Build Up Your Immune Defense with the Right Diet

By Sharon Palmer, RD

Foods that power up your immune system defenses are a hot marketing tool. From food products and dietary supplements claiming to “support immunity” to magazine articles recommending top “immune super foods,” it seems the idea of boosting immune function through diet is everywhere.

Perhaps it’s a good thing, as infectious diseases are the leading cause of morbidity in the world. And as cold and flu season approaches, it’s tempting to buy into the hope of eating your way to a stronger immune system. But is there truth behind the hype?

This is your immune system
The immune system is your body’s protective network that fends off invasion by harmful substances such as bacteria, viruses and chemicals as well as guarding against the development of cancer. When your immune system is healthy, you have multiple barriers to protect against invaders, including your skin, inflammatory responses and specific immune responses such as certain types of white blood cells that destroy pathogens.

Your gut plays a very important role in your immune system. It is the largest immune organ in the body, accounting for 25 percent of your immune cells. More than 400 species of bacteria reside in the gut, and they have important symbiotic (beneficial) relationships with your body. Simin Meydani, DVM, PhD, director of the Jean Mayer USDA Human Nutrition Research Center on Aging (HNRCA) and the Nutritional Immunology Laboratory at Tufts University, calls the gut flora (the collection of microorganisms, mostly bacteria) “the forgotten body organ.”

As you age, your immune system diminishes. Speaking at a December 4, 2009 Tufts Seminar on nutrition and immunity, Meydani reported that there is an increasing incidence of infectious disease (and morbidity and mortality from infectious disease) in older adults because they are more susceptible, the pathogens are more virulent, and there is a negative change in the gut bacteria. To top it off, marginal nutritional deficiencies also are common in older adults.

And once you have an infection, a vicious cycle kicks in: the infection obstructs your ability to boost nutrition because of loss of appetite, fever and diarrhea. Thus your immune system weakens. To make matters worse, scientists now know that nutritional deficiencies can cause a virus to become more virulent.

Nutrition and the immune system
One of the most important ways to maintain a healthy immune system is to maintain good nutrition. “When a pathogen enters the body, the immune system recognizes it and the body produces an army of specialized immune cells to get rid of the pathogen,” explains Meydani. And what do the immune cells need in order to grow in numbers? “Essential nutrients like vitamins, minerals, amino acids and fatty acids,” she adds.
This is the root of the diet-immunity connection. “Nutrients are essential for the optimal function of the immune response and to prompt a defense against pathogens. Deficiencies of nutrients as well as aging can impair the host’s defense and increase the occurrence of pathogens,” says Meydani.

The following nutrients show promise for their effects on the immune system:

Vitamin C
Many people believe fervently that vitamin C boosts their immune function and helps fight colds. Unfortunately, the science on this has been far from compelling, with some studies showing that it enhances immune function and other studies showing no effect. According to the National Institutes of Health (NIH), more than 30 clinical trials have examined the effects of vitamin C on cold prevention, yet overall they do not support a significant reduction in risk.

Still, there’s no harm in pushing vitamin C-rich foods, such as papaya, red peppers, broccoli, strawberries, citrus and tomatoes as part of your healthful diet.

Vitamin E
Its documented role in the immune system has led immunology researchers to study vitamin E. In a 2004 study led by Meydani and published in August 18, 2004 in The Journal of the American Medical Association, vitamin E supplementation was found to reduce the risk of upper respiratory infections, in particular the common cold. Foods packed with vitamin E include nuts, olives and leafy greens.

Studies show that people with low serum zinc levels experience twice the frequency of pneumonia (as well as longer duration and more antibiotic use), compared with those who have adequate zinc levels, according to Meydani. In a 2007 study in the American Journal of Clinical Nutrition, zinc supplementation resulted in 65 percent fewer occurrences of infection. But when it comes to the common cold, Meydani says, “The jury is still out. Some studies show zinc helps and reduces duration of colds, while other studies don’t show that effect.” Zinc food sources include red meat, poultry, seafood, beans, nuts and cheese.

Vitamin D
While the evidence isn’t strong, scientists recognize that the sunshine vitamin may have important functions within the immune system. Research shows that patients with tuberculosis respond better when treated with vitamin D or sunlight. In addition, you can find vitamin D in fortified dairy products, mushrooms, salmon and sardines.

The potential immune benefits of calorie restriction have attracted the interest of researchers (see EN July 2010, “Long-term Calorie Restriction”). In the “CALERIE” study (Comprehensive Assessment of Long-term Effects of Reducing Intake of Energy Study,) overweight participants divided into two groups reduced their calorie intake by 10 percent and 30 percent for six months, resulting in significant improvements in immune response in both groups, with the best effects seen in the 30 percent group.
“I’m not recommending that people who are not overweight would benefit from calorie restriction. Eat enough, but not too much,” urges Meydani. If you decrease calories too much and it results in nutritional deficiencies, it’s not good for the immune system.

These live microorganisms can exert health benefits by improving the number of beneficial bacteria in the gut. “Several studies show that the right types of probiotics increase the immune response,” says Meydani. A study published in the 2007 issue of The Journal of Nutrition found that a fermented milk drink containing the probiotic lactobacillus casei strain Shirota significantly increased natural killer cell (an important immune cell) activity. However, only certain strains of probiotics have been tested for immune benefits; thus, it’s important to look for clinical proof (see EN October 2009, “Healthy Bugs Living in Harmony”).

Curiously, mushrooms have captured scientists’ attention in immune research (see EN October, 2010 “The Magic of Mushrooms as Medicine”). When mice were fed a powder made of white button mushrooms, the natural killer cell activity increased significantly, according to a 2007 study published in The Journal of Nutrition.

The bottom line
So should you take mega-doses of essential nutrients to boost your immune system? “We can’t say with certainty. Sometimes if you supplement at more than the required level of nutrients, the immune function can decrease. It’s not always the case that more is better,” says Meydani.

In addition, nutrients appear to impact people’s immune systems differently. Meydani explains that people can be “responders” or “non-responders” to particular nutrients, based on genetic differences.

Many food companies and the media have made exuberant claims about the ability of particular foods to power up your immune system. It’s important to keep in mind that foods contain a synergy of nutrients that work in unison to provide health benefits versus supplements, which only provide one or two nutrients. The best approach for fostering a healthy immune system is to eat a highly nutritious diet that meets all your body’s needs. Here’s more reason to make every bite count, with delicious, whole foods bursting with nutrients.

Sharon Palmer, RD is a Registered Dietitian and writer covering health, wellness, nutrition, cooking, wine, restaurant reviews and entertainment. She is passionate about environmental issues, eco-friendly culinary practices, sustainability, food safety, humane animal practices and food security. Sharon’s feature articles have been published in a variety of publications, including Better Homes and Gardens, Prevention, Oxygen, LA Times, Cooking Smart, Delicious Living, Food Product Design, Today’s Dietitian, and CULINOLOGY. She is the author of The Plant-Powered Diet: The Lifelong Eating Plan for Achieving Optimal Health, Beginning Today. For more information, please visit

The articles written by guest contributors are the sole responsibility of the individual writers in terms of factual accuracy and opinion and do not necessarily reflect the views of the publisher of this blog.

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