Protect Our Immune System During This Difficult Time – Part I
Author: Xuan Huang, Ph.D.
The author would like to acknowledge Cody Hanick (MS; PhD student in Nutrition Science) and Janelle Schirmer (RD) for providing feedback. The author also wants to thank Alex Zotov and Dr. Nuo Xu for proofreading the article.
Part I: Vitamins
COVID-19 is an infectious disease, and people with compromised immune system are the most vulnerable. Immunity is closely tied to nutrition. Research has found that vitamins A, C, D, E, B6, folic acid, zinc, selenium, copper, manganese, iron, and pre-/pro-biotics have important influences on our immune responses. We want to help you bring the foods that are high in these nutrients onto your table and share with you some other tips to stay healthy during this difficult time. If you are here just for the recipes, here is your fast pass. If you are curious to learn about nutrition and our immune system, read on!
Our Immune System
Before we dive into what foods might better prepare us for fighting pathogens, let’s take a brief detour to understand our immune system. There are two basic types of immune systems: the innate (non-specific) immune system and the adaptive (specific) immune system. The adaptive immune system activates defensive activities that require adaptation of white blood cells that can recognize antigens which our bodies have previously encountered. Because COVID-19 is a novel virus, it is not likely you have any adaptive immune system against it – we will therefore save this topic for another day. Let’s focus on what a healthy body is capable of to defend a virus at its first-time encounter: the innate immune system.
The innate immune system has multiple defenses: healthy mucous membranes of our Gastrointestinal (GI) and respiratory system prevent the entry of invaders; an army of white blood cells including macrophages, neutrophils, and natural killer cells serve as the first responders to stop the invasion and kill cells that have been infected; our bodily inflammatory reactions such as a fever would make our body a hostile environment for some disease-causing pathogens to thrive.
Studies have shown that certain micronutrients are playing a particularly important role in maintaining a healthy immune system. In this first blog, we will focus on vitamins. In the next two blogs, we will discuss minerals, pre-/pro-biotics, spices, herbs, and other considerations.
Vitamin A
Vitamin A was once called “the anti-infective vitamin” in the 1920s. Even though this title is up for debate [1], it is true that vitamin A helps maintain the mucosal surfaces of the respiratory, GI, and genitourinary tracts. This is essential for blocking the entryway for invaders. Further, vitamin A plays crucial roles in the regulation of the differentiation, maturation, and function of cells of the innate immune system [2]. If you want to boost your vitamin A consumption, the highest ranked foods are animal livers and kidneys – but if you are like me, you probably won’t find them appetizing, not to mention that these foods are also notoriously high in cholesterol, which may cause other diseases that we don’t want to die from either. The good news is, many whole plant foods are also very high in vitamin A. In fact, “Vegetables and Vegetable Products” ranked third among all food groups defined by USDA in terms of median Vitamin A nutrient-density (measured by mcg/Cal), shown in Figure 1. Top common choices include carrots, lettuce, spinach, turnip greens, mustard greens, bok choy, collard greens, Swiss chards, butternut squash, and sweet potatoes. You may not know so many green vegetables are high in vitamin A, which is normally associated with carrot or sweet potato’s bright orange color, but it is only because the orange color is masked by the chlorophyll in leafy greens. What about “Dairy and Egg Products”? The food group tops the chart. But in probing the data, we found many products in this category that are high in vitamin A are due to the fact that dairy products are fortified with vitamin A. If you do choose to get vitamin A through milk, for example, make sure to look for the ones with fortification.
Figure 1a. Vitamin A density (mcg/Cal) distribution and boxplot of each food group of USDA SR Legacy food items. Food groups are ranked in terms of median density value, with the top group being the highest. As shown, outliers make the boxplots difficult to read. Figure 1b disables the display of outliers so the distributions can be more easily compared.
Figure 1b. For comparison, the red line is derived density level from the vitamin A Recommended Dietary Allowance (RDA), the average daily intake level estimated to meet the needs of nearly all people in a certain group*. Foods to the right of the red line set us well on the way to meet our vitamin A needs. As can be seen, many vegetables are far exceeding this recommendation level. The red line is also displayed in Figure 1a but is less legible due to the scale of outliers.
