Food headlines contradict each other from one week to the next, so it’s no surprise that many women are unsure what to believe. Low fat or low carb? Grains or no grains? Are eggs fine, or not? Underneath all of that noise, the research on which way of eating best protects against long-term illness has been consistent for thirty years. It points to the Mediterranean pattern, and in the last decade it has also explained why that pattern works so well. A large part of the answer is what your food does for the bacteria living in your gut.

Why the Mediterranean pattern keeps coming out on top

The Mediterranean way of eating is built on vegetables, fruit, beans, lentils, whole grains, nuts, seeds, olive oil and fish, with meat in smaller amounts. A 2024 analysis of 28 studies, covering more than 679,000 people, compared those who followed it most closely with those who followed it least. The closest followers had a 23 per cent lower risk of dying from any cause, and a 27 per cent lower risk of dying from heart disease or stroke. Those are figures from adults in later life.

The reason it works is only partly the nutrients. In a one year trial across five countries, including the UK, 612 people aged 65 and over ate a Mediterranean diet designed for their age group. The bacteria that increased in their guts were the ones that produce short-chain fatty acids, and those same bacteria were linked to lower markers of inflammation, less frailty and better memory and thinking.

You might also like my post: What to eat on the Mediterranean diet

Your gut bacteria do far more than digest your food

The fibre in vegetables, fruit, beans and whole grains is not just roughage. Your own enzymes can’t break most of it down, so it passes through to your large intestine, where trillions of bacteria can. They ferment it into short-chain fatty acids, and one of these, butyrate, is the main fuel for the cells lining your colon. Butyrate also tightens the junctions between those cells, so the barrier between your gut contents and your bloodstream stays sealed, and it encourages the immune cells that keep inflammation in check.

The same bacteria make B vitamins and vitamin K, some of which you can absorb from your colon. And, through the butyrate they produce, they influence how the immune cells in your gut wall behave, which is why the bacteria you feed have an effect well beyond digestion.

This is why fibre intake tracks so closely with long-term health. A series of analyses in The Lancet drew on 185 observational studies and 58 trials. People eating the most fibre had a 15 to 30 per cent lower risk of dying from any cause, and of dying from heart disease, than those eating the least. Between 25 and 29 grams a day gave most of the benefit, and more was better still. The UK target is 30 grams a day, and the average British adult manages about 18.

The modern diet works against your gut bacteria in several ways at once. Low fibre, repeated courses of antibiotics, regular anti-inflammatory painkillers, long periods of inactivity and a diet heavy in processed food all narrow the range of species you carry. As you pass 40, the diversity of your gut bacteria also tends to fall, so what you eat carries more weight, not less.

You might also like my post: Gut Microbiome Diversity After 40

How quickly can food change your gut bacteria?

The change begins within days. When researchers switched volunteers between a diet made entirely of animal foods and one made entirely of plants, the balance of bacteria in their guts began to change within a day, and the change was clear within days. Bacteria that thrive on bile increased on the animal diet, and the ones that break down plant fibre increased on the plant diet.

It means the direction is in your hands, and you don’t have to wait months to see a return on a change. Here are the five changes that make the most difference.

1. Eat fermented foods regularly

Live yoghurt, kefir, sauerkraut, kimchi, miso and traditionally made cheeses all carry live bacteria. In a ten week trial, 36 healthy adults were assigned to either a diet high in fermented foods or one high in fibre. In the fermented food group, the diversity of gut bacteria increased, and the effect was larger in those who ate the most. Nineteen inflammatory proteins in their blood fell over the trial, including interleukin 6, which is raised in type 2 diabetes and rheumatoid arthritis.

The fibre group did not see the same drop in inflammation over those ten weeks, and their diversity stayed where it was rather than rising, although their bacteria did produce more of the enzymes that break fibre down. The researchers concluded that adding fibre on its own, over a short period, is not enough to widen the range of species you carry. Fibre feeds the bacteria you already have, while fermented foods introduce new ones. You need both, and it’s the fermented foods that most of us are missing. I’ve written more about the difference between probiotics and prebiotics, and about how to ferment garlic at home if you’d like to try making your own.

