Sugar and immunity: what you need to know

Wondering how sugar affects your immune system? We often talk about foods supporting immunity, yet rarely consider that they might damage it too. However, eating too much sugar impacts your body’s ability to fight off an infection in both the short and long term.

With many of us suffering from colds, the flu or COVID-19, it’s important that our diets aren’t sabotaging us – and this means reducing our sugar intake.

We spoke to NHS-trained consultant cardiologist and bestselling author Dr Aseem Malhotra to learn more.

Sugar and the immune system

What is sugar and what is it in?

Sugar is the broad name that refers to a group of sweet-tasting simple carbohydrates. There are four main types you’ll likely know of: glucose, fructose, lactose and sucrose.

Glucose is the primary source of energy in our bodies, and what the term ‘blood sugar’ refers to. When we eat and digest carbohydrates, glucose is released into the bloodstream in order to be transported to the cells that need it. Increased blood sugar also triggers the pancreas to produce insulin, which is the hormone that regulates blood glucose levels.

Fructose doesn’t interact with insulin or impact blood sugar levels. Instead, it’s absorbed directly into blood during digestion and is metabolised in the liver.

Lactose and sucrose also contain glucose. That’s because these two are disaccharides, meaning they’re made up of two molecules instead of one. So, sucrose, for example, is 50% glucose and 50% fructose.

Sugar is found far and wide in food, and can occur naturally or be added. Certain fruits and vegetables like apples, grapes or peas contain fructose, while lactose is present in milk. However, it’s the added sugars that you really need to watch.

What are free sugars?

All added sugars are ‘free sugars’. These are sugars that lack fibre and have no nutritional value. Guidance from the NHS states that adults shouldn’t exceed 30g of free sugars – around seven teaspoons – per day, so it’s important to pay attention to what foods they’re in.

You can find these in abundance in treats like cakes, sweets, biscuits and fizzy drinks. Often the level of added sugar in these items will already exceed the recommended daily limit.

“One regular-sized chocolate bar will have about nine teaspoons of sugar,” explains Dr Malhotra. “So, if you have that and no other sugar in your diet all day, you’ll still go over the maximum recommended, after which it starts causing harm over time.”

However, don’t let yourself be fooled into thinking it’s just stereotypical junk food that’s bad. While whole fruits and vegetables don’t count towards your daily sugar intake, things like fruit juices, sweet syrups and smoothies do.

“One glass of orange juice will have about seven teaspoons of free sugar in it, which is similar to a can of Coca Cola – and the body will treat it in the same way,” says Dr Malhotra. “It will have the same level of harm in terms of sugar content.”

However, not all juices are equal. While pure fruit juices can be a problem, making your own juices using raw fruit and vegetable ingredients can be a good way of getting your nutrients. Following the 80/20 rule, which means using 80% vegetables to 20% fruit, will help you keep the sugar content down. Additionally, it can be helpful to sip slowly and almost treat it as a meal in itself.

How does sugar impact the immune system?

The key issue is that sugar is a toxin. This is a chemical substance that damages our cells, and includes things like stress, alcohol and trans fats.

Toxins undermine our immune system by triggering the production of proinflammatory markers called cytokines.

“Anything that is a toxin to the body will affect the immune system and cause inflammation to the body, whether it’s stress, being inactive, excess body fat,” explains Dr Malhotra. “All these things will increase markers of inflammation in the body, so the body feels like it’s under attack.”

Inflammation is part of the body’s natural healing process. It becomes an issue when we constantly trigger this response – exactly what happens if you consistently eat too much sugar.

As a result of excess sugar consumption, chronic low-grade inflammation will develop. This essentially means that it keeps our immune system on constant red-alert. The immune system becomes less robust at dealing with other threats as it’s diverting resources towards dealing with inflammation. This includes infections like colds or COVID-19.

Additionally, regularly eating too much sugar can cause insulin resistance. When glucose levels are too high for a prolonged period of time, cells stop responding to insulin and blood sugar doesn’t go down. Conditions like type-2 diabetes and prediabetes, associated with a weakened immune response, can develop as a result.

A high sugar intake also puts you at greater risk of developing metabolic syndrome, which is the term given to the dangerous combination of diabetes, high blood pressure and obesity. There’s a strong link between poor metabolic health and risk of death from COVID-19.

What you need to know about sugar and COVID-19

Due to its effects on the immune system, sugar has both a long- and short-term impact on the body’s ability to fight viruses like COVID-19.

Research shows type-2 diabetes and hypertension (high blood pressure) are the most common comorbidities in patients with coronavirus, while metabolic syndrome is also a strong indicator of such risk factors. There is a 10-fold risk of death from COVID-19 for those with these conditions.

Worryingly, an estimated seven out of eight adults in the UK have suboptimal metabolic health, according to Dr Malhotra. That’s because it goes beyond just affecting those classed as overweight or obese. Poor metabolic health is the point at which you have too much body fat.

While you might have a technically healthy weight or normal BMI, you could still be at risk. Instead, it’s important to look out for conditions like excess body fat around the waist, insulin resistance and high blood pressure.

As consuming excess sugar can kickstart all of these issues, doing so won’t put you in good stead when it comes to protecting yourself. Based on flu data, it’s also likely that those who suffer from obesity or a chronic inflammatory state have a higher risk of infection, as well as complications. They are also likely to have the virus longer, meaning they could take longer to recover.

Additionally, eating sugar while you’re actually sick can impact your recovery. Glucose doesn’t just provide energy to your own cells. It will give any viruses and bacteria something to thrive off too.

“In essence, if you’re eating lots of sugar, you’re feeding the virus, you’re giving it more potency,” says Dr Malhotra. “The worst thing you can be doing, if you get COVID-19, is to be eating Krispy Kreme donuts.”

How to improve immunity through diet

Want to better protect your immune system against COVID-19? Dr Malhotra recommends two main actions: regularly taking vitamin D and switching to a low refined carbohydrate diet, which includes minimal sugar.

This applies whether you already have COVID-19 and want to support yourself, or if you want to decrease your risk of having a serious or fatal experience of the virus.

“People think there’s really nothing we can do right now, but it’s just scientifically wrong,” explains Dr Malhotra. “If you look at heart disease, dietary changes rapidly reduce the risk of heart problems within weeks, so in terms of infection and resilience, it’s very similar.”

As Dr Malhotra’s recent bestseller The 21-Day Immunity Plan details, you can significantly reduce your risk in as little as three weeks.

“Metabolic syndrome or these risk factors can be reversed within a few weeks of just changing diet, in particular for people who are the most vulnerable,” he says. “For the worst levels of obesity or metabolic syndrome, they have the biggest effect in the shortest space of time.”

Optimising your diet and lifestyle could also make the COVID-19 vaccine more effective. Studies into the flu vaccine show the efficacy is impaired if you’re obese, have vitamin D deficiency or are inactive, and this is likely to be the same for COVID-19.

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