This article is going to call out and demonise refined and simple sugars, the ones in sweet snacks and desserts, fruit juices and soft drinks, for a reason that goes beyond blood sugar spikes: eaten regularly, they feed chronic, low-grade inflammation, and that same disruption is what stops the immune system working efficiently. Even honey and maple syrup count as "simple" sugars in this sense.
What I am not going to do is make you feel bad about whole fruit, the occasional date ball, or a chia pudding sweetened with stevia. As for other natural sweeteners: coconut sugar is still sugar, though it is less refined and contains fibres that soften the blood sugar response — and agave syrup should be avoided like the plague. Agave is a highly refined, high-fructose syrup that strains the liver and contributes to long-term problems including metabolic syndrome and type 2 diabetes. Sorry if that bursts a bubble. On with the article.
Sugar and inflammation
Added sugar, especially the fructose-heavy kind found in soft drinks and many processed snacks, is one of the more consistent dietary drivers of chronic, low-grade inflammation. Part of the mechanism is chemical: excess glucose and fructose react with proteins and fats to form advanced glycation end-products (AGEs), which bind to a receptor called RAGE and switch on inflammatory signalling inside cells. Part of it is metabolic: regular sugar spikes drive insulin resistance and visceral fat gain, and both insulin resistance and visceral fat are themselves sources of inflammatory messengers such as IL-6 and TNF-alpha.
The downstream marker most studies track is C-reactive protein (CRP), the liver's response to circulating inflammatory signals. A 2018 systematic review of intervention trials found that higher sugar intake reliably raised CRP and other inflammatory markers compared with lower-sugar diets, even over a matter of weeks. That inflammatory state is not separate from the immune suppression described below: it is the same disruption showing up in a different measurement.
Sugar and immune function
One study examined how carbohydrates affect immunity — glucose, fructose, sucrose, honey and orange juice specifically. Participants took 100g doses of each while changes in immune function were tracked over 12 hours.
Immune suppression appeared as soon as 30 minutes after ingestion and peaked within the first one to two hours. Fructose was the worst offender, cutting white blood cell activity by 45%; the other sugars reduced immune activity by around 40%. And the effect of a 100g dose — easily reached through fruit juices and soft drinks, or a sweetened coffee alongside a breakfast muffin — lasted at least 5 hours.
One mechanism by which sugar disables immunity is by blocking vitamin C from entering the cell. Vitamin C acts as a cellular antioxidant, protecting white blood cells against the poisonous free radicals they generate while fighting infection. Insulin, the hormone secreted by the pancreas, is known for transporting sugars into cells — what is less widely appreciated is that it also handles the uptake of vitamin C and other nutrients. When blood sugar rises, after a sugary drink or snack, vitamin C uptake is blocked in favour of glucose. This happens because the sugar molecule is chemically very similar to vitamin C, so the two compete for the same receptors. Less vitamin C inside the cell means white blood cells are less active, cell death rates rise, and fewer new immune cells are produced.
The same study found it took a further 5 or 6 hours for intracellular vitamin C to return to its original concentration after 100g of sugar. Which means anyone consuming simple carbs at breakfast, mid-morning, mid-afternoon and evening is suppressing their immune system all day and into the night.
Sugar in context
Fortunately, slow-release low glycaemic index (GI) foods and moderate-GI carbohydrates such as wholegrains and oats do not have this effect on immunity. Eating fat and protein alongside carbohydrates also lowers the overall glycaemic load (GL) of a meal — a smoothie of banana, kale, avocado, nuts and seeds affects blood sugar far less than an apple, pear and orange juice.
Stress, sleep, nutrient status and activity levels all shift the picture from person to person. During physical activity the body does not need insulin to move sugar into cells, so if you are eating your chocolate bar between rounds of star jumps, technically you are fine.
Likewise, for someone whose diet is rich in organic vegetables, wholegrains and healthy protein and fat, sugar's blocking of vitamin C uptake will do less damage than it does to someone short on essential nutrients. The overall glycaemic load of the daily diet matters too: cereal and milk for breakfast, a banana mid-morning, a sandwich at lunch and pasta or potatoes for dinner is not much "sugar" as such, but the high-GI carbohydrates add up quickly.
So where does that leave us? Broadly, a healthy person can get away with dessert once or twice a week, provided their diet is rich in beneficial phytonutrients the rest of the time. But if you are dealing with insulin resistance, PCOS, fatigue, acne, breast cancer or virtually any other chronic disease, giving up sugar is the way to go. The good news is that good behaviour begets good behaviour — the less sugar you eat, the less you want. Two weeks off sugar is enough to reset both taste buds and metabolism, bring inflammatory markers like CRP back down, and set you on course for a nutritious diet low in simple carbohydrates.
References
Cited by the original article:
Stanhope KL, et al. (2013) Adverse metabolic effects of dietary fructose: results from the recent epidemiological, clinical, and mechanistic studies. Curr Opin Lipidol. 24(3):198-206.
Della Corte KW, et al. (2018) Effect of Dietary Sugar Intake on Biomarkers of Subclinical Inflammation: A Systematic Review and Meta-Analysis of Intervention Studies. Nutrients. 10(5):606.
Sanchez A, et al. (1973) Role of sugars in human neutrophilic phagocytosis. The American Journal of Clinical Nutrition, 26(11):1180–1184.
