Muscle weakness is a common side effect of low-carb diets, but it doesn't have to be. The trick is to balance your nutrients carefully -- low-carb doesn't mean no-carb, and your high-protein diet may be lower in protein than you think. According to 2007 study in the American Journal of Clinical Nutrition, it takes two to four weeks to slip into ketogenesis, where your body begins steadily burning fat for energy, so you may just need to wait it out a bit. If you're past that stage, you may need to tweak your diet to regain your strength.
Carbs Too Low
Carbohydrates are made of sugar molecules that provide the majority of your energy. When you digest carbs, they are broken apart into the base molecule, glucose. Your body uses some of that glucose immediately, but it converts the rest to glycogen and stores it in your muscles and liver for later. When you make a sudden demand on your body, like exercising a few hours after a meal, that glycogen gets converted back to glucose and used as fuel. Low-carb diets deplete your glycogen stores by not allowing you to refill them as they empty, which forces your body to turn to fat for energy. It takes longer to convert fat into a usable molecule, so you may not see the muscle response you're used to. It's not that your muscles are weaker, it's just that they're not getting the fuel they need. Even if you're eating low-carb, get about half of your daily calories from carbohydrates.
Protein vs. Fat
When you cut your carb intake, the difference must be made up somewhere. You may think that you've replaced it with protein, but because many protein sources are high in fat and fat has twice as many calories per gram as protein, that may not be the case. If you get less than 15 percent of your calories from protein, your muscles may begin to break down. Unlike the glycogen situation, this means that you actually are weaker. Whether you are using a low-carb diet to lose weight or to build muscle, muscle loss is the opposite of your goal. Track your food intake for a few days to get a clear picture of your protein intake, and follow your diet plan closely. Some plans allow you as much as 35 percent protein, but choose lean sources like poultry and fish to avoid increasing your fat content instead.
Electrolytes
Cutting carbs and eating more cooked meat results in a lower intake of sodium and potassium. A deficiency of these minerals means that your muscles become unable to maintain a nitrogen balance, and may degrade. It doesn't take much -- a 2004 study in "Nutrition and Metabolism" found that as little as 3 to 5 grams of sodium and 2 to 3 grams of potassium per day can allow you to maintain functional muscle mass. Check your multivitamin label to see what it provides, and supplement with a sports drink that contains electrolytes. You can also eat more spinach, which provides both minerals without being high in carbohydrates.
Calories
Once you've checked out all the other issues, make sure you're eating enough calories. It may not be a problem with the low-carb diet, but simply that your body isn't getting enough fuel, period. This can especially be true if you've recently changed your workout routine -- you may be burning more calories, so you may need to eat more calories to compensate. If you're eating low-carb to lose weight, starving yourself won't help you. Creating a deficit of only 500 calories per day will take you down the road of slow but steady, healthy weight loss. Consult your doctor or use an online calculator to determine how many calories you should be eating for your age, size, gender and activity level.
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