This post is a continuation from last week and part II of "The Sustenance Saga", a series of posts that aims to answer a question from my Mum. She wrote,
"Hey, I have a question about lifestyle & immunity. How does your diet (sugar, refined carbs, meat, alcohol intake) and exercise or lack of exercise affect immunity?"
Last week, I wrote about how high-fat, Western diets can lead to prolonged inflammation. This week, we're focusing in on two different parts of this question: sugar, and exercise.
Before I dive further into the details, let's start with this important note: sugar isn't always bad. For instance, humans do require carbohydrates to function well, as you'll see later on. But, like many things in biology, it's all about moderation and balance.
Since we focused in on macrophages last week, let's start off with a closer look at these cells. In Part I of the Sustenance Saga, we went over how macrophages are so-called 'innate immune cells' that have one major job: eating up any dangerous microbes and other microscopic problems that they encounter. Basically, when macrophages see a problem, they eat it. They're basically the emotional eaters of the immune system. However, the story doesn't end there.
Scientists have now classified macrophages into two different categories based on their behaviour. These categories are (rather uncreatively, I must say) termed "M1" and "M2". M1 macrophages are the classical ones that have been in textbooks for years. Their job is pro-inflammatory. When they see something dangerous, they produce lots of cytokines, i.e. molecular mail, that are then sent to other immune cells to draw them in to the site of that danger. M2 macrophages are more unconventional. These ones are more anti-inflammatory, and send out messages that promote tissue repair and tell the rest of the immune system to calm down.
But back to the sweet stuff. Another major difference between these two types of macrophages is that they depend on different food sources to support their activities. And guess which one prefers sugar?
You guessed it - the pro-inflammatory, M1 macrophages rely on glucose, a sugar, for their energy. They're basically fast-food junkies. The anti-inflammatory M2 macrophages, meanwhile, prefer a more "slow food" diet of fatty acids.
Macrophages aren't even the only immune cells to depend on these different energy sources depending on their behaviour. CD4+ T cells and CD8+ T cells, which are cell types that have an immune 'memory', unlike macrophages, also switch from eating fatty acids to eating sugar when they become activated and 'inflamed'.
Interestingly, this is also a two-way street. Indeed, when cells switch up their behaviour to become more inflammatory, they switch to eating sugar. But, having a lot of sugar around can also make cells become more inflammatory.
And, this is where exercise comes in. During exercise, sugar in the bloodstream is used up to provide all the extra energy you need to keep your heart and muscles working harder than usual. This means that there is less sugar lying around to feed your immune cells and skew them towards becoming inflammatory. In other words, if you have the urge to run about for a bit after a sugar rush, it's really not a bad idea to act on that inclination to help your immune system (and the rest of you) relax after the sugar influx.
As with sugar, a balanced exercise programme seems to make immune cells the happiest. There is some evidence that after moderate exercise, immune cells actually work even better than usual, possibly because of changes in protein metabolism. But after too much exercise, the immune system actually can become too relaxed. If you use up all of your energy on extreme exercise, there's nothing left for your immune system to work with. This has been put forward as one of the reasons why extreme athletes are particularly likely to get sick just after a big race.
So, in general, as it usually is, a balance is best. Exercise skews the immune system towards relaxing, forming memory, and regulating itself. Sugar skews the immune system towards attacking and multiplying. Ideally, the immune system will lie somewhere in between these two extremes.
That's all for this time. Until next week, I challenge you to take advantage of the long summer days and get outside! It's good for your immune system.
These articles were my main sources of information in writing this blog post:
Batatinha HAP, Biondo LA, Lira FS, Castell LM, Rosa-Neto JC. Nutrients, immune system, and exercise: Where will it take us?.Nutrition. 2019;61:151-156. doi:10.1016/j.nut.2018.09.019
Christ A, Lauterbach M, Latz E. Western Diet and the Immune System: An Inflammatory Connection.Immunity. 2019;51(5):794-811. doi:10.1016/j.immuni.2019.09.020