The Science Behind Eating Before / After Exercise

The simplest answer to “what should I eat before a workout?” is: something that makes you feel satiated and energized, whatever that may look like for you. If you enjoy your workouts and feel like you’re in a good groove, without overthinking what’s happening on your plate before or after, then stop right here! And keep doing what you’re doing. You’re doing amazing.

The longer, slightly more scientific answer is something with a combination of carbs, protein, and maybe a little bit of healthy fat. The amount will depend on how long you eat before your workout – a larger meal 2-3 hours before or a smaller snack 30min – an hour before +/- some caffeine if it’s in the morning / you drink caffeine.

Here’s a little more background info. During exercise, we use varying amounts of glycogen/glucose (aka carbohydrate energy), fats, and protein. For the real bill nye science guy nerd breakdown, it’s a little something like this: ( 1 )

  • Glycogen and glucose:
    • 35% at rest, 40% light moderate exercise, 95% high intensity sprinting, 70% high intensity endurance
  • Fats:
    • 60% at rest, 55% for light moderate exercise, 3% for high intensity sprinting, 15% for high-intensity endurance
  • Protein:
    • 2-5% at rest, 2-5% for light-moderate exercise, 2% for high-intensity spring, 5-8% high-intensity endurance

So you can see, you’re tapping into mostly glucose and fat stores during exercise, which means that carbs will be important both before and after. In one of my lectures, a professor said “fats burn in a carbohydrate flame,” and this is what prevents ketogenesis. We need an adequate intake of carbs to spare muscle tissue. ( 1 )

BUT WAIT! I hear all of this information about the ketogenic diet and ketosis. What’s the hype?

exercise and ketogenic diet

Under conditions of limited carbohydrate availability in the Keto diet (depletion of muscle and liver glycogen stores), fatty acids are mobilized from adipose tissue as a means of energy, entering into ketosis. This is a fancy way of saying tapping into fat stores for energy. In some animal models, ketone bodies have been reported to improve metabolic efficiency.

In one study performed on well-trained athletes, three weeks of intensified training and mild energy deficit on a low carb high fat (aka keto) diet increased peak aerobic capacity ( 9 ) (21 male highly trained athletes  who underwent high carb diet, periodic carb diet, or low carb/high fat diet for 3 weeks). The diet was associated with the highest rates of whole-body fat oxidation ever reported across exercise of varying speeds or intensities. ( 9 ) However, while those studied and on the diet had substantial increases in fat oxidation, they didn’t see an improvement in athletic performance unless there was consumption of periodic or regular carbohydrate consumption ( 9 ).

Another study looking at low carb ketogenic diet in male athletes over a 12 week span concluded that compared to a high carbohydrate group, the keto-adaptation had enhanced both body composition and fat oxidation during exercise as well as some measures of performance. While there was improvement in VO2max, the power output during work at maximal intensity was compromised on the ketogenic diet, which can be explained by lower muscle glycogen stores and the reduced activity of glycolytic enzymes after the four-week diet intervention ( 3 ) (47 male endurance trained athletes who underwent 12 week keto-adaptation and exercise training).

In yet another study, the ketogenic diet introduced favorable changes in body mass and body composition, as well as in the lipid and lipoprotein profiles. During long endurance exercise at moderate intensity (between 50% and 70% VO2max), a lower RER and lactate concentration were observed (lower RER is a way of measuring increased fat metabolism) ( 8 ). The ketogenic diet resulted in a two-fold increase in resting plasma free fatty acid concentration (8 young, male subjects with 5 years cycling training experience who underwent 4 weeks of either mixed diet or low carbohydrate ketogenic diet).

Compared to a high carbohydrate diet, or a mixed diet with 50%–70% of energy coming from carbohydrate, a high fat diet with 70% of the calories derived from fat significantly increased the contribution of free fatty acid to the total energy expenditure during moderate intensity exercise. This was observed during the first 90 min of the exercise protocol. During the last 15 min of exercise, when maximal intensity was introduced, free fatty acid metabolism was inhibited by glycolysis. In sum, during short duration of moderate exercises on the ketogenic diet, there is free fatty acid metabolism. But during intense exercise of maximal exertion, when there are increases in lactic acid concentrations, there is less free fatty acid metabolism and athletes may have muscle breakdown ( 8)  (8 young, male subjects with 5 years cycling training experience who underwent 4 weeks of either mixed diet or low carbohydrate ketogenic diet)

In overweight women, resistance exercise in combination with a ketogenic diet may reduce body fat without significantly changing lean body mass, while resistance exercise on a regular diet may increase lean body mass in without significantly affecting fat mass. Fasting blood lipids do not seem to be negatively influenced by the combination of resistance exercise and a low carbohydrate diet ( 10 ) (18 untrained women between 20-40, BMI >25, who underwent 10 weeks resistance training in combination with either regular diet or low carb-ketogenic diet).

