The competence of immune systems, like most other biological systems, tend to decline with age. This shows up as increased vulnerability of individuals 70 years of age and older to viral infections and poor antibody response to vaccination (1). However, emerging evidence is indicating that this loss of immune capability is not inevitable. In these days of coping with Covid-19, it is heartening to understand that there are steps we can take to optimize our immune systems so that they are in the best shape they can be to fight off this virus or any other similar health hazards that we may have the misfortune to encounter in the future.
What is the human immune system?
The human immune system is complex and multi-layered. Its workhorses are specialized white blood cells called lymphocytes along with macrophages and neutrophils that are continually on the lookout for health threats such as viruses, bacteria, fungi, parasites and even cancers. Our immunity has two divisions, the first being the innate immune system which is active from the moment of birth. It consists of physical barriers to infection (skin and mucous membranes) and non-targeted extermination of suspicious cells. The acquired immune system is more “intelligent” in that it “learns” which invaders are menacing, attacks and eliminates them, and then remembers them in case of future confrontations.
Cells of the innate immune system act quickly by sensing intruders with hazardous characteristics and immediately neutralizing them. Macrophages and neutrophils surround dangerous cells, engulfing and annihilating them. Specialized lymphocytes called natural killer cells identify treacherous cells, such as those infected by a pathogen or developing cancer cells, without any priming or prior activation and overcome the menace by forcing the cell to commit suicide (apoptosis) or by releasing chemicals into the attacking cells causing them to explode (2).
The majority of the cells of acquired (adaptive) immunity are lymphocytes called B cells and T cells. They can recognize “non-self” invaders (antigens) and generate specific responses to eliminate these impending dangers. B cells produce large quantities of antibodies created exclusively to fight a single antigen. The antibodies disable antigens, often by preventing their ability to bind to receptors on host cells. B cells also mark antigens for destruction, making it easier for other immune cells to locate and destroy them. T cells react directly against foreign antigens, frequently killing an infected cell before the pathogen has had a chance to replicate. During the battle, new specialty T cells are formed whose purpose is to save a lasting memory of the antigen, allowing a rapid targeted response if the same trespasser is detected again. Regular T cells live for about thirty days but memory T-cells hang around for years or even decades, carrying genes that have been epigenetically tagged with details of that specific antigen (3). The acquired immunity response can take a few days to activate and respond to a threat (4).
How do immune systems decline?
Though the human immune system generally continues to function to at least some degree throughout life, both the innate and the acquired immune system can be affected by aging, weakening their responses to threats. Signalling and activation of macrophages and neutrophils may become defective so that they are less able to destroy pathogens. The function of natural killer cells can be compromised and their numbers decreased (5,6). Production of B and T cells can also deteriorate and the quality and function of cells that are produced can diminish (1).
How can we prevent and reverse age-related immune system changes?
Well-nourished individuals do not experience the same age-associated declines in immune response as those who have a poor nutritional status (7). Studies have found that there are key nutrients that are essential for peak function of immune systems. For example, the presence of adequate protein, iron and zinc in the diet predicts better immune function in older individuals (8). Deficiencies of micronutrients (certain vitamins and minerals) as well as of macronutrients (energy and protein) are associated with dysregulation of immune responses in the elderly (9). Supplements containing zinc, selenium and vitamin E have shown an ability to reduce the risk of and reverse some of the immune dysfunction associated with advancing age (9,10).
A review from January 2020 examined the importance of certain nutrients in the functioning of the human immune system. Micronutrients (nutrients required in very small amounts) are active at every stage of the immune response, from the structural integrity of the skin and mucous membranes to the regulation of natural killer cells, the function of macrophages and neutrophils, the production of functioning B and T cells, the creation of antibodies and the function of the gut microbiome. Micronutrients identified in this review that play an important role in immunity include vitamins A, B6, B9 (folate), B12, C, D and E; the macro-mineral magnesium and trace minerals iron, zinc, copper and selenium (11). Daily intake of micronutrients is necessary to keep the immune system in optimal working order. At the end of this article you will find a list of prime food sources of each of these critical nutrients.
