High Intensity Training (HIT)

New study shows fat to be an effective fuel for high-intensity training.

An international team of sports scientists has demonstrated that replacing carbohydrates with fat does not impair high-intensity exercise performance.

The researchers found that restricting daily carbohydrate intake to less than 50 grams, and replacing the majority of those reduced calories with fat, did not reduce the ability to perform high-intensity interval training (HIIT) compared to a control group whose members consumed a typical high-carbohydrate diet.

While previous studies have demonstrated impaired performance when consuming inadequate carbohydrate, the new study allowed participants time to adapt their metabolisms to the very low-carbohydrate diet over a four-week period.

The 18 study subjects were recreational athletes between the ages of 18 and 30. Those consuming the lower intake of carbohydrates were shown to be able to use much more fat and ketones as an alternative energy source.

What may be the first study to examine the effects of long-term reductions in dietary carbohydrate intake on HIIT performance, the four-week study also evaluated cardiorespiratory and metabolic responses in all subjects and found no impairments.

The results challenge the presumed necessity of a high-carbohydrate diet for high-intensity exercise performance. Most athletes, coaches and scientists presume that reduced dietary carbohydrate intake will severely impair these types of workouts, considered as vital components of optimal performance for most sports and general fitness.

However, researchers found no evidence of performance compromise during the HIIT workouts.

The control group consumed a diet that was about 48 percent carbohydrate, 17 percent protein, and 35 percent fat, while the study group followed a very low-carbohydrate diet of about 8 percent carbohydrate, 29 percent protein, and 63 percent fat for the four-week test period.

An important finding was the significant increase in fat oxidation — the increased use of stored body fat for energy. In particular, there was a progressive increase in fat oxidation over the repeated HIIT workouts, which supports the importance of aerobic metabolism for such training.

Traditionally, dietary guidelines have recommended daily carbohydrate intake of 6-10 grams per kilogram of body weight for athletes performing one to three hours of daily moderate- to high-intensity exercise.

The study is titled, “Effects of A 4-Week Very Low-Carbohydrate Diet on High-Intensity Interval Training Responses,” and published this month in the Journal of Sports Science and Medicine. It was performed at the Human Motion Diagnostic Centre & Department of Human Movement Studies, Ostrava University, Czech Republic, and led by Lukas Cipryan. His collaborators included Daniel Plews, Alessandro Ferretti, Philip Maffetone and Paul Laursen.

While traditional nutritional advice is to load up on carbohydrates, science continues to prove that fat is a powerful and healthful fuel alternative for human performance.

To read the full text of the study in the Journal of Sports Science and Medicine, click here.


  • Paul Smith says:

    I’m 53 y.o. male with 2 stents in my artery after massive heart attack 4 years ago. I’m on statins for life. What is your opinion for my case for high fat or keto diet in order to loose weight and gain muscles and stamina?

  • Tyler says:

    Hi – I read Alex Hutchinson’s, “Endure” not too long ago. In his section on Fuel, he calls out a statistic on runner’s currently holding the top 100 times in the marathon. I don’t remember the exact stat off the top of my head, but the majority of these runners prescribe to a carb heavy diet. Do you attribute this to either 1) Slow acceptance to the new fat for fuel method over the last 5 years or 2) Maybe running a marathon for this group does rely on the anaerobic system more, therefore consuming more carbs would account for that? Alex Hutchinson seemed to believe a high Fat diet is best for strictly aerobic exercise whereas there are drawbacks during anaerobic efforts.

    I am just spit-balling ideas as I become more comfortable with the ideas and continue to learn more. Thanks!