Ketosis and exercise summary:

  • the ketogenic diet may be good for fat burning, but may prevent improving athletic performance in male highly trained athletes
  • the ketogenic diet burns fat stores during moderate exercise (50-70% VO2max), but not during maximal intensity exercise in young men
  • in young, overweight women, the ketogenic diet may reduce body fat without significantly changing lean body mass

Exercise and fasting:

The benefits:

In one meta-analysis, acute endurance exercise in the fasted state resulted in elevated blood free fatty acid concentrations (aka using fat as energy) and stable blood glucose concentrations in the first 60–90 min of exercise ( 7 ) (meta-analysis of 19 studies examining acute impact of endurance exercise in fasted vs fed state). Long-term exercise training in the fasted state in healthy subjects is associated with greater improvements in insulin sensitivity, basal muscle fat uptake capacity, and oxidation ( 7). Endurance exercise in the fasted state in healthy subjects is generally assumed to be associated with lower blood insulin concentrations, stable blood glucose concentrations (in the first 60 min), elevated blood free fatty acid concentrations and fat oxidation ( 7 ). Another study reported similar findings. Pre-exercise feeding enhanced prolonged duration of aerobic exercise performance ( 12 ) (meta-analysis of fasted vs fed-state exercise on performance and post-exercise metabolism). Fasted exercise increased post-exercise circulating FFAs compared to fed exercise, suggesting that triglyceride mobilization from adipose tissue is increased, in turn elevating circulating FFA concentrations for potential use as fuel ( 12 ) (meta-analysis of fasted vs fed-state exercise on performance and post-exercise metabolism).

Other studies, however, discuss neutral / no benefits of fasted exercise in comparison to eating before exercise.

While exercising after eating invoked a more substantial and prolonged suppression of appetite than fasted exercise, this did not result in any differences in energy intake between trials and elicited a negative energy balance relative to control in both exercise trials. Another study looking at daily caloric consumption / appetite differed if participants exercised after eating or while fasting (twelve healthy males undergoing 60 min of treadmill when fasting vs non-fasting) ( 11 ). The study demonstrated that there seems to be no additional benefit of exercising before or after breakfast, as both conditions produced a negative daily energy balance compared to a sedentary day. Aka exercise itself is what’s important – not what you eat or don’t eat beforehand (twelve healthy males undergoing 60 min of treadmill when fasting vs non-fasting) ( 11 ).

In another study, twenty healthy young female volunteers were randomly assigned to 1 of 2 experimental groups: a fasted training group that performed exercise after an overnight fast or a post-prandial training group that consumed a meal prior to exercise. Training consisted of 1 hour of steady-state aerobic exercise performed 3 days per week. A meal replacement shake was provided either immediately prior to exercise for the FED group or immediately following exercise for the FASTED group. Both groups showed a significant loss of weight and fat mass  from baseline, but no significant between-group differences were noted in any outcome measure ( 16 ) . These findings indicate that body composition changes associated with aerobic exercise in conjunction with a hypocaloric diet are similar regardless whether or not an individual is fasted prior to training.

Still others demonstrate potential downfalls.

Another study looked at 8 healthy men and compared 36 minutes of high intensity cardio when eating vs non-eating before the exercise intervention ( 5 ). In both cases, the total amount and quality of food eaten 24 hours after the training session was the same. Eating breakfast increased VO2 max (a measure of exercise intensity) and RER (respiratory-exchange ratio) immediately after the exercise.  ***background: respiratory quotient, which typically ranges between 0.7 and 1.0, is an indicator of metabolic fuel or substrate use in tissues; it must be calculated under resting or steady-state exercise conditions. A ratio of 0.7 is indicative of mixed fat use, whereas a ratio of 1.0 indicates the exclusive use of carbohydrates ( 6 ). While exercising the fed group RER was 0.96 (indicating carbohydrate use), whereas the fasting group was 0.84 (indicating more lipid utiliazation). HOWEVER, 12 hours later, the fed group RER was significantly lower, indicating that eating before exercise may influence increased fat burning long after the exercise itself ( 5 ). The RER was lower both at 12 and 24 hours after the exercise session in the fed group than in the fast. Thus, a prolonged effect on substrate utilization occurs with a shift toward lipids with feeding before exercise. Extremely small study done only in men, but interesting nonetheless ( 5 ).

SO, to summarize.

  • Some research in human studies suggests that fasted exercise helps mobilize fatty acid oxidation to be used as fuel better than exercising directly after eating
  • other studies mention no differences in weight when comparing exercising when fasting vs non-fasting.
  • there may be more resting energy expenditure (burning long after exercise) with eating before exercise as opposed to fasted exercise.
  • Which basically means the JURY IS OUT!