Vitamin D is increasingly revealing its importance in innate immunity and its ability to boost immune function against viral diseases. Vitamin D lowers inflammation levels and this particular ability may play a role in the development of cytokine storms, a complication associated with a wide variety of infectious and non-infectious diseases which has turned up in the progress of Covid-19 disease in some people. Cytokines are small proteins important for signalling and communication within the immune system. Emerging research suggest that cytokine storms can be caused by the body’s own immune system overreacting to a virus. In a cytokine storm, cytokines summon immune cells to attack an infection but, for some as yet unknown reason, they fail to turn off the alarm after the danger has passed. Cytokine storms can cause more damage to the host than to the invader it is trying to fight (12). Very recent evidence, from April 8, 2020, suggests that severe Covid-19 is associated with vitamin D deficiency and that normal levels of vitamin D may suppress the development of cytokine storms in Covid-19 patients. Investigation in this area is very preliminary and further exploration is warranted (13). However, other research corroborates this finding. An April 9, 2020 observational study from three South Asian hospitals found that vitamin D deficiency was associated with an eight-fold higher risk of suffering a more severe form of Covid-19 illness compared to patients with sufficient vitamin D (14). In addition, a meta-analysis from 2017 showed that vitamin D supplementation was linked to a significant reduction in respiratory tract infections (15).
Phytochemicals, nutrients found only in plants, also merit consideration in this discussion. Plant cells synthesize secondary metabolites that do not appear to be strictly necessary for the survival of the plant. Known as phytochemicals, these nutrients are produced as a defence for the plant in response to external stimuli such as infection and nutritional or climatic changes. When ingested by humans, phytochemicals show massive therapeutic potential including increasing immune system proficiency and boosting antibody production as well as antioxidant, anti-inflammatory, antithrombotic, antimicrobial, antidiabetic, anticancer, memory enhancing and cholesterol lowering activities. Inflammation and oxidative stress cause immune system consequences such as higher susceptibility to infections and hyperexcitation of the immune system with development of autoimmune disorders or other inappropriate immune overreactions. Phytochemicals are demonstrating strong protection of the immune system against inflammation and oxidative stress (16,17).
Long-term, moderate physical activity in the elderly is associated with several benefits including reduction in infectious disease risk; increased proliferation and effectiveness of T cells; increased antibody response to vaccination; and improvements in both physical and psychosocial aspects of daily living (7). A randomized study from 2012 looked at participants over the age of 50 and showed that an eight-week exercise program completed before the onset of the cold and flu season was associated with 35% fewer episodes of cold and flu and 47% fewer sick days (18). Mechanisms for these beneficial immune system effects remain to be identified (7).
Recent reviews have also recognized that benefits of exercise to the immune system depend on the type and amount of exercise being performed. Healthy quantities of exercise encourage the proliferation of immune cells such as T cells and neutrophils and higher cell-killing activity in natural killer cells. However, excessive exercise can cause a decline in the function of cells in the innate immune system that can result in an elevated risk of infection (19,20).
Implementing an Action Plan
Tweaking nutrition to benefit the immune system
The dietary intake of various micronutrients is inadequate worldwide, including both industrialized and developing countries. Evidence suggests that increased intake of some micronutrients above the Recommended Dietary Allowances (RDAs) might help to optimize immune function. Research has proposed a variety of choices for increasing micronutrient intake, including single nutrient supplements, multiple nutrient supplements and diet (11).
Tackling this issue one nutrient at a time is fraught with problems. Nutrient deficiencies do not occur in isolation. It takes multiple micronutrients acting in a synchrony based on their complementary modes of action to support effective immune function (11). Single nutrient supplements can result in over-supplementation and the possibility of upsetting the important balance between nutrients.
Consideration must also be given to the source and the form of the nutrient being used. Here are some examples of this. The form of selenium found in plants is more available to the body than that found in supplements. Folate, the naturally occurring form of vitamin B9, is more bioavailable than folic acid, the form found in supplements (11). Heme iron, the type found in animal sources, is absorbed more efficiently than the non-heme iron found in plants, but this does not always have the desired end result. Once iron has entered the body there are no mechanisms to get rid of any excess. Humans do however have a means to prevent excess iron absorption, though it only works effectively on non-heme iron, the type found predominantly in plant foods (21).
Bearing in mind all these factors, could diet be the preferred source of the nutrients needed to enhance the efficiency of our immune system? Recent studies are suggesting that this could be the case.
Research from 2010 studied over 1,000 pregnant women and discovered that those eating the most fruit and vegetables suffered 39% fewer viral respiratory infections, an outcome that appeared to arise from both fruits and vegetables instead of either food group alone (22).
