Most endurance athletes significantly increase their consumption of carbohydrates in order to maintain levels of muscle glycogen. This leads to an increased proportion of carbohydrate as an energy source – carbohydrates typically make up ~55-65% of an endurance athletes total calorie intake – which can lead to a significantly reduced proportion of fat intake (<20%). When the proportion of fat intake is reduced (e.g. 20% or less of total calories) there may be a significant reduction in fat stores – particularly intramuscular stores – and this may compromise endurance exercise performance (Pendergast et al., 2000). Research suggests that the levels of intramuscular triglycerides (IMTG) – believed to be a major substrate during fat metabolism – are compromised when the proportion of fat consumed is reduced during a high carb diet (De Bock et al., 2005; Pendergast et al., 2000).
Intramuscular triglyceride stores and endurance exercise
Intramuscular triglyceride stores are sufficiently large to supply around 2,000-3,000 calories during exercise. They are believed to supply around 50% of total fat calories – so ~25% of total calories during moderate intensity exercise. Like muscle glycogen, stores of intramuscular triglycerides also deplete during prolonged exercise (De Bock et al., 2005; Pendergast et al., 2000; Staron et al., 1989; Hurley et al., 1986).
It’s known that prolonged endurance exercise (~2h or more) can cause intramuscular triglyceride stores to drop significantly (Van Loon et al., 2003; Hurley et al., 1986). These stores can remain low for a period of days after prolonged exercise. Not surprisingly researchers have found that a high carbohydrate/low fat diet (~62% carbohydrate and 24% fat) impaired intramuscular triglyceride replenishment when compared with a more normal fat proportion (~49% Carbohydrate and 39% Fat) (Van Loon et al., 2003;). In addition to increasing the rates of triglyceride replenishment a high fat diet may also increase muscle triglyceride stores (~50-75% in type I and type IIa fibres) and enhances the rate of the intramuscular triglyceride breakdown/metabolism – particularly in type IIa fibres (Van Proeyen et al., 2011).
Researchers have also found that a high carbohydrate intake can suppress fat metabolism and inhibit the breakdown of intramuscular triglycerides (Van Loon et al., 2003). In fact the researchers found that carbohydrate ingestion (before, during, or after exercise) inhibited the expression of gene (UCP3) that is believed to play an important role in fatty acid metabolism. Whilst the inhibiting the breakdown of intramuscular triglycerides is a good thing for endurance exercise performance, it may compromise the training effect of prolonged endurance training (e.g. prolonged exercise of >2hours where the aim is primarily to enhance fat metabolism). Interestingly, when subjects on a supervised high fat diet (∼50% carbs, 35% fat, & 15% protein) consumed a high carbohydrate breakfast (675 kcal ~70% carbs) ∼90 min before exercise there was no significant reduction in the breakdown of intramuscular triglycerides (Van Proeyen et al., 2011). This indicates that increasing the proportion of fats, to the level consumed in a more normal diet, may be beneficial by maintaining the rates of intramuscular triglyceride breakdown even in a carbohydrate fed state. Of further interest was that a diet containing just 50% carbohydrate was sufficient for the maintenance of muscle glycogen levels in subjects who exercise for ~5 hours/week at 70-85%HR max.
Therefore it appears to be important that endurance athletes –particularly those competing in events of ~1-2hours or more – consume sufficient dietary fats to increase base levels of intramuscular triglycerides, replenish intramuscular triglyceride stores following exercise, and possibly increase the rates of fatty acid metabolism.
The effects of fat intake on endurance exercise performance
Whilst there appears to be a training benefit to consuming a higher proportion of fat calories than the traditional high-carb low-fat proportion research is not clear as to the benefit of a high fat diet prior to endurance competition. Although its known that a high fat intake can increase the mobilization and metabolism of free fatty acids (Okana et al., 1996;) research has generally failed to find improved exercise performance following a high fat diet (Erlenbusch et al., 2005; Flemming et al., 2003). Most of the studies looking at the effect of high fat diet on exercise performance have generally utilised low proportions of carbohydrates which may have resulted in reduced muscle glycogen levels and therefore limited exercise performance. A more recent study looked at the effects of the consumption of a pre-exercise high fat meal and subsequent small proportion of carbohydrate jelly in a carbohydrate loaded state (Murakami et al., 2012). The researchers found that the combination of 3 days of carb loading combined with the pre-exercise high fat meal and carbohydrate jelly enhanced endurance running performance. It was concluded that a pre-exercise high fat meal and subsequent carbohydrate jelly was favourable nutritional approach for a marathon race provided the athlete is in a carbo-loaded state.
Ensuring you achieve an adequate fat intake
Most endurance athletes consume a high proportion of calories from carbohydrates (~60-65% of total calories) which may result in significantly reduced proportion of total calories from fat (<20% of total calories). Research suggests that in order to maximize intramuscular triglyceride (IMCL) stores, enhance IMCL replenishment rates, and IMCL breakdown/metabolism the proportion of calories from fat should be kept within the normal healthy range (~30-35% of total calories). Importantly, increasing the proportion of calories from fat does not appear to negatively affect glycogen replenishment providing that the calories from carbohydrate are kept above ~50% of total calories and the calorie intake is adequate to meet the daily energy requirements. A guideline would be to consume ~55% of calories from carbohydrates, ~30% from fats and ~15% from protein.
