If the same total calories are consumed by an athlete, does it matter what time of day (timing) or how many meals they are spread across(frequency)?
When we ask this question for athletes we are really asking one (or both!) of two things:
· Does meal timing and frequency impact body composition?
· Does meal timing and frequency impact athletic performance?
These questions are particularly relevant given the current popularity of intermittent fasting; this is where individuals cycle between periods of eating and periods of fasting, either by restricting eating to a limited number of waking hours each day (e.g. an 8-hour time window) or by eating extremely low calories for a certain number of days each week.
In this post, we look at existing research around the impact of meal timing and frequency on athletic body composition and performance, and highlight some of the gaps that remain in our understanding. Before we begin it is worth highlighting that meal timing and meal frequency are distinct, although interrelated. Someone eating only once a day could eat in the morning, or the afternoon, or after training; the frequency is constant (once a day), however the timing is very different.
So, to begin …
Meal timing and frequency on body composition:
In sedentary populations research suggests that total calories consumed, rather than meal frequency and timing, has the significant impact on body composition (Sundfor et al 2018; Kerksick et al 2017; La Bounty et al 2011). In other words, if total calories consumed are equivalent, then body composition is equivalent whether someone fasts intermittently, or eats regularly across each waking day. This appears to be the case whether someone is following a diet that is hypocaloric (‘dieting’), eucaloric (steady state) and hypercaloric (‘overeating’) diets.
In contrast, meal frequency and timing does appear to impact body composition in athletic populations. More research is needed to understand this mechanistically and quantitatively, although we can hypothesise based on our knowledge of metabolism and the impact of training on the body. Most research on athletes has focussed on the impact of meal timing and frequency in hypocaloric diets. The findings to date demonstrate that regular feeding, rather than intermittent fasting, is beneficial to body composition i.e. promotes lower fat mass and higher lean body mass (Iowa et al 1996; La Bounty et al 2011). Indeed, one study demonstrated that in a population of athletes from different sporting disciplines, hours spent in calorie deficit was correlated with body fat percentage (Deutz et al 2000).
Consistent with studies looking specifically at the impact of protein feeding on athletic performance and recovery, it appears that the beneficial impact of continuous feeding compared to intermittent fasting is driven, at least in part, by the regular ingestion of protein (Jager et al 2017; Schoenfeld et al 2018). Protein cannot be stored in the body. If the amino acids from ingested protein are not used for building tissue (such as muscle) or there other functions in the body, they are converted to non-protein substances and stored or burnt for energy. This means that we need to take in protein regularly if we want a constant supply of the essential amino acids for net muscle protein synthesis (Phillips et al 2016; Jager et al 2017; Schoenfeld et al 2018; Morton et al 2018). Given that muscle protein synthesis is elevated for up to 24 hours post resistance training, it would appear particularly important for muscle growth, strength and training adaptations that the body is provided with a regular supply of complete protein across this period (Reidy and Rasmussen 2016). Indeed, studies have shown that eating above a threshold level of protein regularly (~every 4 hours during waking hours) in the period after training results in greater muscle mass accumulation and strength gains that eating it at lower doses and / or lower frequencies (Areta et al 2013; Jager et al 2017; Schoenfeld et al 2018). The ‘threshold’ level of protein will differ between individuals, dependent on their muscle mass, the volume and intensity of the training session, and the number of muscle groups trained, i.e. the number and size of the muscles stimulated for repair and growth.
Consistent with this, pre-sleep ingestion of 30-40g casein protein increases overnight muscle protein synthesis in an athletic population (Madzima et al 2014). Casein is slow digesting, as it curdles upon hitting the acid in our stomach (nice!!), helping explain why it has a persistent overnight effect. Interestingly, it was also shown to increase waking resting metabolic rate without impacting the rate of fat oxidation … in English this means that the rate at which calories were burned were increased, and despite the fact extra calories were taken in (in the form of the casein), stored body fat was oxidised at the same rate as it was if no casein was given (Madzima et al 2014). This finding is consistent with findings of other studies in both athletic and non-athletic populations (Jager et al 2017). This suggests a meal of casein before bed has a beneficial impact on body composition.