An investigation from 2012 found that increased intake of fruits and vegetables improves measures of immune function. This 16-week-long randomized controlled trial was conducted in 83 healthy volunteers aged 65 to 85 who were eating two or fewer servings of fruits and vegetables a day. It compared the effect of increasing dietary fruits and vegetables to five servings a day compared to a control group eating two servings a day on measures of immune function in the participants. Results showed that those eating higher amounts of fruits and vegetables had significantly greater antibody responses to a vaccine against pneumococcal disease administered at 12 weeks into the study (23).
In 2016, a randomized, double-blind clinical trial showed that after four weeks of consuming added plant stanol esters, participants showed noteworthy increases in antibody production after vaccination against hepatitis A virus (24). Plant stanol esters are the plant equivalent of cholesterol (25).
Action plan for obtaining the nutrients required for a well-functioning immune system
Planning a diet aimed at increasing the competence of the immune system is not something that needs any particular expertise. The same foods that are required for proficient immunity are also known to increase health in general. Moreover, adding nutritious foods into the diet has only beneficial side effects.
If you examine the lists of food sources for the nutrients needed by the immune system, you will notice that many foods appear on multiple nutrient lists. For example, leafy greens show up as a good source of vitamin A, B6, B9, C, E, magnesium, copper and iron. Eating broccoli provides vitamin B6, B9, C, E, magnesium and iron. Legumes contain vitamin B6 and B9, magnesium, copper, iron, selenium, and zinc. Nuts and seeds provide vitamin B6 and E, magnesium, copper, iron, selenium and zinc. Even cocoa powder, which is a healthy powdered seed when it is not combined with sugar and fat, is a rich source of magnesium, copper, iron and zinc. In other words, you can eat one food to help satisfy the need for many of the important nutrients you’re looking for in the quest to establish an efficient immune system. Check out these lists and identify the foods that you feel would be a good addition to your diet. Plan to include them in your meals every day.
On top of these already identified nutrients, we can’t forget the many benefits of phytochemicals on the immune system. At the end of this article you’ll also find a list of some important food groups whose phytochemicals are specifically helpful in immunity. Consider also choosing foods from each of these groups as part of your daily food intake.
Tweaking exercise to benefit the immune system
Immune response depends on the intensity and duration of exercise. It is now well understood that not exercising is detrimental to health but it is becoming clear that too much exercise can be negative as well (19).
Current exercise recommendations in many countries suggest that adults should accumulate at least 150 minutes of moderate-to-vigorous-intensity aerobic physical activity every week (30 minutes daily on 5 days a week) in bouts of 10 minutes or more. Moderate-intensity activity is defined as an activity that somewhat increases breathing and induces light sweating (for example, brisk walking at a rate of about 6.5 km/hr or cycling on the flat); vigorous-intensity activity will result in heavier sweating and being “out of breath” (for example jogging or cross-country skiing) (26).
This recommended amount of exercise should be considered the minimum of exercise for health. Studies have found that 150 minutes of moderate-intensity exercise can lower overall mortality rate by 7%. But enjoying more minutes of such exercise is even more beneficial. Walking for 300 minutes a week (60 minutes daily on 5 days a week) can drop overall mortality by 14%. Further increasing that time to 450 minutes a week (90 minutes daily on 5 days a week) decreases mortality rate by 24% (27).
This does not mean that the more time you spend exercising and the more intensity you put into it the healthier it is. There is a sweet spot where the amount of exercise is just right to improve immune defences. Moderate-intensity exercise improves immunity while extreme effort reduces it (28).
Action plan for achieving the exercise required for a well-functioning immune system
How much exercise should you strive for in your life? Choose a moderate-intensity exercise such as brisk walking. Plan to walk five days a week for at least 30 minutes. If you have enough time to increase the length of your walk to 60 minutes, or even 90 minutes, then you will reap more benefits. If you would rather indulge in a more strenuous activity, then keep your weekly exercise time closer to 150 minutes.
Summing it all up…
Aim for a diet consisting of a wide variety of fruits and vegetables. Be conscious of the food groups that will provide the nutrients needed for a robust immune system and try to include them in your diet every day. The beauty of eating foods that improve the immune system is that the same foods enhance health in general.
Pick an enjoyable physical activity that you can do daily. Don’t stress about the length of time available to you for exercise. Remember that it should not be difficult to take 30 minutes out of most days to enjoy a walk or do another activity that you enjoy. If you can find extra time, then by all means make your exercise last longer.
Your lifestyle is an important deciding factor for how well you will be able to fight off illness over your lifetime. A healthy immune system is a significant way to help you through the present pandemic and ready yourself for future epidemics.
