How Low-Carb Diets Can Affect Your Physical Fitness Performance

Lt. Cmdr. Kevin Bernstein, a sports medicine physician at Naval Branch Health Clinic Mayport, conducts a grocery shopping tour at Naval Air Station Jacksonville’s commissary on Oct. 25. (Jacob Sippel/Naval Hospital Jacksonville)

A former Army officer and reservist emailed me to ask about low-carb diets. Here is his question:

"Will a low-carb diet hurt my PFT scores?"

He went on to state that his unit was being called for active duty, and he had to lose some weight to get into "fighting shape." He went on a high-protein and low-carbohydrate diet and lost more than 20 pounds very quickly. However, his workouts, such as runninglifting weights and calisthenics, were not that impressive, and he is not seeing the results he needs.

The low-carb diet eliminates foods, such as fruits and vegetables, loaded with fiber, minerals and nutrients vital to excretion and fighting illnesses. It is never a good idea to eliminate such an essential element of good health completely. Limiting unhealthy carbs (sugars) is always a good idea, but not eating key nutrients will affect your workout.

If you are running fast to score a competitive time on your PFT or doing calisthenics for maximum scores, the body needs carbohydrates for fuel. If you do not have this quick-burning fuel in your body, you will fatigue more quickly. Some experts say that lifting weights, performing calisthenics and running at a fast pace could increase the risk of muscular injury if your glycogen level (stored carbs) is low.

I have recommended that extremely overweight people start a low-carb diet, such as Atkins or the South Beach Diet, but only temporarily since they were too heavy and bloated to move, much less try to run. These diets allowed them to lose more than 20 pounds of fat and retained water in a few weeks by eliminating carbs and going into ketosis.

However, by simply adding a gallon of water a day to your diet and eating fewer salty foods, your body will lose the excess water being retained. I have seen many under-hydrated people simply add water to their diet and have the same results of a 20-pound weight loss.

So the short answer to a long explanation: Yes, having low levels of carbohydrates in your diet will affect your PFT test and daily workouts. If you want to lose weight, do it by reducing calories in and increase calories burned by more cardiovascular and strength training or calisthenics. Also check out my free diet plan -- full of ideas on water intake, food varieties and exercise tips.

Take a look at the "Physical Fitness Test Anxiety" article for tips on taking the PFT and recommended foods to help with performance. If you have any questions, feel free to email me.

Stew Smith is a former Navy SEAL and fitness author certified as a Strength and Conditioning Specialist (CSCS) with the National Strength and Conditioning Association. Visit his Fitness eBook store if you're looking to start a workout program to create a healthy lifestyle. Send your fitness questions to

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