Archives for category: Endurance

Hi there,

Training and competing pose challenges on the body with regards fuel stores, hydration and regulating body temperature. With a few exceptions, very few sports events put as much stress on the body as competing in a marathon. Here are some tips that will help you prepare and recover for your upcoming marathon.

Week Before

1- Rest and Sleep

While not necessarily a nutrition tip, it is vital to rest and sleep well in the week before your event. You have all the hard training done, although you might have a few light sessions planned. Getting at least 8 hours sleep every night will allow your body to recover and be fully prepared for the task ahead.

2- Carb Loading

We have two fuel sources that our body uses during exercise – fat and carbohydrate. At lower exercise intensities (resting, walking, jogging), our bodies prefer to use fat as we have large quantities stored in our bodies.

At higher exercise intensities (hard running), our bodies prefer to use carbohydrate stored in our muscles and liver as it can access it and release the energy more quickly. However, a problem with this is that we have the ability to store only small amounts of carbohydrate in our bodies (300-500g). If you’re trying to perform a personal best (i.e. exercising at a high intensity), there is a possibility of running out of carbohydrate (i.e. hitting the wall, bonking) later in the race. When this happens you will have to slow down, and you might feel dizzy, faint or disorientated.

In order to prevent this happening, you will have to carb-load. Carb-loading involves eating larger than normal amounts of carbohydrate in our to max out our bodies ability to store carbohydrate. This should allow you to run faster for longer.

The recommendations say that you should be eating between 8-12g per kg of body weight. As an example, for a 70kg person, this would mean eating between 560 – 840g of carbohydrate. This is very difficult to do eating a healthy diet.

In order to achieve this, you will need to cut back on eating protein, vegetables, fat and fibrous starchy carbohydrates (e.g. porridge oats, brown pasta and rice). Normally it is a beneficial to eat these types of foods, but they may stifle your appetite when carb-loading, making it more difficult to reach your carbohydrate targets.

You will need to eat more sugary types of carbohydrates (e.g. sugary breakfast cereals, fruit juices, jam, snack bars, sports drinks, smoothies, white flour based foods (pancakes)) and low fibre starches (e.g. white rice, white pasta, white bread).

Eating these types of foods allow you to over-eat and hit your targets. A word of warning – only do this when preparing for a marathon, when doing a long run (18+ miles) or practicing your race nutrition. If you do this on a regular basis, it will more than likely lead to you increasing your body fat.

Finally, I would recommend doing this 2 days before your race, giving your body time to digest and store carbohydrates and get rid of waste. You can then eat a normal diet the day before.

3- Hydration

When your body stores carbohydrates, it does so with water. If you’re eating and storing larger than normal amounts of carbohydrates, you are drawing extra water into your muscle and liver. As a result, you may need to increase your fluid intake. Keep a close eye on your hydration levels.

  • Thirst may increase.
  • Urine may be darker in colour.
  • You may only pass a small amount of urine.

If you experience any of these symptoms then drink a pint on water immediately and then drink 200ml every 20-30 mins until the symptoms above are improved. It is also best to drink with a meal or snack as your body will absorb fluid more efficiently.

Morning of and During Race

4- Breakfast

Marathons normally start early in the morning. Try and eat a breakfast 2-3 hours before the race start. This is highly individual but ensure you get some carbohydrates (e.g. porridge) and a small amount of protein (e.g. boiled egg) at a minimum. Continue hydrating (but not too much).

5- Eating during race

After the first hour of your race, you should be aiming to consume 30-60g carbohydrate per hour during the race to keep your blood sugar levels high, continuing to fuel your run. A typical sports gel will contain 30g of carbohydrate. So will 500ml of a typical sports drink. However, consuming these in large quantities can cause stomach ache, pain and diarrhea in some people. It would be a good idea to vary carbohydrate sources (e.g. ripe banana, energy bar, snack bars) to prevent these uncomfortable side effects.

