Every time that we eat something rich in protein, such as a large piece of meat, our kidneys get work to do, because they have to excrete the end-product of protein metabolism, urea. If someone eats a lot of protein, their kidneys have to deal with a large amount of urea.

After you have eaten a steak, for example, the blood flow through the kidney corpuscles and the pressure there increase temporarily via a mechanism that is not fully understood. A healthy kidney copes with this without a problem.

But you can imagine that a kidney that is already damaged by long-standing, poorly controlled diabetes, for example, is very sensitive to the increased pressure after a protein-rich meal. If too much urea is produced, it can no longer be completely excreted through the kidneys and begins to accumulate in the blood.

Protein is an indispensable part of our diet. The building blocks of proteins, the amino acids, are used to make all our body’s cells, many of our hormones and components of the blood. There are 20 amino acids in total, of which the body can make 12 itself. The others have to be taken in through foodstuffs; they are therefore known as ‘essential’ amino acids.

Both animals and plants can be the source of our dietary protein. Animal protein is consumed as meat, cheese, dairy products, eggs and fish: vegetable proteins are found in legumes, soya, cereals, vegetables, potatoes and in a small amount in fruits. These two types of protein differ in several ways.

Animal protein has a very similar amino acid composition to human protein. It can therefore be used by human organs without any special changes to the chemical structure. However, a distinct disadvantage is that animal protein is nearly always found closely associated with fat.

Because 1 gram of fat contains twice as many calories, and therefore twice as much energy, as 1 gram of carbohydrate or 1 gram of protein, eating animal protein usually results in the consumption of a lot of calories and a lot of fatty acids. This can lead to overweight and a rise in the levels of fatty acids in the blood.

It also seems that animal proteins in particular stimulate blood flow through the kidneys. In this respect, vegetable protein is much healthier. It is often found associated with fibrous substances that cannot be broken down by the human digestive system, which are good for the health of the intestinal tract. Their fat content is usually low and they impose less of a burden on the kidneys.

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. The food pyramid is the easiest way of demonstrating this.

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! If you want to compile a picture of your protein consumption, you should complete a food diary for a day, listing all the things you have eaten. You can then discuss this with a dietician. They can calculate the average protein content of animal and vegetable proteins.

They may also suggest ways in which you can modify your protein consumption. But the goal is not that every person should eat 0.8 g of protein per kg of bodyweight per day or even less. For many of us, that would mean a serious change in our lifestyle and it has not yet been proved scientifically that this would be beneficial in the long term.

Each small reduction in a significantly high rate of protein consumption, however, can reduce the burden on the kidneys. There are situations in which a protein-poor diet, with less than 0.8 g of protein per kg of bodyweight per day, is actually necessary.

If there is already some kidney damage and this gets worse very quickly, the amount of urea in the blood can rise steeply. A protein-poor diet allows the urea concentration to fall again, thereby avoiding kidney toxicity. Such treatment is usually undertaken in the clinic.

Salt and the kidneys

Cooking salt (which chemically is sodium chloride) belongs to the list of nutrients that are essential for the body. It is used, amongst other things, to maintain the function of the nerves, muscles, glands and circulation. The kidneys play a key role in the regulation of the salt content of the body.

If the amount is too high, the kidneys intervene and excessive salt is excreted; on the other hand, if the intake is too low, the urgently required mineral is held back in the body. If the kidneys are damaged, such that they can no longer perform this function efficiently, then the salt content of the body rises.

Because salt exists in the body in its dissolved form, a higher salt content leads to water retention and thereby to higher blood pressure. This rise in blood pressure is highly detrimental for the kidneys. On average, each of us consumes about 9 grams of salt a day.

Someone who likes to season their food can easily consume up to 12 or 15 grams a day! Nutritionists, on the other hand, recommend only 6 grams per day. This amount is absorbed by the body from a mixed diet through the natural salt content of the foodstuffs, without the need for adding more salt. You should remember that the salt content of food is highly variable, so that it is easy to exceed the target amount.