Lists of Top Food Sources of Nutrients Required by the Immune System
A: The RDA (Recommended Dietary Allowance) of a nutrient is the “average daily dietary intake level that is sufficient to meet the nutrient requirement of nearly all (97% to 98%) healthy individuals in a particular life-stage and gender group”. It is easy to look up this reference on-line if you are wondering about RDAs for other age groups (29).
B: The aim for optimizing the immune system is to consider the RDA as the minimum daily intake for each nutrient. Obtaining excess amounts of these nutrients from the diet is generally not a problem, although, there are exceptions as are noted in these lists.
C: Sources for the amounts of nutrients in each food listed are nutritiondata.self.com (30) and ods.od.nih.gov/factsheets/ (31).
D: Where applicable, animal sources of each nutrient will also be listed.
Best Food Sources of Vitamin A
RDA is 900 mcg for adult males and 700 mcg for adult females
½ cup carrots, cooked – 3207 mcg
½ cup butternut squash, baked – 3430 mcg
½ medium sweet potato – 3286 mcg
½ cup mustard greens, cooked – 4426 mcg
½ cup mustard greens, raw – 2940 mcg
½ cup chopped turnip greens, raw – 3186 mcg
½ cup chopped turnip greens, cooked – 1200 mcg
½ cup spinach, cooked – 2830 mcg
½ cup spinach, raw – 422 mcg
½ cup chopped kale, cooked – 2656 mcg
½ cup chopped kale, raw – 1545 mcg
½ cup swiss chard, cooked – 1607 mcg
½ cup swiss chard, raw – 1101 mcg
½ cup beet greens, raw – 1202 mcg
½ cup beet greens, cooked – 1352 mcg
½ cup collards, raw – 1200 mcg
¼ cup collards, cooked – 1156 mcg
½ cup shredded romaine lettuce – 614 mcg
3 ounces beef liver – 6582 mcg
1 large egg, boiled – 75 mcg
3 ounces sockeye salmon, cooked – 59 mcg
Best Food Sources of Vitamin B6
RDA for males is 1.3 mg daily age 14 to 50 and 1.7 mg daily over 50
RDA for females is 1.3 mg daily age 18 to 50 and 1.5 mg daily over 50
1 cup yellow corn – 1 mg
1 cup brown rice – 1 mg
1 cup chickpeas – 1.1 mg
1 banana – 0.8 mg
½ cup prunes – 1 mg
½ cup pistachios – 1.1 mg
½ cup hazelnuts – 0.8 mg
½ cup sunflower seeds – 0.5 mg
½ cup walnuts – 0.3 mg
1 medium baked potato – 0.4 mg
1 cup broccoli or broccoli raab, cooked – 0.4 mg
1 cup shredded red cabbage, raw – 0.4 mg
1 cup spinach, cooked – 0.4 mg
1 cup spinach, raw – 0.1 mg
1 cup turnip greens, cooked – 0.3 mg
1 cup lentils, cooked – 0.4 mg
¼ cup garlic – 0.4 mg
¼ cup peanut butter – 0.4 mg
3 ounces beef liver – 0.9 mg
3 ounces Atlantic salmon, cooked – 0.6 mg
3 ounces sockeye salmon, cooked – 0.6 mg
3 ounces chicken breast – 0.5 mg
3 ounces ground beef – 0.3 mg
Best Food Sources of Vitamin B9 (folate)
RDA is 400 mcg for adult males and females
1 cup edamame – 482 mcg
1 cup cooked navy beans – 255 mcg
1 cup asparagus -268 mcg
1 cup Brussels sprouts, cooked – 156 mcg
1 cup collards, cooked – 177 mcg
1 cup collards, raw – 60 mcg
1 cup spinach, cooked – 263 mcg
1 cup spinach, raw– 58 mcg
1 cup turnip greens, cooked – 170 mcg
1 cup turnip greens, raw – 107 mcg
1 cup mustard greens, cooked – 102 mcg
1 cup mustard greens, raw – 105 mcg
1 cup broccoli, cooked – 103 mcg
1 cup arugula, raw – 20 mcg
1 egg, hard-boiled – 22 mcg
3 ounces halibut, cooked – 12 mcg
1 cup 1% milk – 12 mcg
3 ounces ground beef – 7 mcg
3 ounces chicken breast – 3 mcg
Best Food Sources of Vitamin B12
RDA is 2.4 mcg for adult males and females
Vitamin B12 is an unusual vitamin. It is produced by microorganisms that live in the soil, in natural water courses and in the intestines of humans and animals. In these modern days of ultra clean food and water, we are no longer obtaining vitamin B12 from dirty vegetables and it is produced too far along in our own intestines for us to take advantage of it. Meat-eaters can acquire B12 from the meat they eat but studies show that this does not always translate into sufficient B12 blood levels for health (32,33). It is suggested that, unless you have had your B12 blood levels tested and know that they are at healthy levels, all people over the age of 50 should take a B12 supplement of around 1000 mcg once a week. This amount is much higher than the RDA, however, B12 has a complicated absorption process and this amount ensures that you will get enough. Extra B12 appear to be not harmful and is easily excreted from the body (34,35).
