Nutrition and Immune Function

Every winter, and during every outbreak, sales of immune supplements soar. Vitamin C, zinc, elderberry, echinacea and a long list of combination products promise to strengthen defences. Behind the marketing lies a genuine and important relationship: nutrition profoundly affects immune function. But the relationship is not linear. Correcting deficiency makes a large difference; adding more to an adequate diet usually makes little. Understanding where the evidence lies allows practitioners to focus on what matters.

Malnutrition: the biggest immune problem

Worldwide, the most common cause of immune deficiency is not a genetic disorder or HIV but malnutrition. Protein energy malnutrition impairs barrier function, reduces the number and function of T cells, weakens antibody responses and blunts innate immunity. In children in low income settings, the combination of malnutrition and infection drives a large share of preventable deaths, and each worsens the other: infection increases nutrient requirements and reduces appetite and absorption, while malnutrition increases susceptibility to infection.

In high income countries, malnutrition is less visible but still common, particularly among older adults, people with chronic disease, hospital patients and people with eating disorders or food insecurity. Undernutrition in these groups is associated with more frequent and severe infections and poorer responses to vaccination.

Micronutrients and immunity

The iron example deserves emphasis. Because pathogens also need iron, the body withholds it during infection. A large trial in a malaria endemic area found that giving iron and folic acid to young children regardless of their iron status increased the risk of hospitalisation and death. Supplementation is best targeted to confirmed deficiency, particularly where infections are common.

Obesity and immunity

Obesity is associated with chronic low grade inflammation and impaired immune responses. People with obesity have higher risks of severe influenza and COVID 19, more surgical site infections and, for some vaccines, weaker antibody responses. Improving metabolic health is therefore part of supporting immune function.

Ageing, nutrition and immunity

Immune function declines with age, a process called immunosenescence, while background inflammation increases. Older adults are also at higher risk of undernutrition, protein deficiency, vitamin D deficiency and low zinc status. Ensuring adequate protein and energy, correcting deficiencies and maintaining physical activity are practical ways to support immunity in later life.

The gut and the microbiome

A large part of the immune system is located in the gut, where it is shaped by diet and microbes. Fibre rich diets support microbes that produce short chain fatty acids with immune regulatory effects, and a fermented food diet has been shown in a randomised study to reduce markers of inflammation. Probiotics have been studied for preventing respiratory infections; systematic reviews suggest a possible modest reduction, but the evidence is of low quality and effects vary by strain.

Popular immune products

• Echinacea: trials have produced inconsistent results, with at most small effects on colds.
• Elderberry: small trials suggest possible benefit for colds and influenza symptoms, but evidence is limited and product quality varies.
• Garlic: insufficient evidence for preventing colds.
• High dose multivitamins: little evidence of benefit in well nourished adults.

These products are generally low risk, but they should not replace the fundamentals, and their costs can add up.

Nutrition and vaccine responses

Nutritional status influences vaccine responses. Malnutrition, obesity and some micronutrient deficiencies have been associated with weaker antibody responses. Sleep also matters: short sleep around the time of vaccination has been associated with reduced antibody responses in several studies.

A practical approach

1. Assess nutritional status, particularly in older adults, people with chronic disease and people with restrictive diets.
2. Ensure adequate protein and energy.
3. Correct confirmed deficiencies of vitamin D, zinc, iron, B12 and others.
4. Encourage a diverse, fibre rich diet with fruit, vegetables and fermented foods.
5. Support metabolic health and a healthy weight.
6. Prioritise sleep, activity and stress management.
7. Recommend vaccination as the most effective immune intervention.
8. Discuss immune supplements honestly, noting where evidence exists and where it does not.

An illustrative case

An 81 year old man living alone has had 3 respiratory infections in the past winter and has lost 5 kg. He has been buying several immune supplements. His diet consists mostly of tea, toast and soup. His doctor finds low vitamin D, a low normal albumin and a low zinc level. The plan focuses on protein and energy: meal delivery, fortified foods and an oral nutritional supplement. His vitamin D is corrected, zinc replaced for a limited period, and his vaccinations brought up to date. A physiotherapist starts a simple strength programme. His immune supplements are stopped. The following winter he has one mild cold and his weight has recovered.

Nutrition first

The most powerful nutritional interventions for immunity are the least glamorous: enough protein and energy, correction of deficiencies, a varied diet and a healthy weight. Supplements have modest roles in specific situations. The practitioner’s job is to make sure the foundations are in place before reaching for products.