Which is why it’s more important than ever to engage in behaviors that feel best for you – physically and emotionally! If you’re hungry, eat!

Interestingly, there is some research that caffeine may help with exercise endurance. A meta-analysis reported that when exercise protocol was examined, time-to-exhaustion protocols were significantly greater than non-caffeine consumptions. The results from this meta-analysis confirm the ergogenic effects of caffeine, particularly for endurance testing that use time to exhaustion protocols ( 14 ) (meta-analysis of 40 double-blind studies).

Regardless of what happens before exercise, the science all supports eating within the first few hours after exercise.

Because so much muscle glycogen is used during exercise, it’s important to replace afterwards, as well as fluids and electrolytes ( 2 ) (systematic review of 20 articles describing pre and post workout food consumption). For food consumption after training it is necessary to combine foods rich in proteins of high biological value and moderate carbohydrates for muscle restoration and promotion of other anabolic processes, aka muscle building ( 2 ) (systematic review of 20 articles describing pre and post workout food consumption).

For athletes, for regeneration, repair, and adaptation following the catabolic stress of exercise the main focus is on carbohydrates and amino acids (protein). ( 1 )

  • Carbs: Focus on high carbohydrate, nutrient dense foods
    • Immediate recovery after exercise (0-4 hours) – 1.2g/kg/hour
    • Daily recovery from moderate duration . low-intensity: 5-7g/kg/day
    • Daily recovery from moderate to heavy endurance training : 7-12g/day
    • Daily recovery from extreme training (4-6+ hours per day): 10-12g/kg/day
  • Amino acids post exercise: 6-12g of essential amino acids = 10-20g high quality protein (other sources say 20-30g) – more important for resistance training
  • Hydration!

In one study, data from two studies of 30 marathon runners and 10 triathletes suggest that carbohydrate compared to placebo ingestion help diminish the physiologic stress of exercise, although clinical significance awaits further research ( 4 ) (systematic review).  This stress is sometimes referred to as hormesis, or “how a mild oxidative stress associated with exercise can result in favorable adaptations that protect the body against more severe stresses and disorders derived from physical stress or other etiological origin” ( 13) (article used for background info / definition clarity)

up the antioxidants

High intensity sustained aerobic exercise can create an imbalance between free radicals and antioxidant defenses, but the body naturally increases it’s antioxidant defenses to counterbalance the generation of free radicals.

Nevertheless, eating foods that are high in antioxidants, specifically after a workout can’t hurt.  Here are a few star players that serve as antioxidants:

    • Glutathione: helps regeneration vitamins E and C (abundant in the body)
      • Sulfur-rich foods!
        • Vegetarian sources: broccoli, brussels sprouts, cauliflower, kale, watercress and mustard greens
      • Non-vegetarian sources: beef, fish, poultry
    • Vitamin E: protects mitochondria, lipoproteins, adipose tissue, liver and muscle
      • Almonds, peanuts, sunflower seeds, hazelnuts, pine nuts
    • Vitamin C – used to regenerate vitamin E – provides plasma/ extracellular protection
      • Citrus!! Orange, kiwi, lemon, grapefruit, broccoli, cauliflower, papaya, cantaloupe, strawberries
    • Selenium – needed for glutathione peroxidase functioning (enzyme whose main role is to protect against oxidative damage)
      • Vegetarian sources: brazil nuts, sunflower seeds, brown rice
      • Non-vegetarian: eggs, shrimp, chicken, cottage cheese, beef
    • Carotenoids: lypcopene (may protect against oxidative stress) and alpha/beta caroteine, (yellow, red, pink fruits and vegetables, dark green leafy vegetabels)
      • Lycopene: tomatoes, watermelon, papaya, grapefruit, red cabbage
      • Carotenoids: carrots, squash, grapefruit, oranges, apricots
    • Lignans: flax seeds, soybeans, nuts, berries, rye, bran
    • Organosulfide compounds: garlic, onions, leeks, cruciferous veg
    • Anthocyanins: cherries, blueberries, cranberries, blackberries, raspberries
      • In one journal review, the authors found research supporting the notion that tart cherry consumption attenuates exercise induced muscle damage symptoms after intense exercise bouts. This attenuation seems to be related to the antioxidant and anti-inflammatory properties of anthocyanins and other phenolic compounds present in tart cherries ( 15 ) (systematic review of 6 articles studying well-trained athletes)

Pre-workout snacks:

  • Brown rice cake with peanut butter
  • Hard boiled egg with small sweet potato
  • Berries with cottage cheese
  • WATER!


