Does Low Carb Equal High Fatigue?


I am sure by now with the plethora of low carbohydrate foods on the market and the television advertisements for the Atkins wraps at Subway or the reduced carb friendly Ruby Tuesday’s menu that by now you have eaten at least one or two of these foods. Some of you may even be on a low carbohydrate diet now, but do you wonder if following this lifestyle 24/7 would impede your energy in the gym? After all, we have always been told that carbohydrates are the preferred fuel source for both the brain (all day long) and muscles when exercising. So, if this tenet of sports nutrition is true, how can it be that so many studies have found that restricting carbohydrate in the diet will not necessarily impede exercise performance? Let’s delve into the clinical research and see just what has been found in terms of carbohydrate intake and exercise performance.


Eat fat, burn fat

Some nutritionists and exercise physiologists will tell you that what ever type of nutrient dominates your diet is the one you will predominately burn at rest and even during exercise.  For example, if you eat a diet that is 60% carbohydrate, your body will depend a little more on carbohydrates for energy all day long, however, if your diet is Atkins-esque after a period of “metabolic adjustment” your body will depend more on fat for energy. When someone undergoes transition from a resting state to exercising or from low intensity exercise to high intensity, a change will occur in nutrient metabolism. The amount of carbohydrates or fat that is used is dependent upon various endocrine hormones and some cytokines. Studies show that the more trained you are (in a better trained state), the body will reduce its reliance on carbohydrate and increase fat oxidation – especially from fat that is stores within muscles.

If you are a triathlete, marathon runner or someone who exercises for more than 90 minutes at a time, the amount of carbohydrates that are stored in the muscles and liver is very important and related to how you will perform. These stored carbohydrates are known as glycogen.  In a recent study, scientists from Switzerland fed dualathletes (runners/cyclists) either a high-fat diet or a high-carbohydrate diet for five weeks in a randomized crossover design. All the athletes underwent various performance tests throughout the study that revealed no major differences in power output or running times. The findings strongly indicates that if you are carbohydrate loaded and then follow a high fat diet for five weeks, you will have no adverse effects on your glycogen stores or exercise performance. Thus, high fat diets (which are usually high protein, low carbohydrate) do not impair performance.

Apparently, following a low-carbohydrate diet may impair exercise ability if you are performing repetitive type sports. For example, cyclists undergoing repeated sprints whether on a high-carb or low-carb diet were found to have different results. Those on the high-carbohydrate phase were able to delay fatigue during repeated sprints, however there was no physiological difference in glycogen stores or markers of cellular energy between the diet groups. Thus, the greater fatigue experienced by the low-carb group cannot be explained by a physiological means, but may be a perceived fatigue.


Are women different than men?

This question is not meant to bring up flashbacks to the early 1990s when the “Men are from Mars, Women are from Venus” book was a bestseller, but rather this is to point out that there are well known differences between the genders in metabolism. As a gender, women tend to burn a tad more fat while at rest than men, however, how a women trains and eats can be the difference whether her body maximizes the ability to burn fat all day long.

Many people wrongly believe that if you follow a low carbohydrate diet you will inevitably experience a deficiency in certain vitamins or minerals. To test this scientists put a group of students on a 5% carbohydrate diet (5% carbohydrate 52% fat, 43% protein) which surprisingly enough resulted in the body enhancing its own antioxidant defenses. Furthermore, the cells of the body were better protected on the low-carb diet. The take home message from this study; eat meat, enjoy the low carb and you immune system will not be negatively impacted.


More support for low carb diets…or not?

A study out of Copenhagen examined whether a low-carbohydrate diet that induced reduced (low) glycogen stores would reduce the amount of energy that a muscle cell could form during exercise. If you ask why this is important,let me explain. In the muscle and throughout the body we have “metabolic intermediates”, these metabolic intermediates can be turned into energy (ATP) or stored as potential energy – depending upon our immediate needs. There is some thought within science that this process is affected and partially dictated during exercise by carbohydrate availability. Thus, if there is not much carbohydrates available (as from low-glycogen stores), energy production on a cellular level will be impaired. The end result of the Copenhagen study, being on a low-carbohydrate diet did not negatively impact the ability of metabolic intermediates to be expanded and used for energy through unexpected means. Thus, low-carb diets do not appear to interfere with cellular energy production during exercise.

Weight lifters are always told to drink carbohydrate when they are going to be working out for any real length of time (> 90 minutes). To test whether a high or low carbohydrate diet would have any differential affects in weight trained athletes, scientists provided these diets with a high-carb/low-fat or a low-carb/high fat diet. All participants were tested before and after being on the assigned diets for muscular strength and endurance (so that the researchers could examine both the anaerobic and aerobic effects of the diet when coupled with diet). Can you guess what the researchers found? Wanna bet a muffin or two on the results? Well, surprisingly enough varying the macronutrient ratio or content had no effect on exercise training in either the anaerobic or aerobic tests examined. The take home thought; perhaps the high carbohydrate lovers have it all wrong and we don’t need as much carbos as once thought, even if we do exercise.

Another study out of Copenhagen confirms the thoughts that the body will burn the predominant nutrient you eat, even during exercise. In this study where researchers compared a 65% carbohydrate diet to a 62% fat diet and their relative effects during exercise, the results generally indicate that a low-carb, high fat diet will enhance your ability to spare glycogen during exercise, while forcing your body to increase the amount of fat that is being burned for energy. Not a bad side effect of a low-carbohydrate diet, is it?


Make it a Low-Carb Wrap!

As apparent from the above reviewed studies, following a reduced or low carbohydrate diet (ranging from a low of 8% to a high of ~45%) will not impair your exercise performance on or from a physiological basis. However, additional studies in runners and other aerobic type sports do indicate that your perceived fatigue (emotional/mental, not physical) may occur earlier with this kind of diet. Many scientists believe that as you get in better physical shape that your body will become more adept at using the higher fat portion of your diet for energy and this perceived fatigue would disappear. If you do exercise longer than 90 minutes at a time, there is nothing run with sipping on carbohydrates during the exercise, while following a low-carbohydrate diet with adequate protein the rest of the day. This carbohydrate will mostly be burned during exercise and will not contribute or interfere with your weight loss, body fat reduction attempts.



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Dr. Kalman has been involved in over 100 clinical trials within the pharmaceutical, medical and nutrition fields, having published over 50 abstracts & more than 25 peer-reviewed manuscripts. Dr. Kalman earned a Doctorate in exercise and nutritional biochemistry and is an active member or spokesperson with many organizations (ISSN, NSCA, APS, ACSM, etc.) and a co-founder of The International Society of Sports Nutrition.