Does intake of processed sugar lower the immune system?

Question

Growing up, my parents were very strict about sugar intake, because they had heard "somewhere" that eating refined sugar lowered your immune system function for 12-24 hours.

At the time, this seemed to be anecdotally true to me, because as a kid if I did actually eat something with a lot of sugar (candy, dessert) I would often have a sore throat or stuffy nose the next day.

This article from WebMD seems to support this, but doesn't cite any references:

Eating or drinking too much sugar curbs immune system cells that attack bacteria. This effect lasts for at least a few hours after downing a couple of sugary drinks.

Are there any large studies that show this link? What is the current medical consensus on whether sugar intake makes you more susceptible to infection?

Answer 1

There is some evidence that sugar does suppress the immune system. The study that everyone seems to point to was conducted in 1973, where they fed sugar in 100g doses to people and measured the effect on phagocytosis (where immune cells engulf invader bacteria to destroy it).

The end result was that sugar intake did appear to suppress the ability of the immune system to destroy invading bacteria. This function was suppressed for up to 5 hours in control subjects, while fasting comparisons did not have the same effect. Additionally, they counted the number of neutrophils (type of white blood cells), and they were not diminished, which suggests it was a function blocking mechanism, not that it suppressed the number. So it does limit the effectiveness, but this is not really a suppression of the system.

Part of this may be due to the sugar molecule being structurally similar to Vitamin C (Ascorbic acid), and that it competes for the space and prevents the cells from effectively using Vitamin C to destroy bacteria. (Based off studies Linus Pauling did, but I could not find the study, merely references to it).

There are suggestions that sugar also may contribute to insulin resistence, and that it may contribute to increased inflammation, but these either state more study is needed or are in the incidence of specific other deficiencies (Such as magnesium).