  1. Smoothie: banana (carb), frozen sweet potato (carb), hemp seeds / protein powder (protein), frozen kale (antioxidants), frozen tart cherries (antioxidants), peanut butter (healthy fat), vanilla extract, cinnamon, almond milk
  2. Breakfast bowl: brown rice / quinoa (carb), egg / black beans (protein), roasted broccoli / brussels sprouts (antioxidants), avocado (healthy fat)
  3. Toast (carb) with egg (protein), avocado (healthy fat), and sautéed spinach + tomato (antioxidants)
  4. Huge salad with roasted squash (carb), lentils (protein / carb), quinoa (protein / carb), avocado (healthy fat), roasted brussels sprouts (antioxidants)



( 1 ) Santo. A. Sports Nutrition and Fitness. January, 2016. Powerpoint Presentations.

( 2 )  Silva, P.D. (2018). Food consumption of practitioners of weight training in the pre and post training. Multidisciplinary Core Scientific Journal of Knowledge, 6(6), 108-122.

( 3 ) McSwiney, F.T., Wardrop, B., Hyde, P.N., Lafountain, R.A., Volek J.S., & Doyle, L. (2017). Keto-adaptation enhances exercise performance and body compotision responses to training in endurance athletes. Metabolism, 81, 25-34.

( 4 ) Braun, W.A. & von Duvillard, S.P. (2004). Influence of carbohydrate delivery on the immune response during exercise and recovery from exercise. Nutrition, 20(7-8), 645-650.

( 5 ) Paoli, A., Marcolin, G., Zonin, F., Neri, M., Sivieri, A., & Pacelli, Q.F. (2011). Exercising fasting or fed to enhance fat loss? Influence of food intake on respiratory ratio and excess postexercise oxygen consumption after a bout of endurance training. International Journal of Sport Nutrition and Exercise Metabolism, 21, 48-54.

( 6 ) Deuster, P.A. & Heled, Y. (2008). Testing for maximal aerobic power. The Sports Medicine Resrouce Manual.

( 7 ) Hansen, D., De Strijcker, D., & Calders, P. (2016). Impact of endurance exercise training in the fasted state on muscle biochemistry and metabolism in healthy subjects: Can these effects be of particular clinical benefit to type 2 diabetes mellitus and insulin-resistant patients. Sports Medicine, 47, 415-428.

( 8 ) Zajac, A., Poprzecki, S., Maszcyk, A., Czuba, M., Michalczyk, M., & Zydek, G. (2014). The effects of a ketogenic diet on exercise metabolism and physical performance in off-road cyclists. Nutrients, 6(7), 2493-2508.

( 9 ) Burke, L.M., Ross, M.L., Garvican-Lewis, L.A., … & Hawley, J.A. (2017). Low carbohydrate, high fat diet impairs exercise economy and negate sthe performance benefit from intensified training in elite race walkers. Journal of Physiology, 595 (9), 2785-2807.

( 10 ) Jabekk, P.T., Moe, I.A., Meen, H.D., Tomten, S.E., & Hostmark, A.T. (2010). Resistance training in overweight women on ketogenic diet conserved lean body mass while reducing body fat. Nutrition and Metabolism, 7(17).

( 11 ) Deighton, K., Zahra, J.C., & Stensel, D.J. (2012). Appetite, energy intake and resting metabolic response to 60 min treadmill running performed in a fasted versus postprandial state. Appetite, 58(3), 946-954.

( 12 ) Aird. T.P., Davies, R.W., & Carson, B.P. (2018). Effects of fasted vs fed-state exzercise on performance and post-exercise metabolism: A systematic review and meta-analysis. Scandinavian Journal of Medicine and Science in Sports, 28(5), 1476-1493.

( 13) Ji, L.L., Dickman, J.R., … & Koenig, R. (2010). Exercise-induced hormesis may help healthy aging. Dose Response, 8(1), 73-79.

( 14 ) Doherty, M. & Smith, P.M. (2004). Effects of caffeine ingestion on exercise testing: a meta-analysis. International Journal of Sports Nutrition and Exercise Metabolism, 14(6), 626-646.

( 15 ) de Lima, L.C.R., Assumpcao, C.d.O., Prestes, J., & Denadai, B.S. (2015). Consumption of cherries as a strategy to attenuate exercise-induced muscle damage and inflammation in humans. Nutriticion Hospitalaria, 32(5), 1885-1893.

( 16 ) Jon Schoenfeld, B., Aragon, A.A., Wilborn, C.D., Krieger, J.W., & Sonmez, G.T. (2014). Body composition changes associated with fasted versus non-fasted aerobic exercise. Jounral of the International Society of Sports Nutrition, 11(54). 

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4 Responses

  1. Okay the citations at the end of the blog post….. What! As a STEM undergrad I am SO happy to read this. Thank you for this post.

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