Immune function and dietary sugars

This content is authored by Registered Dietitian, Juliette Kellow.

Understanding the immune system

The immune system consists of a complex network of cells, tissues and organs, including white blood cells, the lymphatic system, bone marrow, spleen and thymus. These work together to detect and respond to potential threats, helping to protect the body against infection and disease. Threats to the body include bacteria, fungi, viruses, toxins, foreign bodies and physical injuries. The immune system also responds to internal threats, such as abnormal changes in cells that can lead to cancer.

According to the World Health Organization (WHO), immunity is defined as the body’s ability to resist disease. It enables the body to defend itself against harmful pathogens through the actions of the immune system (1). There are two main types of immunity:

Innate immunity

This type of immunity is the body’s first line of defence and is present from birth. It includes the skin and thin mucous membranes lining the mouth, nose, airways, lungs and digestive system. These act as physical barriers to help prevent harmful substances from entering the body. It also includes certain immune cells that provide a rapid, but non-specific response to potential threats.

Acquired or adaptive immunity

This type of immunity develops throughout life, after exposure to specific diseases or through vaccinations, which introduce a small, safe amount of a disease-causing substance – or part of it – to stimulate the immune system. Examples include vaccines for COVID-19, measles, flu, shingles, whooping cough, diphtheria, tetanus, polio and human papillomavirus. In response to a specific harmful substance, such as a virus, the body produces antibodies that bind to it and target it for destruction. The immune system then remembers these antibodies, so it can respond quickly and help destroy it if the same threat appears again (2).

Immune system function

While the immune system functions to identify and eliminate threats that may harm the body, the way it responds varies according to the type of threat. For example, viruses, bacteria and allergens all trigger different kinds of immune response.

The immune system and inflammation

One consequence of some immune responses is the release of chemicals that trigger inflammation, typically leading to symptoms such as swelling, redness and pain. Common examples include stings, bites or throat infections. This type of inflammation is temporary, happens quickly and is designed to eliminate the cause of injury or infection and start the healing process.

However, some threats can keep the immune system activated over long periods of time, causing it to continually release inflammatory chemicals (3, 4). For example, a build-up of fatty deposits in artery walls, known as plaque, keeps the immune system on constant alert. This triggers an ongoing inflammatory response, which, over time, can further damage the artery walls, allowing even more plaque to accumulate. This creates a cycle in which increasing plaque triggers more inflammation, which causes more damage (5).

This type of persistent inflammation – often referred to as low-grade chronic inflammation – usually develops without any noticeable symptoms and has been linked to the development of conditions such as cardiovascular disease (6, 7), type 2 diabetes (8), dementia (9), and cancer (10).

Factors that affect immune function

A person’s immune function depends on many factors, including their genetic make-up, previous infections and any vaccinations they have had. Environmental factors such as air pollution can also have an impact.

Many medical conditions are linked to the immune system, too. For example, allergies occur when the immune system overreacts to substances that are normally harmless. Autoimmune diseases such as type 1 diabetes, rheumatoid arthritis and lupus develop when the immune system mistakenly attacks healthy cells in the body (13).

Immune function tends to decline with age, as cells involved in immune responses decrease in number or become less effective. Illnesses, obesity, certain medications and lifestyle also play an important part: smoking, chronic stress, poor sleep, excess alcohol, inactivity, overtraining (exercising harder than your body can recover from) and unhealthy diets can all impair immune function (11, 12).

How diet affects immune function

Many nutrients contribute to the normal function of the immune system, including iron, zinc, copper, selenium and vitamins A, B6, B9 (folate), B12, C, and D. Nutrients such as vitamins A, B2, B3, and B7 (biotin) play a part in maintaining normal skin and mucous membranes, which provide the body’s first line of defence against harmful substances. Vitamin C and zinc are also important for normal skin (14, 15).

Research suggests long-chain omega-3 fats, found naturally in oily fish, may help to modulate the immune system, for example, by influencing immune cell activity. Higher blood levels of these omega-3 fats have also been associated with lower levels of inflammatory substances in the blood, suggesting these omega-3 fats may help to regulate inflammation. However, this is an area that needs further research before firm conclusions can be drawn (16, 17, 18).

