If your body does not have enough carbohydrates and has too much protein, it enters into a state known as ketosis. Ketosis is the accumulation in the blood of ketones (byproducts of fat oxidation) and represents the body’s adaptation to fasting or starvation. The theory behind low-carbohydrate diets is that inducing a constant state of ketosis cause people to lose weight regardless of how many calories from protein and fat are consumed because it causes the body to eventually burn fat for energy. Ketosis increases insulin resistance and glucose intolerance. Insulin resistance is a major risk factor for the development of coronary artery disease, and glucose intolerance has been linked to hypertension and dyslipidemia. Maintaining a state of ketosis can also result in mild dehydration because the kidneys are burdened by having to rid the body of excess nitrogen. This can cause dizziness, headaches, confusion, nausea, fatigue, sleep problems, and worsening of kidney problems.

If you are eating too much protein and not enough carbohydrates, it is likely you are not eating enough fiber. Low intake of fiber can cause constipation, and may contribute to the development of hemorrhoids, diverticulosis, polyps, colon cancer, heart disease, diabetes, and obesity.

High protein intake is also associated with an increased risk of osteoporosis due to calcium loss. When protein is digested, amino acids break apart and pass into the blood making the blood slightly acidic. Since your body needs to have a balanced pH level, calcium is pulled from the bones to neutralize acidity. Therefore, the more protein you have in your diet, the more acidic your blood will be and the more calcium that will be needed. Animal proteins (i.e. meats) are the main culprit of this cycle; grains, beans, vegetables, and fruits act as calcium savers.

In addition to calcium loss, maintaining a low-carbohydrate intake may be associated with an increase in blood pressure with age due to deficiencies of high-carbohydrate, high-fiber foods that protect against hypertension.

Consuming too much protein and not enough carbohydrates also results in poor athletic performance and impaired ability to have an effective workout due to depletion of glycogen stores, which the body burns for energy during exercise; carbohydrates are the body’s main source of glycogen. This is counter-productive, since most people either eat excessive protein under the assumption of either losing weight or building muscle mass and size; however these goals cannot be achieved without a proper exercise program.

Source: Blackburn et al. (2001), Cleveland Clinic Journal of Medicine