Eat healthy, that keeps your resistance up. That is the only corona advice that no one seems to doubt. But is it true? Healthy eating and drinking reduces the risk of heart attack, obesity, diabetes and other diseases, but does it also help against infectious diseases such as corona?

A person’s resistance to infections by bacteria and viruses may be defective. That’s a serious problem. Defects in the immune system that provides our resistance occur, for example, in AIDS, after radiation or chemotherapy and in children with congenital abnormalities of the immune system. You often hear that your resistance is influenced by what you eat. The Nutrition Center says: ‘By eating healthy… your body is better able to fight pathogenic bacteria and viruses.’ What kind of research has been done and are the results convincing?

Many studies have been done on the effect of food and nutritional supplements on parts of the immune system. Such effects are studied in the test tube, in laboratory animals or in volunteers. That is quite feasible, you give people a nutrient or supplement, for example fish oil or a vitamin pill, you take blood and you measure what has changed in some of the thousands of types of cells and proteins that are involved in the immune system.

Resistance cannot be measured

But that doesn’t tell you whether the food or supplement really offers protection against infectious diseases. Resistance cannot be measured and expressed in a number, it is above all a word that feels good. We don’t have enough insight into the immune system to understand exactly which parts need to change and how to protect us against a particular infection. For this it is necessary to investigate the occurrence of the infectious disease itself.

This applies just as much to food as it does to medicines and vaccines. A vaccine can show promising effects in the test tube and in laboratory animals, it can increase the amount of antibodies against corona in the blood in volunteers and show beneficial effects on white blood cells.

But then the investigation is only halfway through; We do not yet know whether the vaccine will prevent corona. This requires experiments in which large groups of people receive the vaccine or a fake injection (placebo). Those experiments are now underway; in a few months or years, we will know which of the dozens of vaccines being researched resulted in fewer people becoming ill from corona in the vaccine group than in the placebo group. In the past, this was often disappointing with vaccines and medicines for other diseases, many of which ultimately proved ineffective or had too many side effects. A noose for the manufacturer, because then the product will not be released on the market.

A favorable exception

It is of great importance for public health to find out whether diet influences the risk of infectious diseases. Yet relatively few such studies appear in top medical journals. A favorable exception was a study by Judith Graat in Kok’s group in Wageningen. They gave 600 elderly people vitamins or a fake pill for a year and examined how much flu or cold they got. The outcome was that vitamins did not help. The research appeared in the top medical journal JAMA. Ten years earlier, Canadian researcher Ranjit Chandra had written in another top journal, The Lancet, published that vitamins and minerals actually reduced respiratory infections in the elderly. Why did he find something else? Because he ripped things off and made up his results. His publications were withdrawn and the justification for the effect of diet on infections was dealt a serious blow.

One possible reason why studies of nutrition and infectious diseases rarely appear in top journals is that they often deal not with the occurrence of the disease itself but with effects on white blood cells. That preference may be because they are faster to do; a study of blood cells takes weeks or months, while a study with disease as a result takes years.

Everything in the balance

There may also be something else at play. A researcher may have built a scientific career based on the promising effects of food or supplements on blood cells or proteins in the test tube, in laboratory animals or in the blood of volunteers. A real disease outcome study would put all that at risk; the outcome of such a definitive experiment is often disappointing, as is the case with pharmaceuticals. That is perhaps why not all nutrition researchers are eager to test whether healthier eating really prevents infectious diseases. That’s why we still don’t know.

A healthy diet reduces the risk of all kinds of diseases, from strokes to tooth decay. Anyone who has eaten and drank healthily for years has thus strengthened his condition and indirectly improved his chance of surviving a corona infection. But don’t think that by filling your supermarket cart with healthy food from now on, you can prevent infectious diseases such as corona.

Martijn Katan is a biochemist and emeritus professor of nutrition at the VU University Amsterdam. For sources and figures see