6-  Hydration

How much you sweat during a race will determine how much fluid you drink. The environmental conditions and the course will play a role in this. You will sweat more in warm, humid condition and/or on a hilly course. Conversely, you will sweat less on a cold, windy, rainy day and/or a flatter course. Some level of dehydration is OK and may be beneficial as you will be lighter and will expend slightly less energy. This is typically up to 2% of body weight. Anything over this may impair performance. It is very difficult to give a recommendation of how much fluid you should drink, As well as the factors listed above, everyone has slightly different sweat rates. At a rough estimate, you should be drinking 400-500ml per hour.

Be careful of drinking too much fluids, as this could mean an early finish to your day!

After Race

7- Immediately after race

Within 0-60 mins of your race finish, you should try to consume a recovery shake containing carbohydrate and protein or if not available, 1-2 bottles of sports drinks. This will help to kick-start refuelling.

8- 1-3 hours after race

You should aim to eat a big meal 1-3 hours after a race. This should contain a large portion of carbohydrates, protein and vegetables. The carbohydrates will continue to refuel the body, protein will help to repair and rebuild muscle and vegetables will contain vitamins and minerals which will help your immune system.

9- 72 hours after race

Continue to eat breakfasts, lunches and dinners containing protein, carbohydrate and vegetables up to 72 hours after the race. One of the big complaints by novice marathon runners is DOMS (Delayed Onset Muscle Soreness). DOMS is muscle soreness which appears two days after the race. By eating regular doses of protein with your meals, this will help with repairing muscle and help to prevent DOMS or at a minimum diminish the muscle soreness.

10- Rehydration

If you have a portable scales then bring it with you to the race. Try and weight yourself immediately before and immediately after the race. You should have lost weight – if you have gained weight then this means you drank too much fluid.

If you lost weight, get the difference and multiply by 1.5. For example, if you lost 3kg then the amount of fluid you need to drink to fully re-hydrate is 4.5L (3 x 1.5). You don’t need to drink all of this immediately, but over the 24 hours after the race.

Word of warning – beer and alcohol don’t count towards rehydration. Drinking large quantities after a race will cause further dehydration and will delay recovery.

I would like to wish you all the best in your upcoming marathon. If you follow the steps above I think you will have an enjoyable race and hopefully hit a P.B.. If you have any questions, please email me at:

Yours in sport,


This is a post I wrote in 2012 for the now defunct Irish Triathlon website. I’ve decided to publish it here as it’s impossible to access. While my thinking has changed slightly over the years, this post could supply basic nutrition info. when preparing and recovering from a long cycle training bout.

Like most beginner cyclist, I made many mistakes at the start due to lack of knowledge. Early morning cycle without breakfast, straight to work and did not eat until lunch resulting in mid-afternoon fatigue. Or I went out on a long cycle, without refuelling properly from the previous day’s session. It would be a momentous struggle to complete the cycle, as tiredness kicked in and I had no food to take on board.

So as not to repeat these common mistakes, I have prepared a guide for your fuel intake for before, during and after your training session. This is based both on my painful personal experience and reinforced by my studies in Strength and Conditioning, Sports Nutrition and my personal experience with advising clients at metabolise

Firstly the some basic physiology; You need energy to fuel all activities and this fuel is provided by carbohydrate, fat and protein. We burn various combinations of carbohydrate, fat and protein, depending on the training intensity, training duration and fatigue level. In a relaxed state and low-medium intensity exercise, we normally burn fat more than carbohydrate. Fat is an abundant source of energy and we have enough fat on our bodies to fuel activity for days or even weeks. As the intensity of exercise increases, our fuel source switches from predominately fat to carbohydrate. Carbohydrate is an excellent fuel source, as it can release energy more quickly than fat. However, a major drawback of carbohydrate as a fuel source is that we have limited supplies in our bodies. At a decent cycling pace, we have enough carbohydrate to fuel 90 minutes of exercise, although this will vary with training status and diet.

