A junk food diet is clearly not healthy. Burgers widen our waistlines, raise our cholesterol levels and tighten our arteries. But scientists now think that even before it shows up as additional pounds on the scale, junk food is changing our bodies in other, surprising ways. It’s actually a form of malnutrition that could be making our immune systems attack our own bodies.
In a recent study published in the Journal of Immunology, scientists at Australia’s University of New South Wales investigated a typical western diet—one that’s high in saturated, monounsaturated and polyunsaturated fatty acids. Specifically, they looked at the diet’s impact on immune cells called T lymphocytes, or T cells.
The researchers fed mice a high-fat diet for nine weeks to see what effect it would have on the T cells before the mice gained weight. The results surprised study leader Abigail Pollock. “Despite our hypothesis that the T cell response and capacity to eliminate invading pathogens would be weakened we actually saw the opposite: the percentage of overactive T cells increased,” she explained .
This might sound great, but having more T cells doesn’t necessarily mean your immune system is stronger. In fact, when the immune system goes into overdrive, it attacks healthy parts of the body, resulting in autoimmune diseases like type 1 diabetes, rheumatoid arthritis and inflammatory bowel disease.
Cell membranes—the bags that hold cells together—are made up of layers of fatty lipid molecules. Looking closely at the T cells, the scientists found that having extra fats in the diet actually changed the amount of lipid molecules in the cell membrane, which in turn, said Pollock, “changes the structure of the cell, altering the responsiveness of the T cells and changing the immune response.”
The team had shown previously that altering the lipid content of T cell membranes affects how they signal and activate each other, but this is the first time the effect has been shown in a living animal. More research is needed, they say, to figure out exactly what’s happening and determine which fats we could avoid to make sure our immune systems don’t go into overdrive.
Indeed, this is not the first study to show that a high-fat diet impacts the immune system; almost 20 years ago, scientists at the University of Oxford in the U.K. studied rats on diets rich in different fats. They found that the lymphocytes of rats fed a high-fat diet rich in palmitic acid grew more, whereas natural killer cells—another type of immune cell—of rats on a high-fat diet rich in stearic acid grew much less.
More recently, scientists at the University of Ulsan in South Korea compared obese mice on a high-fat diet and non-obese mice on a normal diet, and found that the obese mice had significantly lower levels of immune cells, including T cells, in their lymph nodes, where the immune cells wait until they are needed by the body. The lymph nodes near the intestine were much lighter in the obese mice and contained far fewer T cells than those of the control mice.
The scientists concluded that the accumulation of fat around the organs—visceral fat—due to a high-fat diet causes cells in the lymph nodes to self-destruct: “Dietary fat-induced visceral obesity may be crucial for obesity-related immune dysfunction,” they explained.
A high-fat diet won’t only affect your immune system; it could also impair your memory—and that of your kids. When they fed pregnant mice a diet high in lard, scientists at Capital Medical University in China found that the fat in their offspring’s brain was altered. The authors explain in their study: “Our research demonstrated that long-term high lard diet […] changed the brain fatty acids composition and damaged the memory and learning ability of mice.”
What’s in your burger and fries?
A junk food diet is rich in fat, but there are all sorts of other harmful things lurking in there too. Firstly, junk food is highly palatable—it tastes good. This makes us eat more and more, which is an even bigger problem because it’s also very high in calories. A small cheeseburger is 300 calories, the same as a whole (nutritious) meal. And you would rarely just eat a cheeseburger; adding fries (230 calories) and a soda (170 calories) takes the total to a whopping 700.
An excessive calorie intake leads to obesity, which has an impact on the immune system. Norwegian researchers found that being overweight led to inflammation —a sign of an overactive immune response. They studied this on a molecular level, to establish a link between metabolism, inflammation, heart attack and stroke. Their theory is that overeating provides our cells with too much energy and the tiny cellular engines—mitochondria—can stall.
“We believe that long-term stress on the mitochondria may cause metaflammation,” explained Dr. Arne Yndestad of Oslo University Hospital. “A metaflammation is a low-grade chronic inflammation over many years, and unfortunately it’s a condition that’s difficult to detect.”
There’s more. According to World Action on Salt and Health (WASH) we eat on average 8.1 grams of salt a day—about a third more than the recommended 6 grams. Junk food is high in salt; a junk food meal can tip you over the limit of your daily intake.
