For this evaluation I make the distinction between diets that restrict carbohydrate to around 45 percent, and diets such as Atkins and others that would have you eat less than 30 percent carbohydrates or indeed induce ketosis (very low-carb), a condition where fats are converted to chemicals called ketones to fuel the body when stores of glucose are exhausted.
What can go wrong with very low-carb diets?
Carbohydrates. While the body can adapt to some extent to low-carbohydrate, high-fat diets for exercise, weight lifters and others involved in high-intensity exercise need glucose to fuel the intense activities of weight lifting (along with the ATP and phosphocreatine systems). Fat-fueled exercise is possible -- walking and slow jogging are examples -- but as soon as you make demands for high intensity such as sprints, intervals, circuits or heavy lifting, then your performance and your training will suffer. The reason for this is that high-intensity exercise needs fuel supplied to the muscles quickly. Fat cannot supply ‘quick fuel’ sufficiently fast, while carbohydrates can.
For some weight trainers, a low-carbohydrate diet can be problematic in relation to maintaining muscle mass. Protein is not a good substitute for carbohydrate for weight trainers for the following reasons. Although excess protein can be broken down to form glucose, this process is limited and and cannot substitute for carbohydrate’s supply of glucose. In addition, low levels of carbohydrate can stimulate production of the hormone cortisol which breaks down muscle protein to supply the body with glucose – the last thing a body builder or weight trainer wants.
Some experienced weight trainers use a cyclical ketogenic diet where they do very low-carb for a few days and then revert to a high-carb diet to get an insulin rush and, supposedly, build muscle. I'm not convinced of the value of this and I know of no scientific evidence that it works. Don't bother with it.
Protein. Low-carb diets usually supply plenty of protein so insufficiency is not going to be a problem for weight trainers and exercisers. Very low-carb diets can require that up to 35 percent protein is consumed. However, if you’re looking to lose weight with a very low-carb diet you need to be sure that your kidney function is normal. High-protein diets are not recommended for people with chronic kidney disease as this may exacerbate the disease. Very high-protein diets of mostly meat protein may also cause excessive calcium excretion as the body attempts to buffer acidity. Osteoporosis (bone loss) may occur as a result. This may be prevented by a sufficiently alkaline diet high in fruit and vegetables but therein lies the risk. In addition, too much red meat in the diet may increase risks for bowel cancer and possibly type 2 diabetes. High-protein diets may be healthier when red meat is restricted in favor of white meats, low-fat dairy and vegetable protein.
Vitamin deficiencies. On the Atkins diet and other very low-carb diets, excluding whole grains can make it difficult to get enough fiber, magnesium, thiamin (B1), and possibly folate. Without dairy foods, calcium, vitamin D and riboflavin can be in short supply. Restricting fruits and starchy vegetables limits vitamins A and C, potassium and folate. In addition, cholesterol and saturated fat intake exceeds that recommended by the American Heart Association.
Summing up, a very low-carb diet is not suitable for an active person. In fact, if you are very active and exercise four or five days a week at moderate to high intensity, even a somewhat low-carb diet is likely to be sub-optimal. For weight loss and general fitness, moderate low-fat diets higher in carbohydrate are likely to be more suitable.
