Diet and immunity: What’s the connection?

I have a confession to make. Sometimes, I spend too much time on Facebook. In my defence, I don’t post pictures of my lunch, or watch cat videos (well, not very often, anyway ;-)). No, my version of spending too much time on Facebook is getting involved in pointless discussions/arguments with people who hold silly beliefs about nutrition and health.

Just recently, a member of a Facebook group I’m in posted to ask if improving her son’s diet could help with his glue ear (a build-up of fluid in the middle ear that often follows a middle ear infection). Another member of the group – an individual who happens to work in a research capacity for the NSW Department of Health – replied that diet wouldn’t make any difference to her son’s condition unless he was a malnourished child from a developing country, and that she should give him the antibiotics that the GP had recommended as soon as he diagnosed the glue ear.

I couldn’t believe what I was reading. First up, every reputable authority on children’s health advises against using antibiotics for glue ear unless the condition persists for at least 3 months after it’s diagnosed (for example, see the Royal Children’s Hospital Melbourne’s Clinical Practice Guidelines for Treating Otitis Media and their Parent Fact Sheet on Treating Glue Ear, the UK National Health Service’s advice on treating glue ear, and the Cochrane Review ‘Antibiotics for otitis media with effusion (‘glue ear’) in children’).

So just what was a researcher from the Health Department doing, urging a parent to contradict all this expert advice and give her son a treatment that was extremely unlikely to offer any benefit to him, may cause long-term disruption to his gut microbiome, and contributes to the development of antibiotic-resistant ‘superbugs’?

But what I found even more disturbing was the researcher’s insistence that diet makes no difference when it comes to fighting infection, unless you’re suffering from the kind of protein-calorie malnutrition that we in Western countries only see in TV reports on starving kids in Africa. This belief is so wildly wrong, I can’t begin to tell you how perplexed I was that anyone who has read any research at all in the field of nutrition could hold it. Like I said, I wasted WAAAAAY too much of my precious time arguing with this closed-minded person, so I’m going to make sure I didn’t completely waste my time by sharing this important information with you, instead!

For those who love all the sciencey stuff, I have a detailed article on diet and immunity that you can download. That article is several years old now though, and there’s been some very interesting research on the link between diet, the gut microbiome, the immune system and human health since I wrote it.

Here are the points you need to know:

Sweet and deadly

Research from over 40 years ago indicates that concentrated sugar – whether in the form of glucose, fructose, sucrose, honey, or orange juice… but not starches – reduces the ability of white blood cells to gobble up invading bacteria, a process known as phagocytosis (pictured above). Shockingly, the reduced bug-chomping capacity persisted for at least 5 hours after volunteers ate the various kinds of  sugar (1). Think about that for a moment. If you consume the typical Western diet, which contains added sugar in everything from breakfast cereal to biscuits, muesli bars to McDonald’s fries and chai latte to cheesecake, you’re essentially paralysing your white blood cells from the first meal of the day until long after you’ve turned in for the night.

Fat chance of defending yourself

Excess fat intake raises the production of the inflammatory prostaglandin E2. The omega 6 fat arachidonic acid, found primarily in animal products, and overproduced if we eat too many vegetable oils, alters the balance of fats in the membranes of our immune cells, disrupting their function (2).

Saturated fats (which come mostly from animal products, but also occur in large amounts in coconut and palm oils) are even worse, because of their impact on Toll-like receptors, a crucial element of the immune system designed to evaluate what specific type of invader – bacterial, viral, or fungal – is threatening the body (3). Toll-like receptor 4 (TLR4) is designed to sense the presence of bacteria. In order to carry out its job, TLR4 binds to a part of bacteria called lipopolysaccharide, which is mostly composed of saturated fatty acids (4). A high dietary intake of saturated fats causes TLR4 to constantly ring the alarm bell, announcing to the rest of the immune system that a bacterial invader is present and inciting inflammatory activity to combat it (5). This misguided attack on saturated fat causes intestinal  hyperpermeability (‘leaky gut’), further increasing immune dysfunction and worsening infection control (6). It’s also a major factor in triggering autoimmune disease (7).

Being overweight – or even just having a higher than ideal body fat percentage – compromises your immune function in several ways:

Firstly, fat cells are constantly pumping out inflammatory substances including interleukin (IL-) 1, IL-6, and tumor necrosis factor (TNF) (8). Over time, this constant release of substances that should be deployed only when there’s an infection to fight, may result in a ‘boy who cried wolf’ situation – the immune system becomes so blasé about the constant warnings of danger, that it doesn’t respond to a real threat when it should (9).

Secondly, obese people have been found to have significant abnormalities in the number and activity of their infection-fighting white blood cells compared to lean individuals: they have fewer natural killer cells (which kill tumour cells and virus-infected cells), and reduced activity of T and B cells which give the immune system its ‘memory’ – that is, its ability to rapidly respond to bugs that it has encountered in the past, so it’s able to fight them off without you actually getting sick (10).

What this means is that if you’re carrying extra fat mass, you’re more likely to pick up an infectious disease; you’ll feel sicker when you have it (because it’s the inflammatory compounds produced by your immune system, not the bug itself, which cause the malaise, lassitude, fatigue, numbness, coldness, muscle and joint aches, and reduced appetite you suffer when you’re fighting off a bug (11); and you’ll be sicker for longer.

Good bugs, bad bugs

All of us have a diverse mixture of bacteria inhabiting our gut, primarily the large intestine (colon). There are beneficial bacteria (probiotics), which produce vitamins and health-promoting short chain fatty acids such as butyrate; help us to excrete metabolic toxins; and instruct our immune system to fight dangerous invaders while tolerating friendly bugs. Then there are dangerous bacteria (pathogens) which, if they get the upper hand, can produce toxins that can make us sick and even kill us. And then there are bacteria that could swing either way (pathobionts) depending largely on the prevailing food supply that’s available to them.

Broadly speaking, beneficial bacteria prefer to ferment the indigestible portions of carbohydrate (fibre and resistant starch) while unfriendly bacteria thrive on refined carbohydrate, protein and fat (12). So what you choose to eat doesn’t just feed you; it feeds the teeming populations of bacteria that inhabit your gut… for better, or for worse.

Pulling it all together

Back to the woman who posted in the Facebook group I belong to. By pulling the refined carbohydrate, vegetable oils (including saturated vegetables fats such as palm and coconut) and animal products out of her son’s diet, and increasing his intake of fruits, vegetables, whole grains and legumes, she will be essentially deactivating the handbrake that holds his immune system back from effectively combatting the bug that caused his middle ear infection – regardless of whether it’s bacterial or viral. On the other hand, if she gives him antibiotics, she’ll destroy the probiotic bacteria in his gut and further cripple his immune system’s ability to respond appropriately to infection.

Please note that I don’t oppose the use of antibiotics across the board. They are the appropriate prescription for life-threatening bacterial infections. But they’re completely inappropriate, and frankly harmful, for garden-variety middle ear infections and most cases of glue ear. Dietary change should always be trialled for several months before you even think about antibiotics, except in very rare circumstances.