Fat Intake For Endurance Athletes

Endurance athletes often employ a high-carbohydrate diet in order to enhance exercise performance, increase recovery, and maintain levels of muscle glycogen. This leads to an increased proportional ingestion of carbohydrate as an energy source — carbohydrates typically make up ~55-65% of an endurance athlete’s total calorie intake — and a reduced proportion of fat intake (<20%).

In this article we look at how a high carbohydrate intake, coupled with a reduced proportional intake of dietary fat, may compromise endurance training. And how maintaining a normal proportion of fat calories can alleviate this.

Here, we focus on the effects of diet and intramuscular triglyceride (intramuscular fat) stores.

Body Fat stores and endurance exercise

Compared with our glycogen stores, we have substantially larger stores of body fat (triglycerides).

We store this mainly within our adipose tissue (body fat stored under our skin and around essential organs). But we also store this as essential fats (omega-3, 6, etc) within our brain, nerves, bone marrow, and within our organ membranes and cell membranes. And then there’re intramuscular triglycerides — fat stored in droplets within our muscle fibres.

These provide a store of lipid stored within the cytoplasm, where it is readily available for aerobic metabolism within the mitochondria. Importantly, we use intramuscular triglycerides at higher rates than triglycerides released from adipose tissue.

Maintaining these stores is essential for endurance athletes.

One factor here is that when we reduce the proportion of fats in our diet — for instance, a high-carbohydrate diet may reduce this to 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). In particular, endurance athletes may find that a high-carbohydrate, low-fat diet reduces the levels of intramuscular triglycerides (De Bock et al., 2005; Pendergast et al., 2000).

Intramuscular triglyceride (Fat) stores and endurance exercise

Intramuscular triglyceride stores are significant and can supply around 2,000-3,000 calories during exercise. In contrast, most athletes have glycogen stores that can supply between 1,200-2,000 calories — depending on factors such as body size, training history, carbohydrate intake.

As mentioned, intramuscular fat stores are located within your muscle fibres, with larger deposits found within type I (slow oxidative muscle fibres). This is not surprising, since fat metabolism occurs primarily within the mitochondria of slow twitch muscle fibres.

We believe intramuscular triglycerides to supply around 50% of total fat calories during moderate intensity exercise — so ~25% of total calories.

We also know that endurance training can lead to increased intramuscular fat stores. It also leads to increased fat utilisation during endurance exercise. Interestingly, we also achieve increased intramuscular breakdown and utilisation following resistance (Shepherd et al., 2014) and sprint training (Shepherd et al., 2013).

Like muscle glycogen, the stores of intramuscular triglycerides will deplete following endurance exercise (De Bock et al., 2005; Pendergast et al., 2000; Staron et al., 1989; Hurley et al., 1986). And prolonged endurance exercise (~2h or more) will cause our intramuscular triglyceride stores to drop significantly (Van Loon et al., 2003; Hurley et al., 1986).

We compound this when completing multiple days of prolonged exercise. And once low, intramuscular stores can remain low for several days.

High-Carbohydrate diets may impair replenishment of Intramuscular fats

Researchers have observed that high carbohydrate/low-fat diet (~62% carbohydrate and 24% fat), can impair the rate of intramuscular triglyceride replenishment when compared with a more normal carbohydrate/fat proportion (~49% Carbohydrate and 39% Fat) (Van Loon et al., 2003;).

We can counteract this by maintaining a fat intake that’s within the “normal” range for a healthy adult.

By following this approach, we help to maintain the rate of intramuscular fat replenishment. Besides this a high-fat diet may also increases muscle triglyceride stores in type I and type IIa fibres (~50-75%) and enhance the rate of intramuscular triglyceride breakdown/metabolism — particularly in type IIa fibres (Van Proeyen et al., 2011).