Vitamin C
Vitamin C contributes to our immune system in various ways: it enhances our epithelial barriers, acts as an antioxidant, and mediates the inflammation [3]. The two groups richest in vitamin C, as we may all know, are vegetables and fruits. These two groups are so abundant with vitamin C that if you just blindly pick any product from the fresh produce aisle, you don’t have to be too concerned about any vitamin C deficiency. The top choices, if you care to know, are kiwifruits, strawberries, lemon, papayas, grapefruit, orange and lime for fruits, and peppers, bok choy, broccoli, kale, cauliflower, and Brussels sprouts for vegetables. Notably, all but peppers that were mentioned in the vegetable group belong to the cruciferous family. No wonder the cruciferous vegetables make the list of the famous “Daily Dozen” [4], twelve categories of foods recommended to be consumed daily by whole-food plant-based diet evangelist Dr. Michael Greger. By the way, can you believe that on average Americans only consumes 0.9 pounds of kale each year [5]? We were astonished by this statistic when we came across it. We grow kale in our backyard and in its peak season, we have to eat a pound every few days to keep up with it. It is also such an easy-going plant that can even grow over winter in the warm South – that’s another story for another day.
Figure 2b. Vitamin C density (mg/Cal) distribution and boxplot of each food group of USDA SR Legacy food items with outliers display disabled. The red line is derived density level from the vitamin C Recommended Dietary Allowance (RDA).
Vitamin E
Like Vitamin C, Vitamin E is another potent antioxidant, which provides protection from oxidation so that our white blood cells can maintain their membrane integrity when they are busy fighting infections [6]. Fats and oils are high in Vitamin E – top choices are sunflower oil, safflower oil, and grapeseed oil; so are nut and seed products, such as sunflower seeds, almonds, hazelnuts, pecans, and cashews. Vegetables, in terms of vitamin E density (mg/Cal), are even better performers than nuts. Common choices include spinach, broccoli rabe, Swiss chards, turnip greens, and asparagus. Fruits that are rich in Vitamin E includes kiwifruits (second time our fuzzy little friends make the list) and everybody’s favorite: avocados. Peanuts are high in Vitamin E as well, but no, they don’t belong to “Nut and Seed Products” – they are instead legumes – my inner nerd cannot help but mention this.
Figure 3a. Vitamin E density (mg/Cal) distribution and boxplot of each food group of USDA SR Legacy food items.
Figure 3b. Vitamin E density (mg/Cal) distribution and boxplot of each food group of USDA SR Legacy food items with outliers display disabled. The red line is derived density level from the vitamin E Recommended Dietary Allowance (RDA).
Vitamin D
Most immune cells have Vitamin D receptors. Specifically for innate immunity, research has found vitamin D is important for the synthesis of macrophages [7], the white blood cells whose name in translation is literally “big-eaters”. They engulf cellular debris and pathogens. In general, few foods provide adequate vitamin D, but you can see there are several foods in “Vegetable and Vegetable Products” that has Vitamin D levels off the charts. Can you guess what are these? (Imaginary drum roll please… ) They are mushrooms! Crimini, portabella, white, maitake, morel, chanterelle, oyster, shitake, enoki, especially those exposed to ultraviolet light. The group that ranks the highest with vitamin D is “Finfish and Shellfish Products”. The top common choices include salmon, halibut, trout, snapper, swordfish, and mackerel. Though scant in foods, vitamin D is very different than other nutrients because our body can synthesize it through exposure to sunlight – in a way it is arguably a hormone. So during this period of isolation, spend some time in your backyard or balcony, because vitamin D is free.
Figure 4a. Vitamin D density (IU/Cal) distribution and boxplot of each food group of USDA SR Legacy food items.
Figure 4b. Vitamin D density (IU/Cal) distribution and boxplot of each food group of USDA SR Legacy food items after elimination of first-round outliers for better visualization. The red line is derived density level from the vitamin D Recommended Dietary Allowance (RDA).
* The RDA used in calculation for this graph and other graphs are for age 31-50 females. Numbers for other age groups may vary.
[1] Semba, R. D. (1999). Vitamin A as “anti-infective” therapy, 1920–1940. The Journal of nutrition, 129(4), 783-791. https://doi.org/10.1093/jn/129.4.783
[2] Huang, Z., Liu, Y., Qi, G., Brand, D., & Zheng, S. G. (2018). Role of Vitamin A in the Immune System. Journal of clinical medicine, 7(9), 258. https://doi.org/10.3390/jcm7090258
[3] Carr, A. C., & Maggini, S. (2017). Vitamin C and immune function. Nutrients, 9(11), 1211. https://doi.org/10.3390/nu9111211
[4] Greger, M., & Stone, G. (2016). How not to die: discover the foods scientifically proven to prevent and reverse disease. Pan Macmillan.
[5] USDA Food Availability (Per Capita) Data System. Fresh kale: per capita availability adjusted for loss. Accessed March 27, 2020.
[6] Lee, G. Y., & Han, S. N. (2018). The Role of Vitamin E in Immunity. Nutrients, 10(11), 1614. https://doi.org/10.3390/nu10111614
[7] Thompson, J., Manore, M., & Vaughan, L. (2019). Science of Nutrition, 5th edition. Pearson Higher Ed.