Timing of Nutrient Intake Around Exercise


Should I eat before I workout?

Do I need to have a protein shake immediately after?

Does fasted cardio actually increase fat burning?

When it comes to nutrition and exercise, one thing remains clear: timing is MUCH less important than total intake throughout the day. An individual may time out 'perfectly' when they are eating around a workout, but if the total daily amount is not controlled for (too low or high in total calories, low in protein, low in a micronutrient, etc) then progress will likely be hindered. Conversely, if total intake IS adequately accounted for, there is less likely a chance that timing will make a large difference in overall results. However there are some study findings that could suggest potential benefits from the addition of appropriate nutrient timing, given overall intake is already controlled for.

Should I eat before I exercise?

This is a common question but unfortunately with variable answers. This will largely depend on the individual, the activity, and the time of day. The argument in favor of pre-exercise nutrient intake stems from the idea that by increasing serum and stored nutrient availability, that exercise performance can be supported (or possibly improved) and muscle tissue damage can be attenuated as there is an increased supply of energy substrates (namely glucose, glycogen, and amino acids). (Kerksick, et al. 2008) Muscle glycogen is depleted during exercise where longer duration and higher intensity will contribute to a greater decline in these limited fuel stores. (Kerksick, et al. 2008) As glycogen is depleted, exercise intensity and work output also decrease while muscle catabolism increases (proteins are broken down into amino acids to be used for energy). (Kerksick, et al. 2008)

On a side note: Why wouldn't you use fat for fuel if glycogen is depleted?

Fat is primarily burned as a fuel source under conditions of low intensity (where there is plenty of oxygen available to oxidize fatty acids). (McArdle, et al. 2013) This means that fat is typically burned as a fuel while at rest, but after surpassing a threshold of energy output (where there is less oxygen available, like with increasing the intensity and/or duration of exercise), you will burn fuels that are not as highly demanding of oxygen (glycogen/glucose and amino acids).

In general, the percentage of fat and glucose/glycogen contribution to energy looks like:

  • At rest: fat contributes 60% with 35% from glucose+glycogen

  • Light to moderate exercise: fat contributes 55% with 40% coming from glucose+glycogen

  • High intensity exercise: fat contributes ~3 % with 95% from glucose+glycogen

  • Endurance exercise (@ higher intensity): 70% comes from glucose+glycogen with only 15% from fat

  • Protein contributes 2-5% at all intensities

*You are never NOT burning glucose+glycogen for fuel, but you will require a greater percent contribution of carbohydrates as a fuel source with increases in exercise intensity and/or duration.

So how do you ensure that you have enough glycogen to support your activity level?

Maintaining appropriate carbohydrate intake in the diet is the first concern when it comes to optimizing glycogen stores and utilization during activity. (McArdle, et al. 2013) While energy availability is crucial for exercise performance, the advantage of protecting against muscle breakdown by having glucose and glycogen readily available (thus avoiding the need to break down proteins for energy) is another benefit of appropriate carbohydrate intake for physically active individuals. Depending on the activity type and duration, there may be a need to increase total carbohydrate intake throughout the day, and with specific timing of carbohydrate intake around physical activity. (Kerksick, et al. 2008) In those who are training and/or competing at high intensities, additional attention should be paid to specific timing of nutrient intake prior to exercise performance, particularly to optimize fuel and hydration. (McArdle, et al. 2013) *Fasting prior to intense training or competition is proposed to be detrimental as it depletes liver and muscle glycogen which can impair exercise performance.

The International Society of Sports Nutrition supports the theory of ingesting carbohydrate or carbohydrate+protein prior to resistance exercise to maximize protein synthesis as well as prior to endurance exercise to maximize muscle glycogen stores and energy availability. (Kerksick, et al. 2008) The rule of thumb is to give 1-4 hours for digestion, absorption, and replenishment of glycogen stores and where differences in food type can be associated with shorter or longer duration needed for digestion (lower in fiber, fat, and protein will digest and absorb quicker). (McArdle, et al. 2013)

As an example:

  • If strength training first thing in the morning after an overnight fast (i.e. not eating since dinner the night before), the intake of carbohydrates+protein before the workout would likely show the most benefit in energy substrate utilization during the workout and muscle protein synthesis after. (Kerksick, et al. 2008) In this case, as the training session is early morning, then utilizing foods that are quickly and easily digested and absorbed will be best (like low fiber fruits, juices, protein shakes, snack bars, etc).

  • If that same individual is performing an aerobic workout, they could use carbohydrates prior to exercise as an energy source. (Kerksick, et al. 2008) Again emphasizing lower fiber carbohydrate sources to decrease digestion time.