If you are going on a 2 to 3 hour Cycle, then there is a real possibility that you might empty your body’s carbohydrate stores. At which point, the body switches to burning fat. The consequence of this is that you cannot maintain the same training intensity. It also leads to unpleasant physical consequences: marathon runners call it to “hitting the wall” and cyclists call it “the bonk”. Symptoms include tiredness, fatigue and lack of energy.

The good news is that all this can be easily avoided. By making the correct food choice , we can ensure that we start training with high carbohydrate stores, top them up while we cycle and refuel afterwards to prepare for our next training session.


You need to ensure that our body’s carbohydrate stores are full, by eating foods such as potatoes, rice, pasta and bread. As a rule, we should eat brown coloured carbohydrates (brown bread, rice, whole grain noodles and pasta) as opposed to white. Brown coloured carbohydrates are healthier for us in the long term and give a more sustained release of energy. Preparing for your training session should begin the day before. Eat a mixture of carbohydrates for breakfast (e.g. weetabix), lunch (e.g. baked potato) and dinner (e.g. pasta). Also, eat carbohydrate containing snacks (e.g. low-fat yoghurt’s) throughout the day. This ensures that you’re giving your body the fuel it needs and the time it needs to process and store it.

If you’re training early in the morning, you might not have time to eat a proper breakfast. In which case, eat a light snack or carbohydrate containing food before training – something that’s not too heavy or rich. If you’re training later in the day, continue to consume carbohydrates throughout the day. Eat your last meal 2-3 hours before your session.

During Training

Follow the above to make sure that you are prepared for training. However, depending on your training intensity, after an hour and a half, you might begin to feel tired. This is because you’ve burned much of your body’s carbohydrate stores and they’re now running low. Your blood sugar levels may also be dropping as your liver carbohydrate is used up. To prevent this, we need to start consuming carbohydrates in order to keep supplying the body with energy. This is easily achieved, but needs some forward planning.

You may experiment with different foods, but the most popular choices are sports drinks and sports gels. These foods are generally designed to ensure quick absorption of sugar into your bloodstream, fuelling exercise. You should be consuming 60-90g carbohydrate per hour. Gels are generally well tolerated but may have some unpleasant side-effects like bloating. Experiment with different gels and drinks to see what’s suitable for you. There are many cheap alternatives to Gels and sports drinks. Raisins and figs are two natural foods which were used by cyclists in years past when sports gels didn’t exist.

Post Training

So, you’ve completed your cycle, maintained a good intensity and are feeling rather pleased with yourself. Well, you’re not finished yet! The last thing you need to do is to refuel and repair your body so that all the potential improvements in fitness from your cycle can be realised. Firstly, you have a 45-60 minute window after finishing exercise when your body is better able to take up sugar and refuel. What I normally do is drink chocolate milk immediately when I get home. Chocolate milk contains carbohydrate for refuelling, protein for rebuilding the muscles, fluid for rehydration and various electrolytes and nutrients which are important for your health and fitness.

One to two hours after you finish, you can cook and eat a healthy nutritious meal. Although many people prefer pasta, you can eat nearly any carbohydrate containing meal, once it’s low in fat. This will continue the refuelling and rebuilding process and ensures that your fitness will increase. If you trained in the morning, continue to refuel throughout the day by eating carbohydrate containing meals and snacks, every 3-4 hours. Make sure to include protein (e.g. milk, eggs, fish, chicken) in your meals to help repair and rebuild muscle.

I hope you enjoyed this article. Hopefully you will now be properly prepared for your next cycle.

Another important factor for performance, especially in warm conditions, is hydration. Severe dehydration will have a negative impact on performance, causing you to fatigue and slow down. We can generally tolerate a certain level of dehydration (a 2-3% reduction in body weight), but anything greater than this will have a negative impact.