Best Food Sources of Vitamin C
RDA is 90 mg for adult males and 75 mg for adult females
1 cup chopped yellow peppers, raw – 340 mg
1 cup chopped sweet red peppers, cooked – 232 mg
1 cup chopped sweet red peppers, raw – 180 mg
1 cup kiwi fruit – 164 mg
1 cup chopped sweet green peppers, raw or cooked – 120 mg
1 cup broccoli, cooked- 102 mg
1 cup broccoli, raw – 82 mg
1 cup sectioned orange – 98 mg
1 cup Brusssels sprouts, cooked – 96 mg
1 cup halved strawberries – 90 mg
1 cup cubed papaya – 87 mg
1 cup chopped kale, raw – 80 mg
1 cup sectioned grapefruit – 72 mg
1 cup red cabbage, cooked or raw – 52 mg
1 cup chopped cauliflower, raw – 46 mg
1 cup chopped turnip greens, cooked – 40 mg
1 cup chopped turnip greens, raw – 33 mg
1 cup Swiss chard, cooked – 32 mg
1 cup chopped cauliflower, cooked – 28 mg
Best Food Sources of Vitamin D
RDE is 15 mcg for adult males and females; 20 mcg for those over 70
Vitamin D is actually a hormone and is produced in our bodies upon exposure to sunlight. The sun’s rays provide ultraviolet B (UVB) energy, and the skin uses those rays to produce a precursor of vitamin D that is converted into the active form of the vitamin by the liver and kidneys. Vitamin D has many roles in the body including the promotion of calcium absorption, modulation of immune function, cell growth and reduction of inflammation. Very few foods in nature contain vitamin D. Most people meet at least some of their vitamin D needs through exposure to sunlight. Under the right circumstances, 10 to 15 minutes of sun on the arms and legs a few times a week can generate all the vitamin D that we need. Unfortunately, the “right circumstances” are elusive; they change with the season, the time of day, the cloud cover and pollution, the colour of your skin and the latitude in which you live. Even in northern latitudes it is possible to obtain enough exposure to sunlight to form vitamin D in the winter, however, this would not happen spontaneously but would require determination to expose enough skin for a long enough time. Excess Vitamin D is stored in the fat and the liver and can help provide the vitamin during times of less sunlight. Vitamin D production also decreases with age. It is possible to obtain a blood test to let you know if you are someone who should take a supplement. In any case, experts recommend a supplement of 800 to 1,000 IU (1 IU = 0.025 mcg) daily in the winter if you live north of 37° north latitude. This includes ALL Canadians (36,37).