2. Include prebiotic foods often

Prebiotic foods are the ones your bacteria can digest but you can’t. In practice that means high fibre plant foods, and in particular the ones containing inulin: Jerusalem artichokes, chicory, leeks, onions, garlic and asparagus. Oats, beans, lentils, flaxseed and slightly green bananas supply other fibres that ferment well.

The bacteria that ferment these fibres produce the butyrate described above, so each meal that includes them is feeding the cells of your gut lining as well as you. Variety helps here, because different fibres feed different bacteria. Aiming for a wide range of plant foods across the week does more for diversity than eating a lot of one thing.

You might also like my post: What is Flaxseed Good For?

3. Eat plenty of foods rich in polyphenols

Polyphenols are the plant compounds behind the colour and bitterness in berries, red grapes, olives, cocoa, tea, coffee, onions, garlic, nuts and herbs. Only 5 to 10 per cent of the polyphenols you eat are absorbed in your small intestine. The rest travel on to your colon, where your bacteria break them into smaller compounds that you can absorb and use. Some of those compounds are the antioxidants your body puts to work. In return, polyphenols encourage the bacteria you want more of, including Bifidobacterium and Akkermansia, the species linked to a healthy gut lining and lower fasting glucose.

The Mediterranean diet is rich in polyphenols, and extra virgin olive oil is one of its main sources. So is a good handful of fresh herbs, which is one of the easiest additions to make.

You might also like my post: Does Extra Virgin Olive Oil Reduce Inflammation?

4. Keep ultra-processed foods to a minimum

Ultra-processed foods are the packaged products made from ingredients you would not find in a kitchen: protein isolates, modified starches, emulsifiers, sweeteners and flavourings. In 2024, researchers pooled 45 analyses covering nearly ten million people. The people eating the most ultra-processed food had around a 50 per cent higher risk of dying from cardiovascular disease. They also had a 48 per cent higher risk of anxiety, and a 12 per cent higher risk of type 2 diabetes. All three were graded as convincing, the strongest class of evidence in the review, although the researchers rated the quality of the underlying studies as low for most outcomes.

Part of the explanation is what these products do to your gut bacteria. Emulsifiers are added to give sauces, ice cream and baked goods a smooth texture. In a controlled feeding study, healthy adults ate an identical diet with or without the emulsifier carboxymethylcellulose for 11 days. In the emulsifier group, the diversity of gut bacteria fell and the production of short-chain fatty acids dropped. In two people, bacteria were found pushing into the inner mucus layer that normally keeps them away from the gut wall. That encroachment is one of the first signs of gut inflammation.

Artificial sweeteners behave in a similar way. In a trial of 120 adults, two weeks of saccharin, sucralose, aspartame or stevia each changed the bacteria in the gut and mouth, and saccharin and sucralose also worsened the way people handled glucose. When the researchers transferred those people’s gut bacteria into mice, the mice developed the same glucose problems, which shows the bacteria were carrying the effect.

You might also like my post: Artificial Sweeteners and Gut Health

5. Keep red meat low

Red meat is rich in carnitine, and some of your gut bacteria convert it into a compound your liver then turns into TMAO, which is linked with atherosclerosis. In a trial where every meal was provided, 113 healthy adults ate a month each of diets built on red meat, white meat or plant protein. On the red meat diet, TMAO in the blood was more than twice as high as on either of the others, and it fell again within four weeks of switching. How much TMAO you make depends on which bacteria you carry, and that depends on what you feed them.

A small amount of good quality meat is part of the Mediterranean pattern, so the aim is a few times a week rather than every day, with fish, beans and lentils doing more of the work.

You might also like my post: The Hidden Power of the Microbiome in Heart Disease Prevention

Where this leaves you

A diet high in vegetables, fruit and fibre, with fermented foods most days, plenty of polyphenols, and red meat and ultra-processed food kept low, is the same advice the research has been giving for decades. What has changed is that we now understand why it works: your gut bacteria are doing much of the protecting, and they respond to the next meal you give them.

There is no need to wait for a new plan or a new year. Your gut bacteria respond to each meal, so one change at the next one is already a change to how your gut lining, immune system and metabolism are supported.

You might also like my post: 10 Best Foods to Restore Gut Health