What is the optimal drinking strategy? There is a high level of individual variation in how much we sweat during exercise. We all differ based on training status, hydration status, ambient weather conditions, terrain, clothing etc. The best strategy is to measure your own individual sweat rate. This is relatively easy to do.  For your convenience, I have created a sweat rate and re-hydration calculator on my website at the following link:


To calculate your sweat rate, you will need to measure the following:


  • Weigh yourself (preferably nude) before training/competition.
  • Weigh yourself (preferably nude) after training/competition and after towelling down.
  • Measure the amount of fluid consumed.
  • Estimate any urination between before and after weigh in.
  • Duration of training session or competition.


With this information at hand, you can calculate your sweat rate.  Ideally, you will exercise at race pace for one hour (possibly shorter for swimming). This will allow you to estimate your average sweat loss. You can do this for the three different disciplines, as it is likely you will sweat at different rates depending on the type of activity you’re doing. Ideally, you would perform this test during  a warm-up race to make the figure as realistic as possible.


With this information at hand, you can then calculate your ideal rehydration rate. The calculator allows you to factor in an acceptable level of dehydration. This is set at 2%, which is the maximum accepted weight loss without a negative effect on performance.


If you want to take it to a professional level, determine your sweat rates in different environmental conditions (cold, moderate, hot weather) so that regardless of the weather on the day, you will know what your ideal hydration strategy is for those conditions.


One important consideration with hydration is hyponotraemia , which is a potentially serious medical condition. Hyponotraemia is  caused by consuming too much fluid, resulting in a change in the composition of blood plasma and potentially fatal knock on effects (e.g. swelling of the brain). This generally occurs for beginners or slow competitors who consume excess fluids (i.e. drink more fluid than they lose through sweating) during a triathlon. This is one reason why it is a good idea to measure your sweat rate and have a hydration plan in place. As well as keeping you hydrated, it will prevent you from developing hyponotraemia.


One last thing to mention is caffeine, which has the potential to enhance your performance.  Caffeine works by altering your perception of how hard you’re working. I can testify to the power of caffeine as it has helped me finish a long hard ride on more than one occasion.


Caffeine can be consumed in a number of ways – by taking caffeine tablets in measured doses, by eating a gel which contains caffeine or by drinking coffee or Coke. One point to be aware of is that if you are a habitual coffee drinker than you may not get this performance enhancing effect. The best way to ensure that you get a boost is to stop consuming coffee or caffeine containing products 4-5 days before your event. This “wash-out” period will allow you to get the best effect from caffeine when you take it during an event. As with all nutrition strategies, experiment in training to see if this is right for you.

D.I.Y. Energy Drinks for Triathlon

The two best ways to increase your carbohydrate intake are through fluids and food. Fluids have the advantage of hydrating you as well as providing energy. Many sports drinks and water soluble powders are commercially available which are ready off the shelf. However, I prefer to make my own drinks for the following reasons:

  • I can control the amount of carbohydrate, the taste and adjust to my preferences
  • It’s cheaper to make your own.

If you decide to make your own sports drink, you will need to buy maltodextrin or glucose powder and fructose powder. You can buy 1kg packets of maltodextrin (approx. €5) and fructose (approx. €8) online. This will be enough to last you several weeks.

To prepare your own sports drink, you will need the following ingredients:

  • 50g Maltodextrin
  • 25g Fructose
  • 1L water
  • 1g table salt
  • Flavouring – Miwadi/Robinsons

It is important to use the measurements outlined as this is the optimal blend. Too much maltodextrin/fructose may cause discomfort such as bloating, cramping and diarrhoea. Too little carbohydrate and you won’t be delivering the carbohydrate at the optimum rate. The combination of fructose and maltodextrin allows the maximal rate of absorption of carbohydrate compared to using either on its own. Another added benefit is that the addition of fructose allows greater water absorption, promoting greater hydration. Adding a small amount of salt is also important as the sodium in the salt allows greater absorption of water in the intestine.