Increasing evidence suggests that the gut and immune system work together to help protect the body from infection and disease. Studies suggest that a diverse and balanced gut microbiome supports the immune system in many ways, for example by helping to prevent harmful bacteria from taking over, maintaining a healthy gut barrier, regulating immune responses and influencing inflammation (19).

Image of the gut surrounded by healthy food

Consisting of trillions of microscopic organisms—mainly bacteria—in the large intestine, along with the substances they produce, the gut microbiome is influenced by many factors, particularly diet. The strongest evidence supports a dietary pattern that is rich in fibre-containing plants like wholegrains, pulses, fruit, vegetables, nuts and seeds (20, 21). Furthermore, many plant-based foods are also rich in polyphenols – plant chemicals such as flavonoids that are found naturally in plants – which some studies suggest may have anti-inflammatory properties (22).

Research on dietary patterns consistently suggests that a Mediterranean-style diet – rich in wholegrains, fruit, vegetables, pulses, nuts, seeds, fish and olive oil, with small to moderate amounts of meat, eggs and dairy – is associated with healthier immune function and lower levels of inflammation (23). This way of eating typically provides key nutrients such as fibre, long-chain omega-3 fats, and a variety of vitamins and minerals, all of which play important roles in supporting the immune system. A recent review of randomised controlled trials also found the Mediterranean diet was associated with lower levels of several inflammatory markers, suggesting it may help to protect against chronic inflammation, although the researchers acknowledged that further high-quality studies are needed to confirm this (24).

In contrast, less healthy dietary patterns – typically high in saturated fat, refined carbohydrates and low in fibre, fruit and vegetables – have been linked to poorer immune function (25) and higher levels of inflammation (26, 27). However, much of the evidence for this is observational, meaning it shows associations but cannot prove cause and effect. That said, less healthy eating patterns may provide fewer nutrients that support immune function and can often lack fibre, which helps to support a diverse gut microbiome.

Dietary sugars and immune function

In the popular press, high intakes of dietary sugars are often blamed for being harmful to the immune system, causing inflammation or increasing susceptibility to infections. Alongside this, sugar “detoxes” are sometimes promoted to reduce inflammation or “boost” immune function. However, these types of claims are overstated and not supported by scientific evidence.

Immune cells use glucose as a source of energy (they can also use amino acids, which are building blocks of proteins, and fatty acids), with requirements increasing when they are stimulated, for example, when the body is fighting an infection (15). This glucose comes from the breakdown of carbohydrates during digestion and is then absorbed into the bloodstream, where it is transported to cells for energy or is stored for later use. Because glucose provides energy for cells of the immune system, popular claims that sugar is “harmful to the immune system” or that “giving up sugar boosts immune function” are not supported by current scientific evidence.

Evidence linking dietary sugars to inflammation is also limited. While associations between high intakes of dietary sugars (especially from sugars-sweetened drinks) and raised inflammatory markers in the blood have been seen, these are mainly findings from observational studies (28, 29), which are unable to prove cause and effect. Other related diet and lifestyle factors may be responsible for the observed associations and additional evidence from intervention studies are needed in order to draw conclusions (30).

It is also important to recognise that eating and drinking itself triggers some short-term metabolic and immune responses as the body processes nutrients and monitors for potentially harmful substances, but these responses are normal and helpful. Claims that normal short-term rises in blood glucose after eating – sometimes referred to in the popular press as ‘glucose spikes’ – trigger harmful inflammation in otherwise healthy people are not supported by strong scientific evidence. However, studies do suggest that hyperglycaemia (abnormally high blood glucose) – such as in people with prediabetes or poorly controlled diabetes – over a long period of time may affect immune function (31) and be associated with chronic inflammation (32).

Image of sugars on one side, and on the right side, fruit

Summary

There is currently a lack of good-quality long-term research to suggest that high intakes of dietary sugars, on their own, impair immune function or cause chronic inflammation. Instead, scientific studies give greater support to the idea that overall dietary patterns, rather than single foods or ingredients, have a bigger impact on our immune system.

References

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