  • Naveen Ashik says:

    I have been hearing of this LCHF diet scheme for long. I am a fitness enthusiast into long distance running for almost a decade now. The nutrition has picked up a lot of steam these days and I understand the significance where people want to achieve more fitness levels and do well in the amateur sports arena.
    But I have one concern…. In India we were primarily dependent on rice, wheat and lentils for serving our carb/protein requirements and fat via dairy products mainly and nuts/seeds to some extent.
    So if I move from say a 70-20-10(Carb-Protein-Fat) diet to say a LCHF what is %age of daily fat consumption needed?
    Secondly I have major concern on cost bearing on my pocket, how do I really fill my tummy just eating fat based food as it is highly expensive. Just for example 1KG of inorganic Ghee(Melted cow butter) costs around 500-600 bucks as compared to whole grain of rice or wheat or millets which cost less than 100 bucks. If I put nuts or seeds into equation cost is still higher say nuts costs me around 1000 to 3000 bucks a kilogram based on variety Almond, Cashews and Walnuts.
    Ofcourse I add good amount of fruits and veggies to my diet which helps to maintain fibre, vitamin and mineral requirement.
    How do you suggest especially for Asians and in particular country like India for a decent middle class family to meet the LCHF requirements?

    • Hi Naveen –

      Unfortunately I don’t have a good answer for you at the moment. You are asking a very important but very complex question that has lots of ins and outs based on the availability of specific foods in India. The best I can offer is that Dr. Maffetone recently did an article with several co-researchers from India, so I’ve reached out through Dr. Maffetone to see what their best solutions would be.

  • John Engel says:

    For someone who is rebuilding three MAF capacity, what specific parameters should direct how to start and ramp up interval training for the 20% of exercise per week that is above MAF level?

    • Hi John –

      That’s a question with no easy answer. Typically, what I would suggest is to train 100% at the MAF HR until you’ve shown at least 3 continuous months of aerobic improvement (in your speed). Then, I’d start incorporating some strength and intervals, 10 minutes more every week. However, note that elite athletes in some events such as the marathon never get above 5-10% of above-MAF training in their routines (to train for the Olympic Marathon a few years ago, Galen Rupp was doing nearly 100% aerobic training).

  • Han-Lin says:

    What if both groups have the same protein intake? I think it would be more controlled if the low carb group’s caloric ratio was something like 8:15:77.

    For people with kidney problems, it’s important to limit their protein intake. There’s also other reasons why we shouldn’t eat too much protein.

    Animal-Protein Intake Is Associated with Insulin Resistance in Adventist Health Study 2 (AHS-2) Calibration Substudy Participants: A Cross-Sectional Analysis

    Low Protein Intake Is Associated with a Major Reduction in IGF-1, Cancer, and Overall Mortality in the 65 and Younger but Not Older Population

  • A ray of sunshine says:

    Lchf diet can change the ratio of energy supply, but burning more fat requires an additional 15% oxygen. Does that mean that MAF speed will slow down?

  • A ray of sunshine says:

    The decomposition of sugar and fat, the consumption of fat requires more oxygen participation, if the sugar supply can be reduced, a large increase in fat supply, oxygen consumption increases, does it mean that the same VO2max speed will decline?

    • Hi again,

      Yes in theory, but in practice it never works that way.

      Fat-burning does require more oxygen participation for the same amount of energy than sugar. However, the fact that sugar can be burned anaerobically (but fats cannot) means that increased sugar availability means a preference for anaerobic energy production, which over the long-term means a reduction (or a lack of development) of aerobic capacity. Similarly, oxygen scarcity suppresses fat metabolism and increases sugar metabolism. Check out this article for a good explanation:http://circ.ahajournals.org/content/98/13/1350a

      While theoretically less sugar availability means a lower V02max speed (if all other things were equal), things are very unequal between fat and sugar: fat produces much less CO2 per ATP, fat is a functionally limitless resource, and because fat can only be burned aerobically, fat-burning, not sugar-burning, drives aerobic development (and major increases in VO2 max). In other words, a body that is highly tuned to burning fats, which burns more fats at higher speeds, will always be faster, even in contexts such as HIT. You can see this relationship in this study by Paul Laursen: http://bmjopensem.bmj.com/content/1/1/e000047.info

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