Osteoporosis

• Protein is needed to build tissue, but large amounts of protein raise the level of acid in your blood, causing your body to excrete calcium from the kidneys. Over time, this can weaken the bones and lead to osteoporosis, increasing your risk of bone fractures (references 1).
  
  

Kidney Damage

• Loss of calcium doesn't just harm your bones, it can also lead to kidney problems. High levels of calcium in the urine can cause kidney stones to form. These mineral masses must then be passed out of the body in a very painful process (references 2). For individuals with kidney problems, a high-protein diet can cause further damage as the kidneys work to process and excrete the excess protein waste.
  
  

Lack of Nutrients

• Many high-protein diets include large amounts of high-fat foods such as meat, eggs, and full-fat cheese, while severely limiting carbohydrates. Restricted foods include fruits, vegetables, low-fat dairy, and whole-grain breads and cereals, all of which are great sources of fiber, nutrients, vitamins and minerals. Your body needs all of these nutrients not only for everyday function but for fighting off cancer, heart disease, high blood pressure and diabetes.
  
  

High Cholesterol

• The typical foods in a high-protein diet are high in saturated fats. This is the type of fat that clogs arteries and raises LDL cholesterol, the "bad" cholesterol. Over time, the effects of fat can lead to coronary heart disease, stroke, cancer and diabetes. People who follow high-protein diets may consume up to 53 percent of their calories from fat (see references 3), well above the American Heart Association's (AHA) recommended guideline. For people trying to lose weight, the AHA recommends that no more than 35 percent of calories come from fat and less than 7 percent come from saturated fat.
  
  

Heart Disease

• Not only do high-protein diets raise your risk of heart problems, they also deprive your body of the tools it needs to fight off cardiovascular disease. Fiber-rich, high-carbohydrate foods can reduce blood pressure and cholesterol levels; they're an important part of any diet but are especially crucial for people who consume too much saturated fat. When fatty meals replace high-fiber foods, as is the case in high-protein diets, your body is doubly at risk for dangerous heart conditions.