Sports gels are another great way of increasing your carbohydrate intake during the cycle (and run leg) as they are small and easy to carry. As with sports drinks, you should be looking at the different types of carbohydrate contained in the gels. In general, fructose in combination with glucose, maltodextrin or galactose will allow the maximum rate of delivery of carbohydrate to your working muscles. The optimum ratio is two parts maltodextrin,glucose or galactose to one part fructose.

Not to be under-estimated is taste. When out on a long ride, I find that I look forward to eating my gels/bars as a treat. However, many gels are tasteless. I find that eating gels with a pleasant taste gives me an extra psychological lift! One brand that I can recommend is the TORQ brand of gels, which are very tasty and come in a variety of flavours. If sports drinks and gels are beyond your budget, foods such as ripe bananas, figs and raisins are also high in carbohydrate and might be a low cost alternative.


The maximal rate of which your body can absorb and use carbohydrates from drinks, gels and food is between 60-90g/hour. It is a good idea to experiment with different combinations of fluid, gels and foods during training and warm-up races to find the right combination for you. It is important to realise that we are all individuals and what works for your training partner might not work for you.


One common mistake by beginners is to eat too much on the bike leg. When completing the running leg, there is a lot of fluid and food sloshing around in the stomach. This can lead to gastro-intestinal discomfort, with feelings of bloating and nausea being common. Again, use your training to determine the optimal feeding strategy on the cycle leg, without compromising the run leg.


Because, there are many different types of triathlon events, and your training duration varies, the below table will help you to decide which is the best intake rate and type of carbohydrate to ingest.

Event Duration Carbohydrate Requirement Recommended Intake Carbohydrate Type Single Carbohydrate Multiple Types of Carbohydrate
< 30 min None None None None None
30-75 min Very Small Amounts Carbohydrate Mouth Rinse Most forms of carbohydrate OK OK
1 – 2 hr Small amounts Up to 30g/hr Most forms of carbohydrate OK OK
2 – 3 hr Moderate amounts Up to 60g/hr Ingestion of single carbohydrates (e.g. glucose, maltodextrin) OK, but not optimal OK
>2.5 hr Large amounts Up to 90g/hr Only multiple types of carbohydrates (e.g. maltodextrin and fructose) Not optimal OK

Taken from: Juenkendrup, A. (2011). Nutrition for Endurance Sports: Marathon, triathlon and Road Cycling. Journal of Sports Sciences, 29(S1), S91-S99.


Some interesting research that is just emerging is that your digestive system is trainable. By consuming carbohydrate during training, your body becomes better at absorbing and burning carbohydrate as you exercise. Therefore, train as you mean to compete and this includes incorporating race eating strategies into your training.

Hi there,

Last week I posted an assignment I wrote about high fat diets and endurance performance. The post was technical and may be understandable for sport scientists, but may not be easily understood by the lay person. So, in this post, I will try and explain the rationale and translate into practical guidelines.

Firstly, the protocol is that for 5-6 days prior to an endurance event (probably lasting longer than 1.5 hours – a marathon or half triathlon) you load up on fat in your diet. In practical terms, this equates to around 70% of calories from fat. Or, if you have the ability to weigh and monitor your food intake, this equates to 4.6g Fat/kg Body Weight/day. On the day before the event, we then change strategy and consume a high carbohydrate diet (70% calories or around 11g Carbohydrate/kg Body Weight/Day). This has the bonus of topping up our body’s carbohydrate (i.e. glycogen) stores.

The outcome of this strategy is that your body becomes more efficient and adept at burning fat. This is beneficial because we have huge amounts of energy stored as fat in our body, whereas we have limited amounts of carbohydrate. In endurance events of duration longer than 1.5 hours, the limited supplies of carbohydrate in our body may become an issue and may compromise performance. However, if we follow a high-fat diet, we tap into the bodies nearly unlimited supply of fat.