The table below gives guidelines for quantities of carbohydrate, protein and fat based on an intake of 55% carbs, 30% fat, and 15% protein.
|Total Daily Calories||55% from carbs||30% from fats||15% from protein|
|3,000||413g of carbohydrate||100g of fat||113g of protein|
|3,500||481g of carbohydrate||117g of fat||131g of protein|
|4,000||550g of carbohydrate||133g of fat||150g of protein|
|4,500||619g of carbohydrate||150g of fat||169g of protein|
|5,000||688g of carbohydrate||167g of fat||188g of protein|
One important factor to consider is that although the quantity of fat consumed may need to increase above that of a normal diet you should try to keep levels of saturated fat down to those of a normal healthy diet. It is generally recommended that a normal healthy diet should not contain more than 30g of saturated fat a day and this should remain the same for endurance athletes. To increase fat intake without significantly increasing the intake of saturated fats you may wish to include unprocessed vegetable oils such as extra virgin olive oils, cold pressed sunflower oils and olive oil spread instead of margarine. You may also wish to consider consuming 2-3 portions of oily fish per week, (mackerel , herring etc) as these contain the important essential fatty acids omega-3 which are vital for heart health, good circulation, mobility of joints, and may have positive effects on fat metabolism. Some other good sources of healthy fats include olives, mayonnaise, some types of nuts (almonds, walnuts, and brazil nuts), and seeds (sesame, sunflower, pumpkin). You may also wish to consider taking fish oil capsules (containing high concentrations of the important Omega-3 fatty acids EPA and DHA) to ensure you are getting an adequate supply of these important substances.
Summary of fat intake and endurance exercise
- The proportion of total fat calories is often lower amongst endurance athletes than non-endurance athletes due to the increased proportion of carbohydrates in the diet.
- In many cases the proportion of calories from fat may be as low as 20% or even less.
- Research suggests that this may compromise intramuscular fat stores and endurance exercise performance
- In particular levels of intramuscular triglycerides appear to be particularly compromised in endurance athletes with diets that have a low proportion of fat calories (~20% of total calories).
- Intramuscular triglycerides stores are sufficient to supply around 2,000-3,000 calories and appear to contribute to around 25% of total calories during moderate intensity exercise.
- Following prolonged endurance exercise levels of intramuscular triglycerides can drop significantly and appears that the rate of replenishment is impaired in diets with a low proportion of total fat calories.
- Increasing the proportion of fat calories can significantly increase intramuscular triglycerides stores, the rate of replenishment and enhance their breakdown/metabolism during exercise.
- Research has shown that a high carbohydrate diet can suppress fat metabolism as well as the breakdown of intramuscular triglycerides. This may compromise the training benefit of prolonged endurance training. Increasing the proportion of fats appears to counteract this even if you consume carbohydrates before or during exercise.
- There is some evidence that consuming a high fat meal prior to exercise may be of benefit for endurance performance, providing that you have full muscle and liver glycogen stores and also consume carbohydrates prior to exercise.
- Athletes looking to increase their proportion of fat intake to around 30% should ensure that they do not significantly increase the proportion of saturated fats.
- Good sources of healthy fats include oily fish (mackerel, herring etc), some nuts (almonds, walnuts), seeds (sunflower, sesame, pumpkin), vegetable oils (sunflower, olive oil), mayonnaise and olives.
Fat intake and Endurance Exercise References:
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Erlenbusch M, Haub M, Munoz K, MacConnie S, Stillwell B. (2005) Effect of high-fat or high-carbohydrate diets on endurance exercise: a meta-analysis. Int J Sport Nutr Exerc Metab. 2005 Feb;15(1):1-14.
Fleming J, Sharman MJ, Avery NG, Love DM, Gómez AL, Scheett TP, Kraemer WJ, Volek JS. (2003) Endurance capacity and high-intensity exercise performance responses to a high fat diet. Int J Sport Nutr Exerc Metab. 2003 Dec;13(4):466-78.
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Murakami I, Sakuragi T, Uemura H, Menda H, Shindo M, Tanaka H. (2012) Significant effect of a pre-exercise high-fat meal after a 3-day high-carbohydrate diet on endurance performance. Nutrients. 2012 Jul;4(7):625-37. doi: 10.3390/nu4070625. Epub 2012 Jun 27.
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Pendergast D. R., Leddy J.J., and Venkatraman J.T. (2000) A Perspective on Fat Intake in Athletes. J Am Coll Nutr June 2000 vol. 19 no. 3 345-350.
Staron RS, Hikida RS, Murray TF, Hagerman FC, Hagerman MT (1989) Lipid depletion and repletion in skeletal muscle following a marathon. J Neurol Sci 94: 29–40, 1989
van Loon LJ, Koopman R, Stegen JH, Wagenmakers AJ, Keizer HA & Saris WH (2003). Intramyocellular lipids form an important substrate source during moderate intensity exercise in endurance-trained males in a fasted state. J Physiol 553, 611–625.
Van Proeyen K, Szlufcik K, Nielens H, Deldicque L, Van Dyck R, Ramaekers M, Hespel P. (2011) High-fat diet overrules the effects of training on fiber-specific intramyocellular lipid utilization during exercise. J Appl Physiol. 2011 Jul;111(1):108-16. doi: 10.1152/japplphysiol.01459.2010. Epub 2011 May 5.