The long-term health effects of a high-protein/low-carbohydrate diet are unknown. But getting most of your daily calories from high-protein foods like meat, eggs, and cheese is not a balanced eating plan. You may be eating too much fat and cholesterol, which may raise your risk of heart disease. You may also be eating too few fruits, vegetables, and whole grains, which may lead to constipation due to lack of fiber.

Following a high-protein/low-carbohydrate diet may also make you feel nauseous, tired and/or weak. Eating fewer than 130 grams of carbohydrate a day can lead to the buildup of ketones (partially broken-down fats) in your blood. A buildup of ketones in your blood (called ketosis) can cause your body to produce high levels of uric acid, which is a risk factor for gout (a painful swelling of the joints) and kidney stones. Ketosis may be especially risky for pregnant women and people with diabetes or kidney disease. 

Additional Resources:
http://www.medscape.com/viewarticle/414346
http://www.brighamandwomens.org/healtheweightforwomen/special_topics/intelihealth0803.aspx