increased levels of lipolysis (the breakdown of fat) and utilization of fat as a fuel source. A meta-analysis comparing fed and fasted cardio on fat utilization found fasted cardio to provide an increase in fat burning to the tune of about 3 g (about 27 calories) during a workout.
Research observing the effects of fasted exercise over longer periods of time has shown promising effects on carbohydrate and fat metabolism. [11,12,13]
Fasted Exercise and Weight Loss
Because of the increased rate of fat utilization during fasted exercise, fasted exercise is commonly used as a strategy to increase fat burning during weight loss programs. The data on fasted exercise shows some mixed results.
Some data suggests exercise in a fasted state can decrease body weight, fat mass, and 24-hour energy intake. [14,15] Other research shows no significant differences on the effects of fasted exercise and body composition when both subjects are following caloric deficit diets. [16,17,18]
A review of studies examining the effects of fasted resistance training exercise, the researchers noted that subjects were able to maintain levels of lean body mass while reducing their body fat in the range of 0.8 - 2.5%.
Fasted Exercise and Performance
The effects of fasted exercise and athletic performance have been investigated in various studies utilizing different fasting and exercise protocols to mixed results. The research generally shows that fasted exercise doesn’t offer a performance benefit. And studies on the effects of short-term fasting on exercise performance suggest a decrease in physical performance. [21,22,23]
Both fasted and fed training can impact exercise metabolism in distinct ways. Exercise in a fed state shows short-term performance benefits while fasted exercise shows benefits in the longer term for metabolic adaptations.
Because different styles of exercise rely on different energy systems more than others, some workouts might lend themselves better to exercise in the fasted state.
Slow steady state aerobic style exercise relies heavily on fats as a fuel source. Research shows that fat oxidation is greatest at intensities that are 60-65% of VO2 Max, with our reliance for fat as a fuel source declining as intensities increase to 75% VO2 Max or higher. VO2 Max is the maximum amount of oxygen that a person can utilize during intense exercise.
This makes low intensity cardio an effective style of exercising to do while fasting. By contrast, high-intensity anaerobic style exercise relies heavily on the use of carbohydrates as an energy source. This makes sprint training style workouts very challenging while fasting. For your high-intensity workouts, aim to do them during your feeding windows rather than while fasted.
Lifting Weights and Fasting
Resistance training, especially at higher intensities (moderate weight and high volume) can rely heavily on carbohydrates as a fuel source. Whereas lower volume resistance training workouts, like those shorter than 45 minutes in duration and consisting of about 8 working sets, rely less on carbohydrates as a fuel source.
This makes lower volume resistance exercise workouts more ideal for fasted workouts compared to higher volume workouts. For your higher volume resistance training workouts, aim to do them during your feeding windows rather than while fasted.
If you decide to work out while fasting these are some tips to help you train more effectively.
A decrease of 1-2% in hydration status can negatively impact cognitive performance and exercise capacity. It’s important to stay hydrated while exercising, even if you’re fasting. Since we obtain some of our daily fluid intake from foods it can be even easier to be under-hydrated while fasting.
Keep Your Electrolytes Up
Electrolytes play key roles in regulating fluid balance, muscle contraction, and nerve conduction. Since we primarily obtain electrolytes through the foods we eat, it’s also important to be mindful of electrolyte intake during fasts. Use XTEND Original for a zero sugar boost of refreshing electrolytes.
Keep the Intensity and Duration Low
Lower intensity and shorter duration workouts lend themselves better to fasted exercise since exercises at these intensities rely more heavily on fats over carbohydrates. Our glycogen stores are smaller than our fat stores, so our fuel tanks for exercise that burn fat are much larger. If you’re doing a cardio workout, this means doing it at an intensity that’s about 65% of your VO2 Max or lower. If you’re doing a resistance training workout, this means shorter workouts where the rep ranges and total sets are lower.
Mind Your Macros
If your main goal with fasted exercise is to lose weight, it’s important to be mindful of your overall daily caloric intake. Since you’ll need to maintain a net caloric deficit to lose weight, avoid overeating during your feeding periods. Make sure you’re consuming enough protein during your eating period as well to help preserve muscle mass. Aim for lean proteins and sources high in branched chain amino acids (BCAAs).
Intermittent fasting is a popular diet strategy to reduce caloric intake, bodyweight, and body fat %. Fasted exercise has received a lot of research over the years for its effects on body composition and performance. Fasted exercise has shown promising effects on metabolic adaptations to how our bodies process carbohydrates and fats.
The literature on fasted exercise for weight loss is mixed, with some studies showing a benefit and others showing no effect. If you enjoy doing fasted exercise for the purposes of weight loss, it can be used effectively in a weight loss program, just make sure you’re consuming enough protein during your feeding window to maintain lean body mass and stay in a caloric deficit. If you don’t like doing fasted exercise, you can skip it and still lose weight, just make sure you’re in a caloric deficit.
The literature on fasted exercise for performance generally shows it to be detrimental to performance, especially short-term performance goals. However, the metabolic adaptations to fasted exercise have shown promise for some athletes like endurance athletes. Some endurance athletes periodize their nutrition plans following a “train low, compete high” approach where they will do their lower intensity workouts in a fasted (or low carbohydrate) state to promote metabolic adaptations to exercise. And then when it’s race day, they compete with high carbohydrate availability.
Fasted Workouts and Supplements: BCAAs and Whey Protein are popular supplements that complement fasted training plans. BCAAs and Whey Protein can be used during the feeding windows to help ensure that you get a sufficient supply of amino acids to maintain lean body mass.
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