This is an excerpt from Physiology of Yoga, The by Andrew McGonigle & Matthew Peter Huy.
There are many claims about different lifestyle interventions enhancing our immune system, but the concept of boosting immunity actually makes little sense scientifically. In fact, boosting the number of immune cells in your body is not necessarily a good thing. If your innate immune response was constantly stimulated, you would feel permanently unwell with a runny nose, fever, and lethargy. Inflammation has also been linked to depression (Haapakoski et al. 2016). Luckily there is no way to intentionally boost the innate immune system. It is also important to note that for the vast majority of us, the body already produces many more lymphocytes than it can possibly use, and the extra cells remove themselves through a natural process of cell death called apoptosis. On the whole, our immune system already does a remarkable job of defending us against disease-causing microorganisms without needing to be further enhanced. Although some lifestyle interventions have been found to alter some immune system components, currently there is no evidence that they actually boost immunity to the point where the person is better protected against infection and disease.
It is widely accepted that moderate exercise is good for us. Just like a balanced diet, exercise can contribute to general good health and therefore to maintaining a healthy immune system. Exercise promotes good blood circulation, which also helps the immune system to work optimally, and is one of the most widely studied behavioral interventions in terms of its immunomodulatory effects. Studies demonstrate an association between physical inactivity and low-grade systemic inflammation in healthy subjects, while regular exercise protects against diseases associated with chronic low-grade systemic inflammation (Petersen and Pedersen 2005). A study by Martin, Pence, and Woods (2009) suggested that moderate intensity exercise improves the immune response to respiratory viral infections. In a systematic review of the literature, Ploeger and colleagues (2009) investigated the effects of acute and chronic exercise on various inflammatory markers in patients with a chronic inflammatory disease. They reported that while training programs can reduce chronic inflammation in some patients, single bouts of exercise might elicit an aggravated inflammatory response. They suggested that the exercise training-induced response appears highly dependent on the type of disease; the severity of the disease; and the frequency, duration, and intensity of the exercise intervention. The authors also noted that the results of the review reveal a major gap in our knowledge regarding the evidence for safe but effective exercise for patients with a chronic inflammatory disease. A study by Haaland and colleagues (2008) also reported that the intensity of the exercise plays an important role. They suggested that strenuous exercise may cause acute immunologic changes (such as diminished natural killer cell activity), which may predispose to infection in certain individuals.
It is also widely recognized among the scientific community that people who are malnourished are more vulnerable to infectious diseases. However, whether the increased rate of disease is caused by the effect of malnutrition on the immune system is not certain. There are still relatively few studies on the effects of nutrition on the immune system of humans. While a systematic review by Rytter and colleagues (2014) concluded that the immunological alterations associated with malnutrition in children may contribute to increased mortality, the underlying mechanisms are still inadequately understood. The authors also noted that different types of malnutrition are associated with different immunological alterations. They suggested that better designed prospective studies are needed, based on current understanding of immunology and with state-of-the-art methods. In a systematic review and meta-analysis of randomized controlled trials looking at the role of multivitamins and mineral supplements in preventing infections in elderly people, the authors (El-Kadiki and Sutton 2005) concluded that the evidence for routine use of multivitamin and mineral supplements to reduce infections in elderly people is weak and conflicting. Only eight trials met their inclusion criteria, and owing to inconsistency in the outcomes reported, not all of the trials could be included in each meta-analysis. Much more good quality research is needed to give us a better understanding of the role that multivitamins and supplements play here. If you already have a balanced diet that meets the recommended amounts of nutrients, there is no need to take vitamin and mineral supplements, and doing so does not enhance your immune system. If you are malnourished in some way, then taking specific supplements may help you to reach recommended nutritional levels.
While it is important to reaffirm that moderate exercise and a balanced diet are good for us, it is also important to reiterate that there are currently no scientifically proven direct links between lifestyle interventions and enhanced immune function.