An emerging area of research, and one where the picture is not yet clear, is the relationship between cortisol levels and fat storage in athletic populations, and the differences between men and women in this regard. If cortisol (stress) levels are elevated for an extended period of time, fat mass accumulation is promoted, particularly in women. We can hypothesise this is an evolutionary response, designed to extend survival in times of limited resources by conserving and storing energy (fat) in case the famine continues for an extended period. Cortisol levels rise when we are in a calorie deficit, or a so-called ‘starvation’ mode. Cortisol levels are also high initially after we wake each morning. Eating can help bring down cortisol levels, in particular eating carboydrates, due in part to interactions between insulin released into our blood after feeding and cortisol. As such, we can hypothesise that it would be beneficial to body composition to eat soon after waking. At least in females there is some evidence this is the case (Sims 2016). Further research is needed to confirm this, and whether it is similar in men.
As an aside, it is perhaps worthwhile noting that increasing meal frequency has been shown to have a positive impact on blood markers (LDL cholesterol, total cholesterol and insulin), which may have longer term health and body composition benefits – in particular in the case of insulin which promotes fat storage when present at high concentration in the blood (La Bounty 2011).
However, despite all of the above, if restricting the number of hours that they eat in is the only an individual can control not overeating, then it may be still beneficial to do so for body composition.
Meal timing and frequency on athletic performance:
Considering athletic performance, the point made in the section above around meal frequency and muscle mass / strength accumulation is obviously relevant over the long term. This section focusses more specifically on the acute impact of meal frequency and timing on performance in any one day.
High intensity aerobic and anaerobic activity uses carbohydrate as the primary fuel. This is because it is the only fuel that can be ‘burned’ fast enough to provide the energy for high intensity exercise. Fat oxidation, the primary alternative to carbohydrates, occurs too slowly for this; fats are an effective fuel for lower intensity exercise when we can tolerate a lower rate of energy release.
To perform high intensity exercise for an extended period, the body therefore needs sufficient carbohydrate stores to fuel this. The carbohydrate stores in the body is the glycogen stored in the muscle and liver. A well-trained individual eating sufficient carbohydrates can store sufficient glycogen to power up to around 2 hours of high intensity activity (Kerksick et al 2017). The glycogen stores in the liver are used to maintain blood sugar levels for normal activity, not just in exercise. As such, if an individual has not eaten carbohydrate for many hours before exercise they will have used some of their stored glycogen to maintain blood sugar levels in this ‘fasted’ period. This means there will be less glycogen remaining to fuel high intensity exercise.
As a result, consuming a high carbohydrate (up to 2.5g/kg bodyweight, depending how fasted the individual is prior to this point) and moderate protein meal 1-4 hours prior to training is recommended for the consumed food to be digested and converted to glycogen in the liver and muscles (Williams and Rollol 2015; Kerksick et al 2017; see also International Sporting Federation Position Statements). The moderate protein helps the steady release of energy from the carbohydrates as well as helping support muscle protein synthesis when resistance training is involved. To replenish glycogen stores rapidly after a high intensity training session has depleted them, a high carbohydrate and high protein meal is recommended within 1 hour of training; recommendation are up to 1.2g/kg bodyweight carbohydrate and 0.25g/kg bodyweight protein, depending on the intensity and volume of exercise performed and muscle mass of the individual, and therefore extent of glycogen depletion (Jager et al 2017; Kerksick et al 2017; see also International Sporting Federation Position Statements). The addition of protein supports rapid glycogen replenishment, as well as the longer-term muscle protein synthesis. This should be repeated within 4 hours of training. In theory, such an approach of pre- and post- training feeding could still support an intermittent fasting approach to diet, as eating could be concentrated at these times.
At this point, we should also mention the ketogenic diet. This diet mimics fasting in many ways as it removes carbohydrates and therefore the ability to maintain blood glucose levels. Despite what is often stated by proponents of the ketogenic diet, studies performed to date have shown that the ketogenic diet does not enhance endurance performance, and impairs muscle glycogenolysis and energy flux, thereby limiting high intensity energy production (Hawley and Leckey 2015). Proponents argue that the body adapts to better use fat to fuel exercise. Whilst at submaximal exercise capacity a high fat and low carbohydrate diet plus endurance training supports increased effectiveness of using fat as fuel, maximal exercise capacity is inhibited – as this always relies on carbohydrate as fuel (Hawley and Leckey 2015). Based on this, low intensity exercise can be sustained in a fasted state and thus for this meal frequency and timing around training is of lesser importance.
