Of all of our biological systems, the immune system is one of the most complex and amazing. Given that we have daily exposure to countless bugs and viruses it is incredible that we don’t get sick more often than we do, all thanks to our immune system.

As a graduate student I was intrigued by how the immune system worked and specifically, how it responded to training and acute bouts of exercise. I was curious as to why athletes seemed to be more susceptible to getting colds and other upper respiratory illnesses. As an endurance athlete, this also had particular relevance to me.

Exercise and Salivary IgA

IgA is a class of antibodies that is produced in the mucosal system and can be measured in tears and saliva. It is our first line of defense against airborne pathogens, such as those which cause upper respiratory infections (URI). As part of my masters and doctoral research I put college students through various intensities, modalities and durations of exercise and measured their IgA levels pre and post (yes I’ve collected a lot of spit in my time). What we found was that following light to moderately intense exercise, IgA levels did not change.1 However, following very intense (exhaustive) exercise IgA levels were suppressed (up to 24%) for some time post exercise .2 This suggested that following intense exercise there is a period of time (in this case hours) where one could be more susceptible to getting sick. To this day, I don’t exercise intensely prior to getting on an airplane because of that reason.

My advisor also looked at IgA levels of the Nebraska swim team and found that resting IgA levels were chronically depressed over the course of their training season.3 Other studies have found that extended endurance exercise (such as running an ultramarathon) also resulted in depressed levels of IgA and subsequent increased risk of getting an URI. Some studies, however, have also shown that, while post exercise levels are reduced in athletes, resting levels in some athletes are higher on average than those who are sedentary. Some types of exercise, like Tai Chi and resistance training also resulted in higher levels post exercise. The long and the short of it is that when appropriately challenged, the immune system may adapt and improve as a result of exercise, albeit, exercise that isn’t chronically intense. The thinking is that there is a J curve response, too little exercise results in poorer immune function, while excessive bouts of intense exercise may temporarily compromise it.4

What I didn’t know at the time was that diet and certain foods can also affect immune function.

Food and Immune Function

Magic Mushrooms

In a 2012 study researchers had a group of subjects eat blanched white button mushrooms (100g) every day for a week. They then measured their secretion of salivary IgA. Compared to a control group, the mushroom eating group had a 53% increase in their secretion of IgA. IgA secretion further increased by 56% in the 2nd week (after they had stopped eating the mushrooms) and then it dropped to baseline levels by the 3rd week. An added bonus is that mushrooms also exhibit anti-inflammatory properties. Scientists think this is due to a phytochemical in the mushrooms called Pyrogallol.6 Pyrogallol is also found in Amla (or dried Indian Gooseberry) which has also been shown to be anti-inflammatory.7 Based on these findings it is thought that Amla could be helpful in fighting inflammatory lung conditions like asthma.

In a different study, researchers took a group of older adults and had one group increase their consumption of fruits and vegetables per day from an average of 2 or less to 5 or more. They then injected them with the Pneumovax vaccine and measured the immune response. Those eating more fruits and vegetables had a significantly greater immune response to the vaccination.8 This is actually one of the best ways to measure immunity as it measures how well it responds when challenged.

Nutritional Yeast

Enter another immune boosting food, nutritional yeast. Nutritional yeast (or its less tasty cousin, Brewer’s yeast) contains a type of starch called Beta GLUCAN (BG). In fungal sources like mushrooms and yeast, it is in a branched chain form which has been shown to be more biologically active. Researchers tested 10 days of supplementation with nutritional yeast and its effects on immune function in recreational athletes before and after a hard bike ride in hot and humid conditions (heat and humidity further stress the immune system). When they supplemented with BG or nutritional yeast monocyte circulation improved (as compared to a placebo trial) as did LPS stimulation (which mimics stimulation by a bacterial challenge).9 In other words, immune function was enhanced in the group supplementing with the yeast.

To test if this actually resulted in fewer infections, marathon runners were supplemented with BG for 4 weeks starting the day after running a marathon.10 Those who supplemented had significantly fewer upper respiratory infections (24% for the placebo group vs. 8% for the BG group) and upper respiratory symptoms (68% for the placebo vs. 24% for BG group). Those in the BG supplemented group also reported having better overall health status (58% higher) and better mood (lower tension, less anger, less fatigue and better vigor) compared to the placebo group. Note: they supplemented with 500 mg or about a spoonful of nutritional yeast a day. For my Magic Vegetable Smoothie which incorporates nutritional yeast, go here.

All Hail to the Kale

And finally, enter kale. When kale extracts were dripped on white blood cells antibody (IgM and IgG) production was increased in a dose-dependent manner (i.e. the more kale the higher the production up to a certain point). This was true for both raw and cooked kale. Subsequent testing also found this occurred in vivo, albeit in mice. In mice, kale extract was also found to stimulate IgA production.11 While this has yet to be tested in humans, there are indications that other cruciferous vegetables like broccoli are immunostimulatory.12

Vitamin C

As to how well vitamin C can protect us the common cold has been a topic of heated debate ever since Linus Pauling (a Nobel winner) wrote his book Vitamin C and the Common Cold.13 Dr. Pauling personally suffered from frequent colds and found that supplementation with vitamin C seemed to reduce the frequency. Subsequent studies, however, did not support the idea that vitamin C can reduce the incidence of colds, albeit with some exceptions. In one pooled analysis of 29 studies there was a subgroup of 6 studies, all on athletes--runners, skiers and soldiers. In this group, the incidence of getting a cold was halved in those taking vitamin C. The other studies did not show an effect. However, in a later study that lasted over a longer period of time (5 years) those supplementing with 500 mg of vitamin C had a 66% reduced risk of getting 3 or more colds over that time period.

What is clear is that vitamin C may reduce the duration and intensity of cold symptoms. Reported reductions in duration are in the range of 8-18%.14 

There is a biologically plausible reason as to why vitamin C can be helpful. In addition to its antioxidant effects it also has antihistamine effects.15 This can be helpful when fighting congestion and may also help with conditions like asthma and allergies. Vitamin C is also found in phagocytes and lymphocytes (our first line of defense). Finally, it can boost production of interferon. Interferon is a virus fighting chemical.

Vitamin C is not found in eggs, meat, dairy or chicken (I have yet to find a study which shows that these foods boost immunity). It is only found in fruits and vegetables. Just how much do we need? Most animals make their own vitamin C, but humans don’t. According to Dr Steve Blake, a nutritional biochemist, dogs and cats make the least, about 2,100mg per day, while mice make about 20,000mg (all adjusted to 150 lb).16 (in his Nutrition for Immune Power presentation). In a diet rich in fruits and vegetables you can get a good dose (300-500mg) but it couldn’t hurt to supplement at certain times (like during times of hard training or high stress). But use the ascorbated version as that is easier on the stomach than the ascorbic versions.

Vitamin A

Vitamin A is found in yellow, orange, red and green foods. These help to strengthen the mucous membranes and promote the secretion of healthy mucous. It also has numerous immuno-enhancing effects.17 The best way to get vitamin A is from food, not supplements. There are many precursors to vitamin A found in foods--carotenoids, lutein and lycopene are a few. These are also good at quenching free radicals which is a good thing as production of free radicals is increased during times of infection. Pre-formed vitamin A, as found in supplements and some animal foods, does not have antioxidant benefit and can be toxic when taken in excess.

In conclusion

A diet rich in fruits and nutrient rich vegetables of all colors will go a long way towards improving your immune function. Other foods like mushrooms and nutritional yeast appear to help as well. Supplementing with vitamin C on occasion couldn’t hurt either.