Study: Low-Carb, High-Fat Diet Impairs Performance

Low Carb Study Figure 4

No other graphic explains the results of this study better. You can clearly see that the Low Carb, High Fat group actually did WORSE in their performance compared to the other two groups.[1]

There’s an endless debate in the fitness world about the ultimate diet for performance and physique goals. Look hard enough and you can find a diet to fit every whacky nutritional philosophy out there….gluten-free, paleo, “clean” eating, etc….the list goes on and on.

One of the biggest swells in popularity recently is the high fat, low carb diet popularized first by Dr. Atkins and surrounding enthusiasts, and now keto dieters. Proponents of this particular diet champion it for improved aesthetics, mental clarity, energy, and even performance. Their reasoning is that even incredibly lean athletes have an abundance of fat stores in comparison to limited reserves of carbohydrates stored in the body.

However, it’s that last bullet point (performance) that might need to be reconsidered. A new study shows that maybe this low carb lifestyle is substantially less than optimal when trying to maximize performance.[1]

Note: Shout out to Dr. Layne Norton for catching this and posting it with a fair dose of trash-talk!!! Thanks Doc!

The Study

Researchers from the Australian Institute of Sport (AIS) sought to understand the impact of a ketogenic low-carbohydrate, high-fat diet (LCHF) during a 3 week period of intense training on metabolism and performance metrics of world-class endurance athletes (e.g. a seven-time Olympic and World Championship medallist from 2008-2016). 29 data sets were collected from two separate “camps” analyzing the LCHF diet protocol on performance. Participants in the study consumed one of three diets, which broke down as:

  • High Carb (HCHO): Macros = 8.6g/kg C, 2.1g/kg P, 1.2g/kg F
  • Low Carb, High Fat (LCHF): Macros = <50g/d C, 2.1g/kg P, 78% energy from F
  • Periodized Carb (PCHO): alternate days of low and high carb diet
Low Carb Study Table 1

Table 1: Characteristics of elite race walkers (n = 21) in training and diet intervention study (n = 29 cases).[1]

All three diet groups undertook a “beefed” up training regimen that included resistance training, race walking, and cross-training (running, swimming, or cycling). All training sessions were performed in a group setting, monitored by researchers, and recorded by the participants.

Immediately before and after the three week training period, subjects underwent a 3-day “test block” to gather performance numbers and blood markers. The 3-day test block consisted of:

  • Day 1 – treadmill test to assess economy and VO2peak
  • Day 2 – 10km race on a 400m track
  • Day 3 – 25km walk

Now, something interesting to note is that for the pre-treatment test period, all subjects consumed the same carbohydrate-rich breakfast that provided 2g/kg carbs. For the post-treatment test period, only the HCHO and PCHO groups received the carb-rich breakfast while the LCHF group consumed an isocaloric (same caloric density as PCHO and HCHO) meal but it was high-fat, low-carb.

Low Carb Study Table 2

Table 2. Overview of weekly training-diet intervention involving high carbohydrate (CHO) availability (HCHO), periodised CHO availability (PCHO) or low CHO high fat (LCHF) diets in elite race walkers (n = 29).[1]


At the conclusion of the trial, researchers noted that following three weeks of intensified training along with a mild-energy restricted diet, all groups increased peak aerobic capacity (VO2peak) independent of which diet they consumed. Additionally, since all participants were on a mild caloric deficit, there were improvements in body composition (average 1.4kg lost).

The first significant difference among the groups came in the areas of Heart Rate (HR) and Rate of Perceived Exertion (RPE). The LCHF group displayed higher values of these two metrics in the post-treatment test period, which researchers attributed to higher metabolic cost and perceived effort with the LCHF diet.

Low Carb Study Table 5

Table 5. Results of testing on Day 3: 25 km long walk before and after 3-week diet and training intervention in elite race walkers

Additionally, there was also a significant decrease in Respiratory Exchange Ratio (RER) and an increase in the absolute O2 cost of exercise for the LCHF group. The keto-adapted group did have significantly increased fat oxidation compared to the HCHO and PCHO groups, but it didn’t benefit the athletes at all. The increased rates of fat oxidation resulted in reduced O2 economy, meaning, oxygen demand increased for a given speed compared to the HCHO and PCHO groups.

Researchers concluded:

”In conclusion, the results of the present study showed that despite achieving substantial increases in the capacity for fat oxidation during intense exercise, chronic adaptation to a ketogenic low-CHO, high-fat diet impaired exercise economy and negated the transfer of training-induced increases in aerobic capacity into improved performance of a real-life endurance event in elite athletes. In contrast, training with a diet rich in carbohydrate and which provided either high or periodised carbohydrate availability around training sessions was associated with improved race outcomes.“


NutraBio Super Carb Orange Mango

Training with high-end carbs (in this case cluster dextrin) – indeed makes a big difference!

On the outside, it’s not all that surprising – we’ve seen carbohydrates enhance performance, so it doesn’t take a leap of faith to see that extremely low-carb may yield lower performance.

Yet again, we see a part of the mystique of the keto diet get shattered. Living the low-carb lifestyle may be trendy from time to time, and possibly help lose weight and drop body fat for certain types of individuals, but when it comes to maximizing performance, the science is pretty cut and dry — don’t cut carbs — especially around training time!

Carbohydrates provide the most efficient and readily accessible form of energy for the body, and if you’re looking to really excel in sports carbs are your friend, not your enemy!

Back to Layne Norton…

Ironically… we have to wonder — if Layne is the one to be calling others out on their hatred for carbohydrates… then why don’t any of his own Carbon by Layne Norton supplements contain any appreciable amounts of carbs?! Is it time for a carb-based intra to come out from Carbon??

About the Author: Mike Roberto

Mike Roberto

Mike Roberto is a research scientist and water sports athlete who founded PricePlow. He is an n=1 diet experimenter with extensive experience in supplementation and dietary modification, whose personal expertise stems from several experiments done on himself while sharing lab tests.

Mike's goal is to bridge the gap between nutritional research scientists and non-academics who seek to better their health in a system that has catastrophically failed the public.

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  1. Burke, L. M., Ross, M. L., Garvican-Lewis, L. A., Welvaert, M., Heikura, I. A., Forbes, S. G., Mirtschin, J. G., Cato, L. E., Strobel, N., Sharma, A. P. and Hawley, J. A. (2016), Low Carbohydrate, High Fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers. J Physiol. Accepted Author Manuscript. doi:10.1113/JP273230

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