Food sources of Vitamin D:
1 tablespoonful cod liver oil – 34 mcg
3 ounces farmed rainbow trout, cooked – 16.2 mcg
3 ounces sockeye salmon, cooked – 14.2 mcg
½ cup white sliced mushrooms, raw or cooked, exposed to UV light – 9.2 mcg
1 cup 2% milk, fortified – 2.9 mcg
1 cup soy, almond or oat milk, fortified – 2.5 to 3.6 mcg
1 large scrambled egg – 1.1 mcg
3 ounces chicken breast, roasted – 0.1 mcg
Best Food Sources of Vitamin E
RDA is 15 mg for adult males and females
½ cup almonds, dry-roasted – 36 mg
½ cup sunflower seeds, roasted – 25 mg
½ cup hazelnuts – 19.6 mg
1 cup blueberries, wild – 4 mg
1 cup spinach, cooked – 3.7 mg
1 cup spinach, raw – 0.6 mg
1 cup tomato sauce – 3.5 mg
1 cup chopped Swiss chard, cooked – 3.3 mg
1 cup packed Swiss chard, raw – 1.4 mg
2 tablespoons peanut butter – 2.9 mg
1 cup broccoli raab, cooked – 2.8 mg
1 cup turnip greens, cooked – 2.7 mg
1 cup asparagus, cooked – 2.6 mg
1 cup butternut squash, cooked – 2.6 mg
1 cup beet greens, cooked – 2.6 mg
1 cup beet greens, raw – 0.6 mg
1 cup chopped sweet red peppers, raw – 2.4 mg
1 cup sweet red peppers, cooked – 1.5 mg
1 cup broccoli, cooked – 2.4 mg
1 cup mustard greens, cooked – 1.7 mg
1 cup mustard greens, raw – 1.1 mg
1 cup blackberries, raw – 1.7 mg
1 cup carrots, cooked – 1.6 mg
1 cup mangos, sliced – 1.4 mg
1 cup kale, cooked – 1.1 mg
1 kiwi fruit, medium – 1.1 mg
1 cup shredded radicchio – 0.9 mg
1 cup chopped collards, raw – 0.8 mg
1 cup tomatoes, raw – 0.8 mg
Best Food Sources of Magnesium
RDA is 420 mg for adult males; 320 mg for adult females
½ cup almonds, dry-roasted – 320 mg
½ cup cashews, dry-roasted – 296 mg
½ cup peanuts, oil-roasted – 126 mg
1 cup chopped spinach, cooked – 157 mg
1 cup spinach, raw – 23.7 mg
1 cup chopped Swiss chard, cooked – 150 mg
1 cup chopped Swiss chard, raw – 29.2 mg
1 cup black beans, cooked – 120 mg
1 cup edamame, cooked – 100 mg
¼ cup cocoa powder – 108 mg
1 cup beet greens, cooked – 97.9 mg
1 cup beet greens, raw – 26.6 mg
1 cup brown rice, cooked – 84 mg
1 cup kidney beans, canned – 70 mg
1 cup soy milk – 61 mg
1 cup kale, raw – 59 mg
2 tablespoons peanut butter – 49 mg
1 medium potato, baked with skin – 43 mg
1 medium banana – 32 mg
1 cup chopped broccoli, cooked – 24 mg
½ cup raisins – 23 mg
1 slice whole-wheat bread – 23 mg
1 cup arugula – 9.4 mg
1 medium apple – 9 mg
1 medium carrot, raw – 7 mg
1 medium zucchini – 3.6 mg
3 ounces Atlantic salmon, farmed, cooked – 26 mg
1 cup milk – 24 mg
3 ounces halibut, cooked – 24 mg
3 ounces chicken breast, roasted – 22 mg
3 ounces ground beef – 20 mg
Best Food Sources of Copper
RDA is 900 mcg for adult males and females
¼ cup cocoa powder – 2200 mcg
¼ cup cashews, dry-roasted – 1258 mcg
¼ cup sunflower seeds, toasted – 615 mcg
1 cup potato, cooked – 800 mcg
1 cup asparagus, raw – 700 mcg
1 cup tomato puree – 700 mcg
½ cup mushrooms, shiitake – 600 mcg
½ cup mushrooms, brown or crimini – 200 mcg
1 cup chickpeas, cooked – 578 mcg
½ cup tofu, firm, raw – 476 mcg
1 ounce dark chocolate, 70 to 85% dark solids – 501 mcg
1 cup beet greens, cooked – 400 mcg
1 cup beet greens, raw – 100 mcg
1 cup Swiss chard, cooked – 300 mcg
1 cup Swiss chard, raw – 100 mcg
1 cup spinach, cooked – 300 mcg
1 cup chopped kale, raw – 200 mcg
