Oct. 27 (UPI) -- Increasing vitamins A, E and D through diet changes or supplements reduces a person's risk for breathing and respiratory conditions, including flu and COVID-19, a study published Tuesday by the journal BMJ Nutrition Prevention & Health found.
People who consumed recommended amounts of the three key nutrients were less likely to develop the flu, colds, chronic obstructive pulmonary disease and asthma, the data showed.
Research has linked vitamin D, in particular, with boosting immune system function, and being deficient in the nutrient has been found to increase a person's risk for severe COVID-19.
Vitamins A, E and D -- as well as vitamin C -- are all considered micronutrients, meaning they are needed in relatively small doses to live.
"Nationally representative data continue to remind us that micronutrient deficiencies are far from a thing of the past, even in higher income nations," Sumantra Ray, executive director of the NNEdPro Global Center for Nutrition and Health in England, where the research was conducted, said in a statement.
"Despite this, micronutrient deficiencies are often overlooked as a key contributor to the burden of malnutrition and poor health, presenting an additional layer of challenge during the COVID-19 pandemic," said Ray, who was not directly involved in the study.
Nutrition plays an important role in cutting the risk of several infections, although exactly how it boosts immunity is complex and not fully understood, according to the study authors.
Major dietary sources of vitamin A include liver, whole milk and cheese, as well as carrots, dark green leafy vegetables and orange-colored fruits, while vegetable oils, nuts and seeds are primary sources of vitamin E.
Adequate intake of vitamin D through diet is more difficult to achieve, given that it is not found naturally in most foods, though it can be acquired by spending time in the sun. But people often take supplements to ensure adequate levels of the vitamin, the researchers said.
Despite health agencies and organizations publishing daily intake guidelines for these and other vitamins and nutrients, researchers at Oregon State University estimate that 94% of people older than 4 in the United States have less-than-adequate levels of vitamin D.
Similarly, 89% have below-recommended levels of vitamin E, while 44% have have lower levels of vitamin A than recommended, according to the Oregon State analysis.
For the new BMJ study, researchers explored whether consuming the vitamins from either diet or supplements is linked to the prevalence of respiratory complaints.
The study included a nationally representative sample of adults in Britain, where an estimated 20% of people have low vitamin D and over 30% of older adults do not achieve the recommended nutrient intake.
Researchers analyzed data from 6,115 adult participants in the 2008-2016 National Diet and Nutrition Survey Rolling Program. The program is an ongoing assessment of eating habits and overall health by collecting information on all food and drink consumed from around 1,000 randomly selected people living in private households annually.
All 6,115 participants completed "diet diaries" that tracked their food and drink intake for three or more days.
Study participants reprted 33cases of respiratory complaints and not necessarily diagnosed by a healthcare professional, the data showed.
Those reporting respiratory complaints generally were older and less likely to regularly regularly vitamin A, E, C or D supplements, according to the researchers.
None of the adults with respiratory complaints reported taking a vitamin C supplement, which researchers said makes it impossible to determine any associations with respiratory conditions.
But vitamin A and E intake from both diet and supplements was associated with a lower prevalence of respiratory complaints among the study participants, the data showed.
Similarly, vitamin D intake from supplements, but not from diet, was associated with fewer respiratory complaints, the researchers said.
"Our findings are consistent with the hypothesis that supplementation is critical to ensuring adequate vitamin D status is maintained and potentially indicate that intake of vitamin D from diet alone cannot help maintain adequate vitamin D status," they wrote.
"Further research is required to assess the implications of the current study in the context of the current [COVID-19] pandemic," they said.