What’s the problem? Salt is implicated in making our immune systems go haywire. In three separate studies published in Nature , scientists showed that high salt intake increase production of T cells, taking them to harmful levels. One of the studies showed that feeding mice with multiple sclerosis a high salt diet made their disease progress faster.
One of the researchers, Dr. Vijay Kuchroo of Brigham and Women’s Hospital in Boston, Massachusetts, points out that the evidence is not yet definitive, but “is building a very interesting hypothesis [that] salt may be one of the environmental triggers of autoimmunity” and that it’s not yet possible to predict the effect salt has on autoimmunity and the development of diseases like type 1 diabetes.
As if that wasn’t bad enough, it’s a bleak picture for kids too. In 2015, WASH carried out an international study on the salt content of children’s meals, concluding that 8 out of 10 meals had more than 1 gram of salt per serving, tipping the maximum recommended serving per meal.
“The more salt you eat as a child, the more likely you are to have serious health issues in later life,” said Prof. Graham MacGregor, WASH chairman and professor of cardiovascular medicine at Queen Mary University of London. “This can include high blood pressure, increased risk of stroke, heart disease, osteoporosis and kidney disease. That is why it is vitally important that children do not get used to the taste of salt.”
Unfortunately, our kids might be at a disadvantage from the outset: the gut bacteria they inherit can have an impact on their immune health. Babies get their gut bacteria—their microbiota—from their mothers at birth and this continues to be shaped through breastfeeding. The microbiota is known to have an impact on our immunity; Spanish researchers found that children with type 1 diabetes had different gut bacteria compared to healthy children.
In his review “Fast food fever : reviewing the impacts of the Western diet on immunity,” Dr. Ian Myles of the National Institutes of Health gives an overview of what we know about the effects of a western diet on the immune system. The bacteria found in children’s intestines can have a surprisingly big impact, he says. “Our bodies are a kind of mini-ecosystem, and anything that disturbs our bacteria can alter our health in profound ways,” he said in an interview with Time.
They might also be inheriting a genetic preference for unhealthy food. Researchers at Imperial College London have identified a gene linked to cravings for high-calorie foods . They asked 45 people to look at photos of high- and low-calorie foods and rate how appealing they were, and monitored their brain activity. They found that people with a particular genetic change near a gene called FTO who preferred high calorie foods had more activity in parts of the brain linked to pleasure. This, say the researchers, means people with this genetic trait will find it harder to avoid junk food. And if it’s written in our DNA, there’s a chance we can pass it on to our kids.
Even more reason to stop bombarding them with adverts for junk food. Children’s food choices are strongly influenced by advertising, and many countries are already responding by restricting advertising to kids.
Switching off the TV is one way to cut junk food cravings and intake, and therefore protect our immune systems. And the same researchers who found the genetic variants that cause food cravings also found a dietary supplement that can stop those cravings. But there are plenty of other things that can boost our immunity.
The problem with junk food isn’t just the bad ingredients it contains; it’s also the good ingredients it doesn’t contain. You won’t find vegetables like broccoli, cauliflower and cabbage in a fast food meal, but a study by U.K. scientists revealed that these vegetables switch on immune cells in the intestines, which they say explains the link between diet and immunity.
Junk food is also low in fiber, which is known to boost immunity. According to one study , dietary fiber encourages friendly bacteria to grow in the intestine. These lactic acid bacteria can boost immunity , so eating foods that help them grow—like fiber-rich prebiotics, fermented vegetables and raw garlic—could strengthen your immune system.
Immune-boosting antioxidants are also hard to find in your hamburger (unless it’s topped with mushrooms), so eating plenty of sweet potato, elderberry and low-fat yogurt will help.
In his review on fast food fever, Dr. Myles wrote: “While today’s modern diet may provide beneficial protection from micro- and macronutrient deficiencies, our over abundance of calories and the macronutrients that compose our diet may all lead to increased inflammation, reduced control of infection, increased rates of cancer, and increased risk for allergic and auto-inflammatory disease.”
So the next time you’re tempted by a meal deal, consider cooking up some vegetables with chicken and a refreshing yogurt and berry dessert instead. Your immune system—and your memory—will thank you for it.