1 cup asparagus, cooked – 298 mcg
1 cup whole-wheat pasta, not packed, cooked – 263 mcg
½ cup avocado, raw – 219 mcg
1 cup chopped tomatoes, raw – 110 mcg
1 cup shredded radicchio – 100 mcg
3 ounces beef liver – 4000 mcg
3 ounces oysters – 3600 mcg
3 ounces lobster, cooked – 1600 mcg
3 ounces crab, cooked – 624 mcg
3 ounces Atlantic wild salmon – 273 mcg
Best Food Sources of Iron
RDA is 8 mg daily for adult males; 18 mg daily for females from 18 to 50; 8 mg daily for females over 50
¼ cup cocoa powder – 20.2 mg
3 ounces dark chocolate, 45 to 69% dark solids – 7 mg
1 cup potatoes, boiled – 13.6 mg
1 cup white beans, cooked – 8 mg
1 cup lentils, cooked – 6 mg
1 cup kidney beans, cooked – 4 mg
1 cup chickpeas, cooked – 4 mg
1 cup spinach, cooked – 6.4 mg
1 cup Swiss chard, cooked – 4 mg
1 cup Swiss chard, raw – 0.6 mg
1 cup beet greens, cooked – 2.7 mg
1 cup beet greens, raw – 1 mg
¼ cup cashews, oil-roasted – 4 mg
¼ cup pistachios, dry-roasted – 2 mg
1 cup tomatoes, cooked – 4 mg
½ cut tofu, firm – 3 mg
1 cup green peas, boiled – 2 mg
1 cup broccoli, cooked – 2 mg
1 tbsp turmeric – 2.8 mg
1 tbsp oregano, dried – 1.2 mg
1 tbsp cilantro, fresh – 0.7 mg
1 tbsp sage, dried – 0.6 mg
¼ cup green onions – 1 mg
1 slice whole-wheat bread – 1 mg
1 cup brown rice, cooked – 1 mg
¼ raisins, seedless – 1 mg
1 cup whole-wheat spaghetti, cooked – 1 mg
1 cup red leaf lettuce, shredded – 0.3 mg
3 ounces oysters, cooked – 18 mg
3 ounces beef liver, cooked – 5 mg
3 ounces beef, cooked – 2 mg
3 ounces chicken, roasted – 1 mg
1 egg, hard-boiled – 1 mg
Best Food Sources of Selenium
RDA is 55 mcg for adults; upper daily safe limit is 400 mcg
It is noteworthy that Brazil nuts are extremely high in selenium and eating too many of them can lead to toxicity. The amount of selenium per nut is also quite variable. Limit intake to a maximum of four or five nuts a week.
1 Brazil nut – 96 mcg
¼ cup sunflower seeds, dry-roasted – 26 mcg
1 cup brown rice, cooked – 19 mcg
1 slice whole-wheat bread – 13 mcg
1 cup baked beans – 13 mcg
1 cup oatmeal, cooked – 13 mcg
½ cup mushrooms, brown or crimini – 11. 3 mcg
1 cup spinach, cooked – 10 mcg
¼ cup cashews, dry-roasted – 6 mcg
¼ cup walnuts – 6 mcg
1 cup lentils, cooked – 5 mcg
1 cup green peas, boiled – 2 mcg
1 cup bananas, mashed – 2 mcg
1 medium potato, baked with skin – 1 mg
3 ounces halibut, cooked – 47 mcg
3 ounces ham, roasted – 42 mcg
3 ounces beef, roasted – 33 mcg
3 ounces chicken, light meat, roasted – 22 mcg
1 egg, hard-boiled, large – 15 mcg
Best Food Sources of Zinc
RDA is 11 mg daily for males; 8 mg daily for females; upper safe daily limit is 40 mg
A – Zinc toxicity can occur from a relatively small amount of zinc. However, excess amounts of zinc are unlikely to occur through the diet. If taking a supplement, exercise caution so as not to exceed 40 mg of zinc daily (38).
B – Nutrients known as phytates are present in whole-grain breads, cereals and legumes. Phytates can bind to zinc and inhibit its absorption. To increase zinc bioavailability, soak foods such as beans, grains and seeds in water for several hours before cooking. Leavening products such as yeast break down phytates so consuming leavened grain products also increases the availability of zinc from the diet (38).