All well and good. But there is a caveat. By eating a high fat diet and turning up our bodies ability to burn fat as a source of energy, we compromise our body’s ability to burn carbohydrate. There is a trade off. Research has shown that high intensity performance is compromised in this scenario. Carbohydrate is a great source of energy at high exercise intensities, whereas fat isn’t. So, as a result, when we really start to push it in training or competition, our bodies ability to release energy from carbohydrate is dampened.

So, on a practical level, this is what I suggest. If finishing is the main outcome for you, then a high fat diet may help you to avoid the unpleasant consequences of “hitting the wall” or “the bonk”. You should be able to utilise your body’s store of fat to complete the event.

If, however, you have designs of winning or placing highly and you will really push yourself during the event, then I would suggest not following this strategy as you may not be able to give that high intensity spurt towards the end. In this case, I would advise some other strategies which will train your body to burn fat without compromising high intensity performance.

One of these strategies is the “train-low, compete high” scenario. In this, you train when your bodies carbohydrate levels are low. This trains your body to become more efficient at burning fat without compromising your ability to burn carbohydrate at high intensity.

This can be achieved in a number of ways:

  • Training first thing in the morning without having a breakfast.
  • Training twice a day. Eat very little carbohydrate between your first and second training session so that your carbohydrate stores are depleted.
  • On long training runs/cycles, do not consume any carbohydrate.

However, these are adavanced tactics. They should not be used by beginners and should only be used by advanced athletes sparingly (i.e. once a week).

Thanks for reading,

Kevin – Metabolise Sports Nutrition

Performance and Health Implications of High Fat Diets on Endurance Performance.

Athletes experiment with nutritional and training strategies to give themselves a competitive edge over rivals. Although high carbohydrate (H-CHO) diets are typically recommended for endurance athletes, some athletes have ignored this advice, consumed high fat (H-FAT) diets and have competed successful. Mark Allen and Jonas Colting are examples of two tri-athletes who have successfully followed H-FAT diets and won Iron Man and Ultra Man World Championships. Sparing limited glycogen reserves and increasing fat oxidation may be a viable strategy for improved endurance performance.

One of the first studies examining the effect of a high fat (H-FAT) diet on performance was undertaken by Phinney, Bistrian, Evans, Gervino & Blackburn (1983). Five endurance trained athletes consumed either a H-FAT (85% energy) or high carbohydrate (H-CHO) (66% energy) diet for four weeks. During a performance test after dieting, there was a decrease in the Respiratory Exchange Ratio (RER), a three-fold drop in glucose oxidation and a four-fold reduction in muscle glycogen use on the H-FAT diet, indicative of an increased rate of fat oxidation. Similar results were reported by other investigators (Goedecke, Christie, Wilson, Dennis, Noakes, Hopkins & Lambert, 1999; Helge, Watt, Richter, Rennie & Kiens, 2001).

Although H-FAT diets promote fat metabolism and glycogen sparing, one of the problems encountered by early investigators was that muscle glycogen was substantially reduced after a high fat diet due to low carbohydrate intake (Lambert, Speechly, Dennis & Noakes, 1994), which may negatively affect endurance performance (Bergström, Hermansen, Hultman, & Saltin, 1967). Therefore, later research utilised a dietary periodisation strategy, whereby glycogen loading would be undertaken after a period of H-FAT consumption, to replenish glycogen stores. Burke, Angus, Cox, Cummings, Febbraio, Gawthorn, Hawley, Minehan, Hargreaves & Hawley (2000) reported the effects a shorter adaptation period (5 days) with CHO restoration, comparing H-FAT (4g/kg/day) and H-CHO (9.6g/kg/day).  During 2 hours steady state exercise at 70% VO2 max, RER was reduced, fat oxidation increased and CHO oxidation decreased. Thus, even with CHO restoration and availability, exercising muscle preferentially oxidised fat as a fuel substrate.