  • However, if an individual is working out in the afternoon or evening (like after eating throughout the day at breakfast, lunch, snack, etc) the likelihood that they will need to eat before exercise is much lower. In these cases, there is energy (calories) coming in throughout the day, and far enough ahead of exercise that there should be enough a.) time to digest and b.) calories to fill fuel stores to foster activity.

  • If there is an extended period of time between the last meal eaten and physical activity (like longer than 4 hours), then a small snack (primarily carbohydrates+protein) may be beneficial, not only for immediate energy made available (an increase in blood glucose), but for the psychological and physical benefits. [Most of us are useless when we are hungry.] *The suggested range for nutrient intake pre-exercise (for high intensity training or competition) is 1-2g/kg carbohydrate and 0.15-.25g/kg protein 3-4 hours before competition/exercise which would equate to at least 68g carbohydrates and 10-17g protein in a 150lb adult, which is essentially the same as a protein bar + piece of fruit, or a smoothie with a scoop of protein added, etc. TAKE NOTE: these foods should still be counted towards the total daily intake, as well as any nutrients taken in around a workout: they still count!

What about protein after a workout?

The most commonly heralded advice that coincides with resistance training is to take in protein (like a whey protein shake) immediately after training. (Schoenfeld, et al. 2017) This advice is in reference to the 'metabolic window' where it is suggested that within 45 minutes post-exercise, that the intake of amino acids (via a protein shake) can increase muscle protein synthesis (more-so than at other times of the day) as there is increased blood flow to the exercised muscles, and thus increased transportation of amino acids to these specific areas that require rebuilding and recovery from the muscle-damaging work of resistance training. (Schoenfeld, et al. 2017) However, this theory has been questioned where findings show that the so-called 'metabolic window' is not necessarily this tight window of time where post-exercise anabolism is most affected, but rather may be a much wider interval for nutrient intake, closer to hours post-exercise. (Schoenfeld, et al. 2017) The debate continues to be conflicted where some study findings show greater muscle protein synthesis when nutrients (protein, carbs and fat) are taken in closer to exercise (10min post-exercise) compared to several hours later (3 hours post-exercise), but other studies show that there IS still muscle protein synthesis associated with nutrient intake, regardless of the length of time post-exercise. (Schoenfeld, et al. 2017) One key variable that has been shown across studies is that when protein intake is controlled for, there is no significant difference in muscle protein synthesis associated with specific timing (1 hr post-exercise vs 2+ hr post-exercise) which can allude to the notion that it is overall protein intake that is more important for muscle protein synthesis rather than specific timing of a post-exercise protein shake. (Schoenfeld, et al. 2017)

One dietary component that is noteworthy when it comes to post-exercise nutrient timing is carbohydrate intake with protein around a workout. In this study, intake of 40g whey protein with 43g carbohydrate (glucose) and 7g creatine either immediately before or after a workout compared to early morning or late evening (for 10 weeks while following a resistance training program) showed significant increases in lean body mass and type II muscle fibers (fast twitch/explosive muscle fibers) compared to the delayed nutrient intake. (Schoenfeld, et al. 2017) However consumption of 42g protein and only 2g carbohydrate showed NO significant differences in lean body mass between before or after exercise versus delayed nutrient intake (early morning or late evening). (Schoenfeld, et al. 2017) This suggests that it is NOT specifically the timing of protein around a workout, but rather the combination of protein+carbohydrate around a workout that holds the most merit in increasing lean body mass. This difference in carbohydrate content being associated with lean body mass suggests a role in the nutrients acting together versus just protein alone AND further supports how adequate muscle glycogen can be protein-sparing (helps to eliminate protein degradation). Furthermore, when comparing intake of a protein shake (25g whey protein, 1g carbohydrate) either before or after resistance training, the effects were similar for body composition (like lean body mass, fat mass, etc) and strength, regardless of whether the protein was consumed before or after exercise. (Schoenfeld, et al. 2017)

But doesn't that contradict the theory behind the 'metabolic window'?

Yes, exactly!

In essence, these studies suggest that it may not matter much whether you are taking in a protein shake before or after a workout, as long as you are meeting your suggested requirements for protein and carbohydrate. However as stated above there is some merit behind taking in protein+carbohydrate around the workout. *Whether it is before or after is less important, but rather depends on the individual, length of time before last meal, time of day for exercise, etc.

Does this mean that protein shakes are not beneficial then?

Protein shakes (like whey protein) can absolutely be beneficial, particularly for the athletic population because:

  • With increased protein needs (athletes, physically active individuals, recovery/rehab/injury,etc) it becomes increasingly more difficult to obtain enough protein through dietary intake alone (based on difficulty of physically eating that much protein, costs, food preparation, etc).

