The key questions are:
By Dr. Sinead Roberts, PhD, www.feedfuelperform.com
Does meal timing and frequency impact body composition?
Does meal timing and frequency impact athletic performance?
Meal timing and meal frequency are distinct, although interrelated, topics. Using an extreme example to demonstrate the distinction: someone eating only once a day could eat in the morning, or the afternoon, or after training. In other words, the frequency can be constant (once a day), but timing very different.
In this summary, I focus principally on meal frequency in the context of intermittent fasting (eating across a limited number of waking hours per day) versus regular feeding through waking hours. After addressing this in each section I comment more specifically on timing of meals, particularly around training.
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 (Kerksick et al 2017; La Bounty et al 2011). In other words, if total calories consumed are equivalent, intermittent fasting and continuous feeding result in equivalent body composition. This has been shown for hypocaloric (‘dieting’), eucaloric (steady state) and hypercaloric (‘overeating’) diets.
Meal frequency and timing does appear to have an impact on body composition in athletic populations, although more research is needed. The research that has been done on athletes has focussed on the impact of meal timing and frequency when athletes are eating a hypocaloric diet, i.e. when eating reduced calories to lose weight. In these conditions the evidence suggests 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; ae Bounty et al 2011). Indeed, one study demonstrated that in a population of athletes from different sporting disciplines, the more time spent in calorie deficit in a day, the higher the relative body fat percentage (Deutz et al 2000).
It appears it is the regular eating of sufficient protein to maintain muscle protein synthesis that is, at least in part, responsible for the beneficial impact of continuous feeding compared to intermittent fasting in athletic populations (Jager et al 2017; Schoenfeld et al 2018). Protein cannot be stored in the body – if it is not used for building tissue (such as muscle) when consumed, it is 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 is ultimately dependent on the individual, their muscle mass and number of muscles trained and therefore stimulated for repair and growth.
It is also noted 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 benefits – in particular in the case of insulin which promotes fat storage when present at high concentration in the blood (La Bounty 2011).
We do also know that if cortisol (stress) levels are elevated for an extended period of time, fat mass accumulation is promoted (due to interactions with other hormones, including insulin). We also know that cortisol levels are high upon waking, and that feeding brings down cortisol levels (due to interaction with insulin). There is evidence that it is beneficial to body composition and energy to eat soon after waking, at least in females (due to the additional interactions with oestrogen and progesterone) (Sims 2016). Further research is needed to confirm this, and whether it is similar in men.
Having said 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 used to power lower intensity exercise when carbohydrate is not available.
As such, to perform high intensity exercise for an extended period, the body needs sufficient carbohydrate stores to provide the fuel for 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 in order 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 in order 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 actually impairs muscle glycogenolysis and energy flux, thereby limiting high intensity energy production. 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.
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