High Carbohydrate Availability Suppresses Fat Breakdown and Metabolism

Researchers have also discovered that a high carbohydrate intake can suppress fat metabolism and inhibit the breakdown of intramuscular triglycerides (Van Loon et al., 2003). Here, the researchers observed that carbohydrate ingestion (before, during, or after exercise) inhibited the expression of gene (UCP3) which we believed to play an important role in fatty acid metabolism.

Importantly, this 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 minutes before exercise there was no significant reduction in the breakdown of intramuscular triglycerides (Van Proeyen et al., 2011). Suggesting, that high carbohydrate availability may have suppressed the mechanisms involved in triglyceride breakdown.

Maintaining Sufficient Fat Intake is Important

This shows that increasing the proportion of dietary fats — to match the proportion consumed in a traditional diet — may be beneficial for endurance, by maintaining the rates of intramuscular triglyceride breakdown, even in a high 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. This will help to:

  • Increase base levels of intramuscular triglycerides.
  • Replenish intramuscular triglyceride stores following exercise.
  • May help to increase the rates of fatty acid metabolism.

Fat intake and 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 it’s known that a high fat intake can increase the mobilisation and metabolism of free fatty acids (Okana et al., 1996;) research has failed to find improved exercise performance (Erlenbusch et al., 2005; Flemming et al., 2003).

Most of the studies looking at the effect of high-fat diet on exercise performance have utilised low proportions of carbohydrates. This may have resulted in reduced muscle glycogen levels and therefore limited exercise performance.

In one study, researchers 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 carbo loading, combined with the pre-exercise high fat meal and carbohydrate jelly, enhanced endurance running performance.

They concluded that this was a favourable nutritional approach for a marathon race, provided the athlete is in a carbo-loaded state.

Achieving Adequate Fat Intake for Endurance Athletes

Most endurance athletes consume a high proportion of calories from carbohydrates (~60-65% of total calories) which may reduce the proportion of total calories from fat (<20% of total calories).

Research suggests that in order to maximise intramuscular triglyceride (IMCL) stores, enhance IMCL replenishment rates, and IMCL breakdown/metabolism, we should keep the proportion of calories from fat within the normal healthy range (~30-35% of total calories).

Importantly, increasing the proportion of calories from fat does not appear to affect glycogen replenishment — providing that we keep the calories from carbohydrate 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 some guidelines for quantities of carbohydrate, protein and fat based on an intake of 55% carbs, 30% fat, and 15% protein.

Total Daily Calories55% from carbs30% from fats15% from protein
3,000413g of carbohydrate100g of fat113g of protein
3,500481g of carbohydrate117g of fat131g of protein
4,000550g of carbohydrate133g of fat150g of protein
4,500619g of carbohydrate150g of fat169g of protein
5,000688g of carbohydrate167g of fat188g of protein

Moderate The Intake of Saturated Fats

One important factor to consider here is although the quantity of fat consumed may need to increase above that of a normal diet, keep the amount of saturated fat within those of a normal healthy diet.

It’s recommended that a normal healthy diet should not contain more than 30g of saturated fat per day. Ideally, 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 healthy options such as:

  • Unprocessed vegetable oils such as extra virgin olive oils.
  • Cold pressed sunflower oils and olive oil spread instead of margarine.
  • Aim to consume 2-3 portions of oily fish per week, (mackerel, herring, sardines, 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 excellent 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.


  • The proportion of total fat calories is often lower amongst endurance athletes than non-endurance athletes, because of the increased proportion of carbohydrates in their diet.
  • Here the proportion of calories from fat may be as low as 20% or even less.
  • Research suggests that a low proportion of dietary fat may compromise intramuscular fat stores and endurance exercise performance
  • Intramuscular triglycerides stores can supply around 2,000-3,000 calories and appear to contribute around 25% of total calories during moderate intensity exercise.
  • Following prolonged endurance exercise, levels of intramuscular triglycerides can drop significantly.
  • When our diets supply a low proportion of total fat calories, this may impair the rate of triglyceride replenishment.
  • 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 and 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, should ensure that they do not significantly increase the proportion of saturated fats.
  • Excellent 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.


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