Does Sugar Weaken the Immune System?

The short answer: it depends.

The immune system is continually responding to endogenous and exogenous stimuli including sugar. Whether from the foods we eat, our sleep and exercise habits, stress, relationships, or the air we breathe, the body is always looking for ways to maintain homeostasis. A 2018 review showed that 83 research studies concluded that a diet rich in fruits and vegetables actually helps improve immune function. Conversely, a diet rich in processed foods and human-made sugars can suppress the immune system. However, it’s not always that simple.

Is Sugar an Immunosuppressant?

Sugar may or may not act as an immunosuppressant, depending on different conditions in the body. As a general rule, excessive sugar consumption depletes the body’s nutrient balance, which triggers a cascade of inflammation and metabolic disruption. However, saying sugar is always bad for our health might be overgeneralized. For example, the effects of sugar on the immune system differ depending on whether the host is infected with a bacterium or a virus.

Phagocytosis & the Immune System

To understand sugar’s role in the immune system, we need to review phagocytes. Phagocytes are cells that protect the body by engulfing bacterial waste and small foreign particles. The phagocytosis process differs from autophagy because phagocytes consume large particles like bacteria, while autophagy is the taking up and repurposing of internal organelles such as mitochondria. Phagocytes work to destroy particles like bacteria that could infect the host. There are five main types of phagocytes:

  • Neutrophils
  • Monocytes
  • Macrophages
  • Mast cells
  • Dendritic cells

The Effect of Sugar on Phagocytes

A research study was carried out to observe the effect of simple carbohydrates on neutrophils’ phagocytic capacity. Blood was drawn after an overnight fast and then at intervals of 0.5, 1, 2, 3, or 5 hours. The blood was then placed on slides and incubated in Staphylococcus epidermidis.

Participants were given 100g oral portions of glucose, fructose, sucrose, honey, or orange juice. Immediately after consuming simple carbohydrates (approximately one to two hours), there was a significant decrease in the phagocytic index, which means the phagocytes’ immune function was suppressed. The effects lasted up to five hours post-feeding, confirming that the consumption of simple carbohydrates impaired phagocytes’ function. On the other hand, a fast of 36 or 60 hours significantly increased the phagocytic index (P<0.001). However, sugar did not affect the number of neutrophils.

Feed a Cold, Starve a Fever?

Ruslan Medzhitov, professor of immunobiology at Yale University,  challenged the ancient wisdom of starving a fever and feeding a cold. He explains that both viruses and bacteria can cause a fever. Medzhitov’s research has brought him to the following conclusion:

“Starve a bacterial infection and stuff a viral infection.”

He explains that we are not feeding or starving the bacteria or viruses, rather modulating the infection’s inflammation. Depending on the source of inflammation, fasting has different consequences.

Medzhitov warns against oversimplifying the effects of eating when the body has contracted a bacterial infection or a virus. 

Medzhitov and his researchers force-fed mice suffering from food-poisoning due to the bacteria Listeria monocytogenes, and the mice died. To find out what micronutrients were responsible for their demise, the Yale team split the micronutrients and fed them separately.

Interestingly, the Listeria mice survived when they were force-fed fats and proteins. However, when the infected mice consumed carbohydrates or glucose, they died. Starving the mice of sugar while they were infected with Listeria kept them alive.

Conversely, when Medzhitov and his team force-fed mice had the influenza virus, they found that they were more likely to survive. The team at Yale concluded that glucose is required for survival in viral inflammation; however, it is lethal when treating bacterial inflammation.

Diet Can Shift the Microbiome in 24 Hours

Consuming carbohydrates rich in sugars (such as dates) encourages the proliferation of good bacteria such as Bifidobacteria while reducing tumor cells’ growth. Thus, suggesting that labeling sugar as “bad” and an immunosuppressant may be inaccurate.

Conversely, animal models showed circadian disorganization and loss in microbial diversity when fed a high-fat or high-sugar diet. The changes in the microbiota due to the diet shift may significantly alter inflammatory markers. Shifts in circadian rhythm in mice provided high-fat, high-sugar for 10 weeks also resulted in intestinal permeability and reduced microbial diversity, making them more susceptible to injury and disease.

Effects of Fructose on the Body

Fructose is a common additive in processed foods. In fact, fructose now accounts for 10% of caloric intake in the United States. Fructose itself is not bad, but the overconsumption of it that can impact the body. When consumed in small doses, fructose can be broken down in the colon. On the other hand, high amounts of fructose (e.g., over half a can of sugary soda) can damage the liver.

Moreover, the ability of the body to break down fructose is dose-dependent and influenced by genetics. The availability of protein GLUT5, which can be a genetic factor, impacts how well a person can tolerate fructose. For some, excess fructose can lead to bloating, abdominal pain, and bowel discomfort, as bacteria in the gut ferment the fructose, resulting in off-gasses.

Artificial Sweeteners & High Fructose Corn Syrup (HFCS)

The process used to make HFCS was developed in the ‘70s, so it’s a relatively new addition to the human diet. HFCS is made from genetically modified enzymes and corn. HFCS led to young rats suffering from anemia, high cholesterol, and heart hypertrophy when fed to male rats. These rats didn’t reach adulthood, and their female counterparts were infertile.

Other artificial sweeteners, such as Saccharin (Sweet’N Low®, SugarTwin®), Acesulfame K (Sunett®, Sweet One®), and Sucralose (Splenda®), were previously thought to be a non-caloric sugar alternative. However,  artificial sweeteners have now been found to alter gut microbiota and drive glucose intolerance.

The Bottom Line

Sugar in all its forms impacts the immune system, mostly when consumed in high doses. However, it’s not accurate to assume that sugar is bad for the immune system in all cases. When it comes to viruses, glucose was shown to be beneficial potentially. On the flip side, a bacterial infection requires abstinence from sugar. Human-made sweeteners, like aspartame and HFCS, can also have serious deleterious effects on the immune system, especially on infants, and, as such, should be avoided.

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