How Low-Carb Diets Can Hurt Your Weight Training

Don't Do Low-Carb to Get Best Results

Cable curl exercise
Cable Curls. (c) Paul Rogers

Low-carb diets like the Atkins diet are popular for weight loss, even though many people aren't able to stick to them. A low-carbohydrate diet has been defined in various ways, but usually implies less than 40% of energy intake of carbohydrate, with many versions much lower in carbohydrate than this. Can low-carb diets hurt your weight training performance? The answer is likely to be 'yes.'

Why Carbohydrates Are Important for Athletes, Weight Trainers and Heavy Exercisers

Carbohydrates (carbs) are the main fuel of exercise.

Carbohydrate from foods like breads, cereals, sugars and fruit and vegetables is stored as glucose in muscle and liver in a form called "glycogen." Carbohydrate is the premier fuel for fast and intense exercise like running fast or lifting heavy weights. Fat cannot fuel these intense activities with the same efficiency. Unlike fat, glucose storage is quite limited and that's why extreme exercisers like marathoners can virtually run out of glucose and have to slow down dramatically, even though fat is still available as a fuel.

Why High-Performance Exercisers Can't Substitute Fat and Protein for Carbohydrate

One of the inevitabilities of low-carb eating is that you need to replace that carbohydrate with fat or protein. There are no other choices -- even though on weight-loss diets you need to eat less total energy.

Fat as fuel. When researchers tried high-fat eating for athletes, they found that the body did adapt to an increase in fat consumption by prioritizing fat use as body fuel.

However, the downside of this was that even when they ensured sufficient carbohydrate was stored for high-intensity exercise, the "rate" at which glucose was available to muscle had been depreciated. That is, the high-fat diet had actually slowed down the athlete for activities that required a lot of instant power -- like a sprint finish or accelerating up a hill in a bike race.

Weight lifting has similar, high-power requirements. You want that glucose (carbohydrate energy) available as fast as possible. (The only faster fuel is phosphocreatine, which only last for ten seconds or so.)

Protein as fuel. Compared to carbohydrate (glucose) and fat, protein is not a major fuel source even though it can be broken down into glucose, which can be used for fuel when other sources run low. However, this is an inefficient pathway to glucose. Another key point to consider is that when liver or blood glucose gets low, your own muscle will be broken apart -- first into amino acids, then to glucose. Your body knows that it must protect your blood glucose at all costs, or else you can faint from hypoglycemia, just like a diabetic. Muscle, in a way, is a permanent storage site of available glucose. If you want to protect muscle from this process, then you need to have adequate glucose available from other sources to prevent muscle being broken apart. A low-carb diet makes you more susceptible to this degradation of muscle, although careful exercise planning and nutrient cycling might make this less likely.

How Low-Carb Diets Can Prevent Strength and Muscle Building

In a recent comparative study, two groups of novice weight trainers were followed. Subjects were randomly assigned to a low carbohydrate (3.4 g/kg), higher protein diet (1.5 g/kg); or a high carbohydrate (5.0 g/kg), lower protein diet (1.2 g/kg). Both diets exceeded the recommended protein intake. (The low-carb group works out to less than 40% carbohydrate energy.)

In the results, the low-carbohydrate group had a greater strength loss after exercise compared to the high-carb group. The low-carbohydrate exercisers also had less efficient use of protein for muscle repair and rebuilding after training. The authors concluded that dietary carbohydrate, compared to protein, may be a more important nutrient to the novice weight lifter when recovering from exercise-induced muscle damage.

Low-Carb, High-Protein Diets May Affect Bone Density

One of the reasons weight training and weight-bearing exercise has become popular among middle-aged women is that this is a preferred exercise for increasing bone mineral density and preventing osteoporosis.

Whether low-carb diets adversely affect bone mineral density, and are therefore a risk factor for osteoporosis, is a hotly debated question. The theoretical evidence for adverse effects of high-protein, low-carb diets is somewhat sound, though, depending on how stringently observed certain dietary precautions are when following such diets.

Researchers from Purdue University studied women from 43 to 80 on various weight-loss diets. The results showed that women who ate high-protein diets with a large amount of lean meat lost bone mineral density compared to women on other types of diet plans.

A possible reason for this is that diets high in meat and protein are more acidic and may therefore require calcium from existing bone to buffer this acidity. On the other hand, diets too low in protein might also affect bone density adversely. Low-carb diets tend to be high in meat proteins because the lower the carbohydrate, the higher the amounts of protein required to fulfill energy requirements.

Eating more vegetable protein in low-carb diets might offset any negative bone effects by minimizing acidity.

Summing up: If you're on a weight-loss diet, removing refined carbs and sugars is always a good idea, but going too low-carb and too high in meat proteins may not be the best policy. For heavy exercisers, a low-carb diet is likely to impair your best performance.


Stellingwerff T, Spriet LL, Watt MJ, et al. Decreased PDH activation and glycogenolysis during exercise following fat adaptation with carbohydrate restoration. Am J Physiol Endocrinol Metab. 2006 Feb;290(2):E380-8.

Benjamin L, Blanpied P, Lamont LS. Dietary Carbohydrate and Protein Manipulation and Exercise Recovery in Novice Weight-Lifters. JEPonline 2009;12 (6):33-39.

Campbell WW, Tang M. Protein Intake, Weight Loss, and Bone Mineral Density in Postmenopausal Women. J Gerontol A Biol Sci Med Sci. 2010 Jul 6.

Bergqvist AG, Schall JI, Stallings VA, Zemel BS. Progressive bone mineral content loss in children with intractable epilepsy treated with the ketogenic diet. Am J Clin Nutr. 2008 Dec;88(6):1678-84.

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