Therefore, H-FAT diets promote fat oxidation and glycogen sparing during exercise. How might this work? Many of the cellular adaptations associated with H-FAT diets include increased enzymes involved in beta oxidation (Helge & Kiens, 1997), increased fatty acid transporters (Glatz, Luiken & Bonen, 2010) and increased mRNA concentrations of proteins involved in fatty acid transport and metabolism (Cameron-Smith, Burke, Angus, Tunstall, Cox, Bonen, Hawley & Hargreaves, 2003). Therefore, H-FAT diets up-regulate the metabolic machinery for the transport and oxidation of fats into the muscle cell and mitochondria.

How might H-FAT diets affect performance? Helge, Richter & Kiens (1996) divided subjects into H-CHO (65% energy) or H-FAT (62% energy) group for seven weeks followed by carbohydrate restoration in week 8. Subjects trained 3-4 times per week during the study period. After 7 weeks, time trial to exhaustion (TTE) at 81% of pre-training VO2 max increased from a mean of 35 mins to 102 mins in the H-CHO and 65 mins in H-FAT, with the improvement in performance significantly greater in the H-CHO group versus the H-FAT group. Even with a CHO restoration protocol in week 8, TTE improved slightly in the H-FAT group (77 mins) but was still significantly less than H-CHO TTE.

Other studies have demonstrated improvements in performance on H-FAT diets. Phinney et al. (1983) demonstrated that although there was no difference in time to exhaustion (TTE – cycle ergometer test at 62-64% of VO2 max) between the H-FAT and L-FAT diets, TTE increased by four minutes in H-FAT compared to baseline. However, closer inspection of the results revealed that one athlete had an abnormally large increase in TTE in week five while the other subjects either had no change or a decrease in performance.

Lambert et al. (1994) demonstrated an improvement in a cycle to exhaustion at 50% Peak Power Output (PPO) after a two week H-FAT (70% total energy) compared to H-CHO (74% total energy). There was no difference in cycle to exhaustion at 85% PPO. Carey, Staudacher, Cummings, Stepto, Nikolopoulos, Burke & Hawley (2001) examined the effects of H-CHO (11 g/kg/day  CHO, 1 g/kg/day FAT) or an isoenergetic high-fat diet (2.6 g/kg/day CHO, 4.6 g/kg/day FAT) diet for 6 days followed by CHO restoration on time trial performance. After cycling for 4 hours at 65% peak VO2 uptake, subjects on the H-FAT diet maintained an 11% non-significant (P=0.11) higher power output in a 1 hour time-trial task compared to H-CHO.

Therefore, the results from the effects of H-FAT diets on performance are equivocal. However, one observation is that performance in low and medium intensity exercise is enhanced. This might be expected, as substrate oxidation at these intensities is predominately fat (Van Loon, Greenhaff, Constantin-Teodosiu, Saris & Wagenmakers, 2001). If we look at exercising at higher intensities, a different picture emerges.

Stepto, Carey, Staudacher, Cummings, Burke & Hawley (2002) compared a three day H-FAT (4.6g/kg/day) or H-CHO (11g/kg/day) diet on high intensity interval exercise (8×5 min bouts at 86% VO2 peak) and reported higher Rate of Perceived Exertion (RPE) in the H-FAT versus H-CHO. Havemann, West, Goedecke, Macdonald, St Clair Gibson, Noakes & Lambert (2006) examined the effects of a six day H-FAT (68% energy) or H-CHO (68% energy) diet followed by carbohydrate restoration on a 100km TT interspersed with one kilometre sprints. This may be a more realistic scenario for elite performance compared to constant lower intensity performance trials. The one kilometre sprint power output was significantly lower in the H-FAT diet compared with the H-CHO diet.