Food sources of zinc;
¼ cup pumpkin seeds, dried – 4.4 mg
¼ cup sunflower seeds – 4 mg
¼ cup cashews, dry-roasted – 3.2 mg
¼ cup peanut butter – 2.6 mg
¼ cup almonds, dry-roasted – 1.8 mg
¼ cup hemp seeds – 3 mg
2 slices whole-wheat bread – 2 mg
2 tbsp flaxseed – 1.2 mg
2 tbsp chia seeds – 1 mg
1 cup tofu, firm – 4 mg
1 cup baked beans – 5.8 mg
1 cup adzuki beans, cooked – 4.1 mg
1 cup lentils, cooked – 3 mg
1 cup chickpeas, cooked – 2.6 mg
1 cup black beans, cooked – 1.9 mg
1 cup kidney beans, cooked – 1.8 mg
1 cup oatmeal, cooked – 2 mg
¼ cup cocoa powder – 1.5 mg
½ cup mushrooms – brown, crimini – 1.3 mg
½ cup shiitake mushrooms, cooked – 1 mg
1 cup peas, frozen, cooked – 1 mg
3 ounces oysters, cooked – 74 mg
3 ounces Alaskan King crab – 6.5 mg
3 ounces beef chuck roast – 7 mg
3 ounces loin pork chop – 2.9 mg
3 ounces chicken, dark meat – 2.4 mg
Best Food Sources of Phytochemicals
Cruciferous Vegetables (broccoli, broccoli raab, cauliflower, kale, Brussels sprouts, arugula, cabbage, watercress, turnips, rutabaga, kohlrabi, cabbage, Chinese cabbage, collard greens, Chinese broccoli, bok choy, horseradish)
Cruciferous vegetables are part of the Brassicaceae (also called Cruciferae) family of plants. They are rich in glucosinolates which stimulate the immune system, prevent oxidative stress and reduce cancer risk (39). In addition, cruciferous vegetables are rich in a dietary compound that maintains the population of intraepithelial lymphocytes in our intestines. These lymphocytes are a specialized type of white blood cells that act as the first line of defence for the thin membrane lining our gut, producing antibodies against pathogens and repairing the gut lining if it is damaged (40).
Cruciferous vegetables are best eaten raw as their health properties are susceptible to heat.
Alliums (onions, garlic, scallions, shallots, leeks, chives)
Allium plants are rich in organosulfur compounds which are beneficial for immunity.
Garlic appears to enhance the immune system through stimulation of immune cells such as macrophages, lymphocytes and natural killer cells (41).
Onions contain quercetin which is able to modulate the immune system (42).
About 70% of our immune system resides in and around our gut. Whenever the beneficial bacteria in the gut microbiome are encouraged, our immune system benefits. Onions are high in inulin, a prebiotic fiber that is a preferred food for the beneficial bacteria living in our gut microbiome. These bacteria produce short chain fatty acids (SCFAs), important nutrients that we cannot produce on our own. Fiber, such as inulin from onions, nourishes the bacteria, which produce SCFAs and these in turn have positive effects throughout the body, supporting a well-functioning immune system, discouraging pro-inflammatory bacteria, encouraging absorption of beneficial minerals and other nutrients, improving insulin sensitivity and preventing “leaky gut” (43).
When it comes to alliums, only a small amount is needed. A half of a garlic clove or one slice of onion is enough to confer benefit.
Phytochemicals in alliums are activated when the vegetable is cut into, so let the pieces sit after cutting for about ten minutes so that their nutrients can develop more potency.
Whole grains and legumes
These two food types are excellent sources of fiber for a healthy gut microbiome along with vitamin B and zinc that have proven immune function benefits.
Anthocyanins are potent antioxidant phytochemicals that are associated with reduced risks of cancer, central nervous system disorders and immunity problems (44). Anthocyanins are found in red and purple fruits and vegetables but they are highest in berries such as wild blueberries, black and red raspberries, blackberries, Açaí berries and black currants.
A 2011 study found that ingestion of 250 gm of blueberries daily for 6 weeks resulted in increased numbers of natural killer cells (45).
Research from 2016 discovered that blueberry consumption decreased the incidence of upper respiratory tract infections by 33% compared with a control. Also, the total number of sick days was decreased by 40% (46).
The main active phytochemical in green tea is a catechin called epigallocatechin-3-gallate (EGCG). EGCG enhances immune function through a protective effect on the innate immune response to viral infection (47). EGCG also has benefits for the reduction of blood pressure, increasing blood flow to the brain and enhancing cognition.
Drink at least 1 cup of green tea every day to gain its benefits.
The membrane lining our gut is only one cell thick. Its first line of defence against toxins entering the body from the outside world is an antibody called IgA (immunoglobulin A). Consumption of mushrooms has resulted in a 50% increase in IgA secretion after eating about a cup of cooked white button mushrooms for a week. This increased IgA level remained for a further week after stopping the ingestion of the mushrooms (48). As an added bonus, mushrooms are anti-inflammatory, a benefit that seems to come from the phytochemical, pyrogallol, that they contain (49).
It appears that even a very small amount of mushroom (about 1 ounce) has significant benefits.
Include mushrooms in your meals a few times a week.
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This article contains such a fountain of wisdom and easy to follow suggestions. Well written!
Thanks, Heather! I’m so happy that you are enjoying these articles.
I always love to read your articles Deb. I learn so much… Thanks
Thanks Ruth. I’m very happy that you enjoy them!