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The kidney: an organ with many functions

The kidneys have many tasks and functions to fulfil. Amongst other things, they clear the blood of waste products that arise from the metabolic processes of various cell types. They also excrete toxins that are taken in with food, as well as breaking down and excreting drugs. In addition to this function as a ‘clearing house’, the kidneys regulate the water content of the body and the composition of the blood salts, namely the concentrations of sodium, potassium, calcium and phosphorus. Finally, the kidneys make hormones, which are essential for certain bodily functions, e.g. renin, which regulates the blood pressure, and erythropoietin (‘Epo’), which is needed to make new blood cells. For all these tasks to be fulfilled, a good blood supply is essential. The renal arteries, which carry the blood to the kidneys, arise as thick branches directly from the body’s largest artery. A person’s entire blood content passes through the kidneys every 20 minutes. After passage through the kidneys, the blood is returned to the circulation through a large collecting vessel, the renal vein.

What happens when you eat too much protein?
Healthy kidneys can cope with us eating too much protein, but if they are already damaged, the additional protein burden over a long time is not good. The reasons for this have already been explained – too much protein in the diet leads to a rise in the amount of urea and an increase in the blood flow through the kidney corpuscles, which damages them. What is completely wrong in this situation is an old dietary recommendation that is still heard sometimes – that if someone is losing a lot of protein through his or her urine because of kidney damage, he or she should consume more protein to compensate. The opposite is true!

How much protein do you need?
It has been calculated that a person needs about 0.8 gram of protein per kilogram (g/kg) of bodyweight each day to provide the body with the necessary amino acids. Thus, a person who weighs 70 kg needs about 56 g of protein every day. That is not very much – if you eat a piece of chicken for lunch, for example, you have already consumed about 36 g of protein, which is two-thirds of your daily requirement. If you then eat two slices of Emmental cheese for supper, the body has already had enough protein. Any more that you take in through milk, bread or vegetables is extra. The risk of eating too little protein is very small. Dietary studies have shown that each of us consumes on average about 1.6 g of protein per kg of bodyweight per day, which is twice as much as is necessary. People with diabetes often consume a lot of protein because it used to be recommended that they should eat more protein instead of carbohydrate, to help control the blood sugar level. According to current opinion, this is wrong. A healthy diet – and this holds true for those with diabetes as well as for everyone else – should consist mainly of carbohydrate (about 50–60% of total calorie intake), with about 30% coming from fat and only about 15% from protein.