Further research is needed to understand the mechanisms and nuances in different athletic populations. Based on the research performed to date the evidence suggests that continuous feeding rather than intermittent fasting is typically more beneficial athletic performance and body composition – particularly those athletes who are undertaking resistance training such as CrossFit and weightlifting, or who are eating a hypocaloric diet (as a result of the impact of regular protein ingestion on the capacity for net muscle protein synthesis). For those individuals focussed on high intensity cardio based activities and who are not eating a hypocaloric diet, meal frequency (i.e. continuous versus intermittent) may be less important, if the carbohydrate meal timing is carefully planned.
Areta, JL, Burke, LM, Ross, ML, Camera, DM, West, DWD, Broad, EM, Jeacocke, NA, Moore, DR, Stellingwerff, T, Phillips, SM, Hawley, JA, Coffey, VG. Timing and distribution of protein ingestion during prolonged recovery from resistance exercise alters myofibrillar protein synthesis. J. Physiol. 591:2319-2331. 2013
Deutz, RC, Benardot, D, Martin, DE, Cody, MM. Relationship between energy deficits and body composition in elite female gymnasts and runners. Med. Sci. Sports Exerc. 32(3):659-668. 2000.
Hawley, JA, Leckey, JJ. Carbohydrate dependence during prolonged, intense endurance exercise. Sports Med. 45:S5-12. 2015.
Iwao, S, Mori, K, Sato, Y. Effects of meal frequency on body composition during weight control in boxers. Scan. J. Med. Sci. Sports 6(5):265-272. 1996.
Jager, R, Kerksick, CM, Campbell, BI, Cribb, PJ, Wells, SD, Skwiat, TM, Pupura, M, Ziegenfuss, TN, Ferrando, AA, Arent, SM, Smith-Ryan, AE, Stout, JR, Arciero, PJ, Ormsbee, MJ, Taylor, LW, Wilborn, CD, Kalman, DS, Kreider, RB, Willoughby, DS, Hoffman, JR, Kryzkowski, JL, Antonio, J. International Society of Sports Nutrition position stand: protein and exercise. J. Int. Soc. Sports Nut. 14:20. 2017.
Kerksick, CM, Arent S, Schoenfeld, BJ, Stout, JR, Campbell, B, Wilborn, CD, Taylor, L, Kalman, D, Smith-Ryan, AE, Kreider, RB, Willoughby, D, Arciero, PJ, VanDusseldorp, TA, Ormsbee, MJ, Wildman, R, Greenwood, M, Ziegenfuss, TN, Aragon, AA, Antonio, J. ISSN exercise and Sports Nutrition Review Update: research and recommendations. J. Int. Soc. Sports Nut. 14:33. 2017.
La Bounty, PM, Campbell, BI, Wilson, J, Galvan, E, Berardi, J, Kleiner, SM, Kreider, RB, Stout, JR, Ziegenfuss, T, Spano, M, Smith, A, Antonio J. International Society of Sports Nutrition position stand: meal frequency. J. Int. Soc. Sports Nut. 8:4. 2011.
Madzima, TA, Panton, LB, Fretti, SK, Kinsey, AW, Ormsbee, MJ. Night-time consumption of protein or carbohydrate results in increased morning resting energy expenditure in active college-aged men. Br. J. Nutr. 111:71–7. 2014.
Morton, RW, Murphy, KT, McKellar, SR, Schoenfeld, BJ, Henselmans, M, Helms, E, Aragon, AA, Devries, MC, Banfield, L, Krieger, JW, Phillips, SM. A systematice review, meta-analysis and metaregression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. J. Sports Med. 52:376-384. 2018.
Phillips, SM, Chevalier, S, Leidy, HJ. Protein “requirements” beyond the RDA: implications for optimising health. Appl. Physiol. Nutr. Metab. 41:565-572. 2016.
Reid, PT, Rasmussen, BB. Role of Ingested Amino Acids and Protein in the Promotion of Resistance Exercise–Induced Muscle Protein Anabolism. J. Nutrition. 146(2):155-183. 2016.
Schoenfeld, BJ, Aragon, AA. How much protein can the body use in a single meal for muscle-building? Implications for daily protein distribution. J. Int. Soc. Sports Nut. 15:10-15. 2018.
Sims, ST. Roar. Rodale Books. 2016
Sundfor, TM, Svendsen, M, Tonstad, S. Effect of intermittent versus continuous energy restriction on weight loss, maintenance and cardiometabolic risk: A randomized 1-year trial. Nutr. Metab. Cardiovasc. Dis. 28(7): 698-706. 2018.
Williams, C, Rollo, I. Carbohydrate Nutrition and Team Sport Performance. Sports Med. 45:S13-22. 2015.