  • Sometimes it is difficult to actually stomach eating food before or after a workout, and thus having liquid meals (like a protein shake) make for an easily absorbed source of nutrients without the gastric distress that can occur from digesting foods around exercise.

What should be made of more concern, particularly in athletes that train at glycogen-depleting capacities, is replenishing glycogen stores via carbohydrate intake post-exercise (or around exercise) as well as hydration status. (McArdle, et al. 2013) However, the process of refilling glycogen stores and rehydrating takes HOURS, so really the food choices made throughout the day are going to play a large role in overall recovery from the workout, i.e. NOT solely based what was ingested immediately around the workout.

What about fasted cardio for increasing fat loss?

The theory behind performing cardio (or any exercise really) while in a fasted state (like first thing in the morning) is that by not adding energy intake from food, that you can instead utilize your stored fuel (like fatty acids from adipose tissue) to foster your activity. Further support behind this theory is based on the idea of continuing a catabolic state that occurs during fasting. While fasting, serum glucose coming from food is low (hours after a meal) so stored fuel (from liver glycogen and fatty acids) are utilized to maintain blood glucose concentrations. It is proposed therefore that by inducing a greater energy need (like exercise) that you can further increase this catabolism (i.e. burn more fatty acids).

The big caveat to this theory however is the notion that cardiorespiratory exercise may help to release fatty acids from their stores (lipolysis) but this is NOT the same as oxidizing those fatty acids for energy. (Recall where oxygen is needed in high amounts to oxidize fatty acids, so with increasing intensity and duration of exercise, there is less oxygen available to do this.) In this case, you may be releasing stored fatty acids, but without adequately burning them, those circulating fatty acids will likely re-sequester back into storage. Furthermore, as mentioned above, as exercise intensity and duration increases, so does reliance on glycogen/glucose for fuel. (This is why is it proposed to be detrimental to exercise performance to train in a fasted state.) This is also where the practice of fasted 'steady-state cardio' gains popularity where it proposes that low to moderate intensity exercise (like walking, light jogging, stair-master, etc) allows you to continue to utilize fatty acids for fuel during exercise while still in that catabolic state (from fasting) which would theoretically lead to greater overall fat burning potential.

Sounds great, right?

Interestingly, a study that compared fasted vs fed steady state cardio on changes in body composition found NO significant difference between the two conditions. (Schoenfeld, et al. 2014) In this study, a sample of young healthy women were randomized to either a fed condition (where they received a meal replacement shake prior to exercise) or a fasted state (where they exercised while fasted and received the same shake after exercise). (Schoenfeld, et al. 2014) Both groups followed a hypocaloric diet and performed one hour of steady state cardio exercise three days a week during the four week study. (Schoenfled, et al. 2014) Both groups showed significant decreases in weight and fat mass from baseline, but with NO differences between groups. (Schoenfeld, et al. 2014) This study further supports the notion that the timing of food intake (having a shake before vs after exercise) bears little weight over body composition changes compared to the TOTAL DAILY INTAKE. In this study, ALL the women were following a hypocaloric diet (a 500 caloric deficit for each individual, with 1.8g/kg body weight for protein intake, 25-30% of calories from fat, and the remaining calories from carbohydrate) AND an exercise regimen. (Schoenfeld, et al. 2014) It should therefore come as NO surprise that both groups saw significant changes from baseline. The big takeaway here: the fasted steady state cardio, which is proposed to be superior for fat-burning potential, showed no differences in body composition than fed-state. [The meal replacement shake contained 250 calories, 40g carbohydrates, 20g protein, 0.5g fat]

Take from this what you will, but never stop learning.

References:

Kerksick C, Harvey T, Stout J, et al. International Society of Sports Nutrition position stand: Nutrient Timing. Journal of the International Society of Sports Nutrition. 2008; 5:17. https://www-ncbi-nlm-nih-gov.proxy.lib.fsu.edu/pmc/articles/PMC2575187/

McArdle W, Katch F, Katch V. Sports and Exercise Nutrition. 4th edition. Philadelphia, PA. Lippincott, Williams and Wilkins. 2013.

Schoenfeld B, Aragon A, Wilborn C, et al. Body composition changes associated with fasted versus non-fasted aerobic exercise. Journal of the International Society of Sports Nutrition. 2014; 11(1):54. https://www-ncbi-nlm-nih-gov.proxy.lib.fsu.edu/pmc/articles/PMC4242477/

Schoenfeld B, Aragon A, Wilborn C, et al. Pre- versus post-exercise protein intake has similar effects on muscular adaptations. PeerJ. 2017; https://www-ncbi-nlm-nih-gov.proxy.lib.fsu.edu/pmc/articles/PMC5214805/