Whether it’s chicken soup to help fight a cold or garlic to fortify against the flu, tradition has it that food can influence our immune response. But does this idea hold up scientifically? According to Prof. Philip Calder, Professor of Nutritional Immunology at the University of Southampton (UK) and winner of the Danone International Prize for Nutrition, growing evidence shows diet can indeed influence the human immune response. In a phone interview with GMFH editors, Calder explained how data from his lab and others has identified three ways in which nutrition can affect the body’s immune response.
The first link between food and immunity, says Calder, is the most basic: the immune system needs a source of energy. “The immune system is highly metabolically active. It needs a lot of fuel like any active tissue,” he says. “The fuels the immune system needs are the normal sort of fuels that all tissues use for energy: carbohydrate, fat, protein, and so on. We have to have enough of the fuels to allow the immune system to go about its business.”
“Also, one of the cornerstones of the immune response is an increase in the number of cells engaging in the response. That’s cell proliferation… making new cells.” He goes on, “If you’re making things, of course you need building blocks. All of this stuff that’s happening is involving metabolic pathways and biochemical reactions. Typically those pathways, which are converting one thing into another (for example, amino acids into protein), those processes involve enzymes. And those enzymes usually require cofactors… things like vitamins and minerals.”
Therefore, Calder says, “To mount an immune response you need a good supply of micronutrients—vitamins and minerals—and macronutrients, the energy sources like carbohydrate, fat, and protein.” This is well illustrated in extreme cases like malnutrition, where people may lack some of these basic building blocks. “For this reason, people who are malnourished or have nutrient deficiencies, have poor immune responses,” notes Calder. “They are not able to deal with pathogens so easily, so they get infected. They can’t deal with the infection, the infection takes over, and they get very sick.”
“The second thing is that quite a number of nutrients seem to play roles in regulating how immune cells are responding to immune signals,” Calder says. That is to say, when the immune system encounters ‘intruders’ like bacteria, viruses, or parasites, its response is tailored to the thing it encounters; the system has different manners of responding using the cells at its disposal. According to Calder, data show that nutritional signals help shape this response: immune cells decide to what extent they will obey or ignore instructions, depending on what they see around them. “Exactly what immune cells are doing depends on the nature of the signals they’re receiving,” he says. “Things like vitamin A, vitamin D, are regulators of the immune response. Omega 3 fatty acids regulate some aspects… they are acting as nutritional signals that control the way the immune response is happening.” Calder mentions probiotics as another dietary component that can serve as this kind of nutritional signal.
The third way nutrition appears to control immune responses, says Calder, is through the gut microbiota. “The nature of the microbiota, because of its known interaction with the immune system, plays a role,” he says. “Because nutrition particularly affects the gut microbiota, there could be an indirect route between nutrition, the gut microbiota, and the host immune response.” That is, when you eat, you could be shaping your gut microbiota and thus influencing how it ‘talks’ to your immune system. This influence of nutrition on immunity, Calder points out, may have particular importance in health.
Modifying your diet to improve your immune response seems like a distinct possibility now that scientists are learning more about these mechanistic links. But in spite of the traditional advice about chicken soup and other foods, scientists so far know only the broad strokes of how immunity and nutrition are linked. Ahead, Calder and other researchers will learn much more about the diet-immune link and how it varies from person to person. He says, “What the differences are between people, what they really mean, and how the microbiota is interacting with the host—these are not particularly well defined at the moment. That is an area which does need to be explored a lot more.”