Too much sugar won't directly weaken your immune system, but consuming too many calories might

• There is no scientific evidence to suggest that consuming too much sugar will directly weaken your immune system.
• However, sugary foods and beverages contain a lot of calories and too many calories may adversely affect immune function.
• Processed foods high in sugar often lack fiber, which is important for maintaining a healthy microbiome, which also plays a key role in immune function.
• While eating too much sugar might not directly affect your immune system, it could indirectly weaken immune function if you're consuming too many calories from processed foods and beverages.
• This article was medically reviewed by David S. Seres, MD, Director of Medical Nutrition and Associate Professor of Medicine, Columbia University Irving Medical Center.

Too much sugar is linked to a number of adverse health effects including obesity, prediabetes, and fatty liver disease. However, the notion that sugar weakens the immune system is shaky, at best. And scientists say sugar's role is far more complicated.

Implicating sugar

The idea that sugar weakens the immune system likely arose in the early '70s, when a study was published reporting that phagocytes, a type of white blood cell that kills bacteria and pathogens, were less active in people who had recently consumed straight sugar or sugary foods, including honey and orange juice. The measurements were taken within 5 hours of sugar consumption.

However, over the last 4.5 decades, this study's results have yet to be replicated and there are no other studies proving that sugar directly impacts the immune system. In fact, the average healthy adult, will clear simple sugars from their system within two hours, says Peter Mancuso, an associate professor of nutritional sciences at the University of Michigan.

"It's only people with diabetes where [blood sugar levels] could be high enough to impair immune function. Even a liter of Coke a day would be unlikely to impair immune function," Mancuso says.

Sugar, inflammation, and the immune system

Sugar's role in immune suppression is more likely related to total caloric intake. "If you have an excess sugar intake you may also have an excess calorie intake, which leads to obesity," says Mancuso. "[This] is associated with a chronic state of low-grade inflammation."

It is believed that chronic inflammation might overwhelm your immune system because the body is already putting up a sustained battle against the inflammation, leaving it less capable of responding to other threats. This can impair how cells and organs function, increasing the risk of a range of diseases including heart disease, diabetes, and asthma.

Some research shows an association between consumption of fructose — a component of table sugar that is extracted from sugar cane and beets — with asthma, chronic bronchitis, and arthritis. But associations should be interpreted with extreme caution as they do not prove cause and effect.

To that point, Mancuso says, "studies that show associations between a certain level of sugar intake and inflammation may not account for a lack of fruits, vegetables, nuts, and fish, which are known to reduce inflammation."

So a bad diet may play an equal role as excess sugar when it comes to chronic inflammation. And since we generally don't eat straight sugar — except for some sweetened beverages — it's difficult to differentiate the two.

Sugar and the microbiome

Emerging research into the healthy bacteria that support your immune system, called the microbiome, is also giving hints as to how eating too much sugar, in lieu of healthier foods, might impact immunity, says Caroline Childs, a lecturer in nutritional sciences in the University of Southampton in England, who researches the effect of nutrition on immune function.

About 70% of our immune system is associated with monitoring and responding to our gut bacteria. "You grow different colonies depending on the diet you eat. These bacteria live on the waste products of our foods, and their favorite food is fiber," Childs says.

Gut bacteria help convert fiber into short-chain fatty acids, which have anti-inflammatory effects. They also help the body produce vitamins, including up to 86% of our daily vitamin B6 requirement, which plays an essential role in immune system support. Therefore, when we swap out the fiber for sugar, we fail to reap the immune-boosting benefits these microorganisms offer us.

Researchers are still exploring how a healthy microbiome affects overall health, but preliminary studies so far lend credence to the notion that a healthy diet plays a key role.

How much sugar is too much?

The Centers for Diseases Control and Prevention recommends that added sugars account for no more than 10% of your daily calories. Yet, Americans consume roughly 15% of their daily calories from added sugars, most of which come in the form of sweetened beverages and cereals.

These are often referred to as "empty" calories because they don't add nutrients to your diet like essential vitamins and minerals. Processed foods often contain added sugars in large quantities. That's why it's important to look at labels for added sugars.

A single serving of Honey Nut Cheerios with one cup of skim milk, for example, contains 20 grams of added sugar, or 4% of a 2,000 calorie diet. Or, if you're a Starbucks fan, a grande caramel frappuccino contains 55 grams of sugar. That's 220 calories, or 11% of a 2,000 calorie diet — above the recommended limit.

When eating sugar choose whole foods

Eating more whole, unprocessed foods in place of sugary ones ensures you are consuming key nutrients and fibers.

While fruit has sugars, it also contains fiber, which slows down the digestion rate, keeping blood sugar levels more stable. That's why eating dessert after a meal, one that preferably includes plenty of vegetables, might be better for your body and your teeth than a sweet alone.

"Fiber is nature's balance system," says Ian Myles, an immunologist and chief medical research officer with the National Institutes of Health. When you eat an apple, for example, it contains fiber which may counteract inflammatory effects from the sugar. But if you process that apple into juice, then "you strip the fiber, and the counterbalance effect is lost," Myles says.

Avoid artificial sweeteners

Substituting artificial sweeteners may cut calories, but research has actually linked them to obesity, cardiovascular disease and type 2 diabetes. It's not clear why yet, but these additives may change the way we process food, and lead us to crave real sugar. There is also some evidence in animal studies that consuming them alters gut bacteria, with unknown health effects.

It's probably better to stick with the sugar from whole foods like fruits and vegetables in small doses, Childs says: "Perhaps we need to start thinking about sugary treats as something we have more rarely than a daily occurrence. We might think of Coca Cola the way we think of champagne, as something we don't drink every day."

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