Nutrients, immunity and Covid.

Food manufacturers, food celebs and wannabe diet experts often discuss the immune boosting properties of some food or nutrient. Our immune system is a highly complex and  orchestrated network of mechanisms that help us fend off infections and the entry of unwanted foreign matter into our bodies. So, when the issue of “boosting” the immune system is raised, it is not unreasonable to ask what part of the immune system you expect to be boosted. Do you want those antibodies (A type) that line the exterior of the body such as the gut and lungs to be smarter at keeping foreign bodies out? Do you want to boost the ability of the body to produce circulating antibodies (G type) to foreign matter that have gained entry into the body? Do you want to boost the so-called cytokine storm that wreaks havoc in Covid? Do you want to boost those antibodies (E type) that are involved in allergic reactions?

Many nutrients are involved in different parts of the immune system  and a deficiency can cause a significant impairment of that system . But how do you define deficiency? Firstly, measuring dietary patterns to ascertain micronutrient status is a waste of time. At the level of the individual, such measures are totally unreliable. They may give a broad sweep but they lack the definition needed to allow us make any prediction of biochemical deficiency. Secondly, when micronutrient deficiency is measured in dietary terms, it is against particular dietary reference values, the ones you might see on a packet of breakfast cereal.  With these, we can have cut-off points, below which population intakes of a nutrient would be a cause for concern. Many with intakes below that minimum threshold of dietary intake may show biochemical deficiencies but, such is the variation in human nutrient requirements, that many with intakes below that value will be perfectly healthy. 

Cut-off levels, based on blood biochemical values, can sharpen our expectation that an individual is prone to some nutritional disorder and in such instances, nutritional supplements and dietetic advice will help restore these blood value to normal. As normality is restored, the many biochemical pathways that depend on the nutrient in question will be fully operational. But here lies the hub of the problem. Once normality is reached, further intake of the nutrient will have no effect. The petrol tank of a Jaguar has 50% greater capacity than that of a Fiat Punto. Fitting an additional petrol tank will not make the Punto overtake the Jag. When a biochemical need is met, that’s it. 

So should we routinely use nutritional supplements? If someone is restricting their food intake to manage their weight, it might be wise to use them. Equally, as we get older, our appetite falls as does our ability to absorb vitamin B12, and again, their use may be a good insurance. And there are specific clinical needs for certain vitamin supplements such as folic acid in pregnancy, multi-micronutrients in cystic fibrosis or calcium for low bone density. Routine use of nutritional supplements is perfectly acceptable but with the current pandemic, there has been a plethora of recommendations for supplementary intakes of individual minerals and vitamins.

In the present pandemic,  many immune related nutrients have come to prominence, none more so than vitamin D. Recommending vitamin D supplements to boost the immune system begs the question: Is it possible that the immune system of your target population is actually limited by another immune related micronutrient. If you don’t know that, focusing on one vitamin is pretty foolish. Besides vitamin D, many other micronutrients are involved in protecting us from unwanted visitors. Vitamins A and C are centrally involved in maintain the integrity of outward facing barriers such as the lung, the gut and the skin. Vitamin E, zinc, iron, selenium and copper all play a role in the systemic immune system. Fish oil type fatty acids can play a significant anti-inflammatory role, the opposite to immune boosting. So, focusing on one nutrient doesn’t seem to make sense. 

The case has been made that Vitamin D plays a particularly significant role in Covid. A first point to note is that vitamin D is a fat soluble vitamin which is preferentially stored in fatty tissue. As people become overweight and obese, blood vitamin D levels fall as it is shunted into fatty tissue. Body fat is among the highest drivers of Covid complications. A study published by the UK Biobank showed that whereas those with Covid had lower levels of blood vitamin D than those who were healthy, these differences disappeared when the data were adjusted to take account of confounding factors, of which obesity was the most important. 

A recent randomized clinical trial published in the Journal of the American Medical Association examined the effect of boosting Vitamin D status in Covid patients. One group received a single high oral dose of vitamin D which led to a doubling of their blood vitamin D levels. The control group received a placebo which had not effect on their blood vitamin D levels.  The treatment with vitamin D had no statistically significant effect on in-hospital mortality, admission to intensive care  or requirement for mechanical ventilation. Three recent UK reports from government advisory agencies (the National Institute for Clinical Excellence, the Scientific advisory Committee on Nutrition and Public Health England) concluded that there is presently no evidence to support a specific role for vitamin D in the prevention and treatment of vitamin D .  

Eating a healthy diet with lots of variety of food type and food colour, maintaining a healthy weight and keeping fit don’t make headlines. But they save lives.