What we eat—or don’t eat—can affect our immune system. In a study conducted to determine the effect of the consumption of brightly colored vegetables on the immune system, the subjects ate basically no fruits and veggies for the first two weeks. Then, they drank one and a half cups of tomato juice every day for two weeks, followed by two weeks of carrot juice, and then two weeks of spinach powder. Within just two weeks of a fruit- and veggie-deficient diet, immune function plummeted. However, just one and a half cups of tomato juice a day brought subjects back from the ashes.
It didn’t take five servings a day—just one tall glass of tomato juice produced results. The carrot juice alone didn’t seem to help as well, however, nor did the powder equivalent of about one serving of spinach. This tells me two things: how remarkably we can affect our immune function with simple dietary decisions and that not all veggies are alike.
When this study was repeated looking at other immune markers, the tomato versus carrot appeared more evenly matched. There is one family of vegetables, however, that we definitely don’t want to miss out on. Inflammation and leaky gut can occur all because of an absence in our diet of AHR ligands—in other words, cruciferous vegetables, including cabbage, collards, cauliflower, kale, Brussels sprouts, and broccoli.
Do people who eat healthier actually get sick less? Those who eat more fruits and vegetables appear to have a lower risk of getting an upper respiratory tract infection like the common cold, whether they’re otherwise vegetarian or not. Even just one added apple a day may help keep the doctor away. The common cold is usually so innocuous, though, so why not test against something stronger?
Researchers have also looked at more serious respiratory infections like influenza. Studying the relationship between various risk factors and influenza-related hospitalizations in the United States, they found that a 5 percent increase in the prevalence of obesity was associated with a 6 percent increase in hospitalization rate. Physical inactivity had worse outcomes, resulting in a 7 percent increase in hospitalizations. Low fruit and vegetable consumption, however, had the most impact, increasing flu-related hospitalization rates by 8 percent.
The common cold isn’t always innocuous. For instance, a cold during the first trimester of pregnancy is associated with a number of birth defects, including anencephaly, one of the worst, which causes a fatal malformation of the brain. More recent data suggest that the cold-related fever is the real culprit, as anti-fever drugs appear able to prevent the possible birth defects caused by the common cold.
It’s best, of course, not to get sick in the first place. One thousand women and their diets were followed before and during pregnancy. It was found that “[w]omen who consume more fruits and vegetables have a moderate reduction in risk of [upper respiratory tract infection] during pregnancy, and this benefit appears to be derived from both fruits and vegetables instead of either alone.” Whole fruits and vegetables provide a natural balance of all sorts of things that may improve our immune function in a “complementary, combined or synergistic manner that could account for the protective effect observed from high consumption of both fruits and vegetables”—or maybe that’s the only way they got enough in their diet. The women who appeared protected in this study were eating nearly nine servings of fruits and vegetables a day, compared with only five servings of fruits or four of veggies. This suggests that the arbitrary five- or six-a-day minimum may be insufficient for effective immune function.
For example, in one famous study, elderly individuals were randomized into groups that ate either five servings of fruit and veggies a day or two servings a day. The five-a-day group showed an 80 percent improved antibody response to their pneumonia vaccination compared to the two-a-day group. Even though only about 30 percent (12 out of 40 people) of the five-a-day group reached their target levels of servings, they still did six times better than the two-a-day group. But maybe eight, nine, or ten servings a day would have worked even better.
Michael Greger, M.D.
PS: If you haven’t yet, you can subscribe to my free videos here and watch my live, year-in-review presentations—2015: Food as Medicine: Preventing and Treating the Most Dreaded Diseases with Diet, and my latest, 2016: How Not to Die: The Role of Diet in Preventing, Arresting, and Reversing Our Top 15 Killers.