The immune system is continually responding to endogenous and exogenous stimuli. 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 helps improve immune function. Conversely, a diet rich in processed foods and artificial 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, triggering 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 infecting the host 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 protect the body by engulfing bacterial waste and small foreign particles. The process of phagocytosis differs from autophagy. Since phagocytes consume large particles like bacteria, autophagy is the taking up and repurposing of the 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 

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 but rather modulating the inflammation resulting from the infection. Depending on the source of inflammation, fasting has different consequences. 

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 the growth of tumor cells. Thus, suggesting that labeling sugar as “bad” and an immunosuppressant may be inaccurate.

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

Effects of Fructose on the Body

Fructose is a common additive in processed foods. It now accounts for 10% of caloric intake in the United States. Fructose itself is not bad, but overconsumption of it that can impact the body. Fructose can be broken down in the colon when consumed in small doses. High doses of fructose, on the other hand (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 the 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)

People have been using this process to make HFCS since the ’70s. So it’s a relatively new addition to the human diet, creating it from genetically modified enzymes and corn. When fed to male rats, HFCS led to young rats suffering from anemia, high cholesterol, and heart hypertrophy. 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 but have now been found to alter gut microbiota and drive the development of glucose intolerance.

The Bottom Line Sugar, in all its forms, impacts the immune system, especially when consumed in high doses. It’s inaccurate to assume that sugar is bad for the immune system in all cases. It shows glucose to be potentially beneficial to humans regarding viruses. On the flip side, a bacterial infection requires abstinence from sugar. Artificial sweeteners, like aspartame and HFCS, have serious harmful effects on the immune system, especially on infants, and, as such, should be avoided.