Protein is a buzzword in the diet and fitness industry, and with good reason. It is essential to your body, and vital for growth. The general guideline is to get between 15 and 35 percent of your calories from protein, but some people push it even further. Protein is not a case of "more is better" -- continuously eating more than the recommended amount can cause health problems -- and won't provide additional benefits over a normal amount.
The first symptom you'll notice is dehydration. When you replace most of your carbs with protein, you lose water weight. Carbs bind to water in a 1:4 ratio when they're stored in your body, so as your body burns off those stored carbs for energy, the water molecules get released and excreted. Eating too much protein means you're not replacing your spent carb stores, so the water isn't replaced either. You might notice some quick weight loss, but you won't perform as well athletically, and your skin will become dry and rough over time. You are also more at risk for acute dehydration during hot weather or during strenuous activity, so drink plenty of fluids and keep a lookout for symptoms such as dizziness, extreme fatigue and nausea.
When your carbohydrate stores are all used up, your body turns to fat for energy. Most high-protein diets are also high in fat, so there is plenty to go around. The problem is that the breakdown of fat into a usable molecule produces ketones, which your brain breaks down to make the fat breakdown process more efficient. This process is called ketosis, and its purpose is to protect the protein in your muscles from being broken down so you don't lose muscular strength. Symptoms of ketosis include fatigue, irritability and nausea. Many people experience a decrease in appetite during ketosis, which might lead to near-starvation.
Exacerbation of Pre-Existing Kidney/Liver Disorder
The metabolism of protein produces lots of waste, which gets filtered by the liver and kidneys. If you are a healthy person with normal kidney and liver function, you probably won't have a problem. But if your liver or kidney function is even a little bit off, too much protein can exacerbate the problem quickly. The more protein you eat, the worse your situation can get. It's the protein itself that stresses the kidneys, but it's the ketosis that stresses the liver. Although eating too much protein is not the best thing for your health, you might be OK if you're healthy, as long as you take in enough carbs to avoid ketosis.
Other effects of eating too much protein aren't directly related to protein, but to the diet itself. Protein intake must be increased at the expense of another nutrient, generally carbs. When you severely restrict your carb intake, you miss out on fiber, leading to constipation. Most people who follow a high-protein diet eat mostly animal proteins, which can be high in saturated fat -- this increases your risk of heart disease. If your high-protein diet is also high in fat, it increases your risk of ketosis and also contributes extra calories. Some people actually gain weight eating mostly protein because they don't keep track of calories, and they eat high-calorie, fatty proteins.