Stellingwerff, Spriet, Watt, Kimber, Hargreaves, Hawley & Burke (2006) measured the effects of either a five day H-FAT (4.6g/kg/day) or H-CHO (10.3g/kg/day) diet, followed by carbohydrate restoration on 1 min sprint performance. Pyruvate Dehydrogenase activity was significantly reduced at rest and during low and high intensity exercise and estimated rates of glycogenolysis were reduced in H-FAT condition. This suggests that H-FAT diets work not through glycogen sparing but through glycogen impairment.

Intramuscular Triglycerides (IMTG) have been identified as an important fuel substrate during exercise, even at power outputs approaching 85% VO2 max in trained athletes. If undertaking high training volumes on consecutive days, athletes may need to consume higher than normal fat intakes (35-57% of energy) in order to replenish IMTG (Spriet & Gibala, 2004). Decombraz (2003) has suggested consuming a H-CHO diet in the initial 6-8 hours of recovery, with fat content increasing thereafter.

The World Health Organisation (2003) has recommended fat intakes of 15-35% of total energy intake for optimal health. Long-term H-FAT diets are associated with development of obesity, coronary heart disease and certain cancers (Manore, Meyer, & Thompson, 2009) and thus would not be recommended as a lifestyle choice. Acute fat intake results in transient suppression of muscle glucose uptake and muscle glycogen synthesis (Boden, 1997) and in the long-term may lead to insulin resistance, although this may be attenuated in endurance trained athletes (Goodpaster, He, Watkins & Kelley, 2001). However, training while fasting, during periods of hyper-caloric H-FAT intake, can improve whole body glucose tolerance and markers of insulin sensitivity (Van Proeyen, Szlufcik, Nielens, Pelgrim, Deldicque, Hesselink, Veldhoven, & Hespel, 2010). Training 3-4 times per week over seven weeks on a H-FAT (62% E) diet compared to a H-CHO (65% E) has been shown to reduce Natural Killer cell activity (Pedersen, Helge, Richter, Rohde & Kiens, 2000). This may compromise innate immunity and increase the risk of athlete infection. Therefore, there may be several adverse health consequences with consuming H-FAT diets.

From the research reviewed, it would be prudent to form the opinion that H-FAT diets, while increasing the ability to oxidise fat and spare glycogen at lower exercise intensities, reduces the ability to oxidise glucose at high intensities. Given that the outcome of all Olympic endurance events are decided at exercise intensities above 85% VO2 max (Joyner & Coyle, 2008), a compromised ability to oxidise glucose at high intensities would have negative consequences on elite performance. Long term H-FAT intake may compromise adaptations to training and may negatively affect health, although some of these ill effects may be attenuated by training. Reviews of the relevant literature do not recommend high fat diets to enhance performance (Hargreaves, Hawley & Jeukendrup, 2004) or training (Burke, Kiens & Ivy, 2004).

Using different nutrition practices (e.g. carbohydrate restriction before/during/after training – Hawley & Burke, 2010; Van Proyen, Szlufcik, Nielens, Ramaekers, & Hespel, 2011) or performing high-intensity interval training (Burgomaster, Howarth, Phillips, Rakobowchuk, MacDonald, McGee, & Gibala, 2008) may replicate many of the cellular and metabolic adaptations associated with H-FAT diets. High rates (1.75g/min) of exogenous carbohydrate oxidation can be achieved during exercise using multiple transportable carbohydrates (Jeukendrup, 2010), negating the need to protect glycogen stores. Short-term carbohydrate loading can increase muscle glycogen stores two-fold (Fairchild, Fletcher, Steele, Goodman, Dawson & Fournier, 2002). Strategies other than H-FAT diets are available to athletes to promote fat oxidation and boost glycogen stores without the need to compromise high intensity performance or health.


Bergström, J.,  Hermansen, L., Hultman, E., & Saltin, B. (1967). Diet, muscle glycogen and physical performance. Acta Physiologica Scandinavica, 7(2-3), 140-150.

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