Protein is needed for growth and development of a healthy body. Protein is also included in development of antibodies which help to preserve a strong and functioning immune system. Protein consists of amino acids, which are divided as essential and conditionally essential. Body must get essential amino acids from the food, the conditionally essential amino acids can be synthesized by the body itself.
Protein from animal sources includes more essential amino acids than protein from vegetable sources. Unfortunately many animal sources have too much fat. The best animal sources are eggs, dairy (quark, cheese, cottage cheese), fish, poultry and meat.
The best vegetable sources are pulses, nuts, seeds and grains. Soy, rice and rye have also a good amino acids content. Vegetable sources like grains might not have all the essential amino acids. The deficiency can be covered with a small amount of protein from animal sources, for example make your morning porridge with milk, add cheese to pasta etc.
Severe protein deficiency could lead to edema, weakness of muscle and changes in hair and skin. It often occurs with energy deficiency which comes from overall nutrient deficiency.
However eating too much protein could burden kidneys and liver and may lead to podagra and allergies. The energy from daily protein should not be over 20% of daily food energy for a longer period of time.