How to improve gut health naturally: why your gut bacteria need MACs

You've probably heard of fibre. You might have heard of prebiotics. But MACs, microbiota-accessible carbohydrates, are a more precise way of describing what your gut bacteria are actually looking for in your diet.

Not all carbohydrates qualify. Not all fibre behaves in the same way. And the gap between what we think we're eating and what our microbiome can actually use is often wider than people realise.

Here’s what MACs are, why they matter, and how they fit into how to improve gut health in a way that actually works in real life.

What are microbiota-accessible carbohydrates?

MACs are carbohydrates that your digestive system cannot break down, but your gut microbes can. Unlike simple sugars or refined starches, which are absorbed in the small intestine, MACs travel through to the large intestine intact.

Once there, your gut bacteria ferment them. In the process, they produce short-chain fatty acids (SCFAs), including butyrate, propionate, and acetate, compounds that play important roles in your health.

How to improve gut health: why short-chain fatty acids matter

SCFAs are involved in several aspects of how your body functions day to day:

• Gut lining integrity: butyrate is the primary fuel for the cells lining your colon. A well-nourished gut lining helps maintain a strong barrier between your gut and your bloodstream.

  
  

• Immune regulation: SCFAs interact with the immune system and help regulate inflammatory responses. Given how much of your immune system is based in the gut, this has wider implications than many people expect.

  
  

• Blood sugar regulation: SCFAs are linked with improved insulin sensitivity and more stable blood sugar responses over time.

  
  

• Microbial balance: regularly feeding your microbiome supports a more diverse and resilient community of bacteria.

Diets consistently low in MACs are associated with reduced microbial diversity and a higher risk of metabolic and inflammatory conditions over time, including type 2 diabetes, cardiovascular disease, obesity, and certain cancers.

The research in this area has grown significantly over the past decade, and one message is becoming clearer: if you don’t feed your microbes, they struggle to support you.

MACs are not the same as the 'fibre' on a food label

This is where the nuance matters. The fibre figure you see on a cereal box or protein bar tells you how much fibre is there, but not what kind it is or how your gut bacteria will respond to it.

Processing can change the structure of fibre and how it is fermented in the gut. And isolated fibre additives, often listed as inulin, chicory root extract or soluble corn fibre, don’t behave in the same way as the mix of structurally complex carbohydrates found in whole plant foods.

While some of these added fibres can still feed gut bacteria, they tend to act differently and often support a narrower range of microbes compared to whole foods.

MACs are far more likely to come from whole, minimally processed plant foods, where fibre remains part of a wider structure that supports a broader and more balanced microbial response.

MAC-rich foods worth knowing about

The good news is that these foods tend to look very ordinary. No specialist products or supplements needed.

Legumes

Lentils, chickpeas, black beans, and split peas are among the most MAC-dense foods available. They are also versatile, affordable, and easy to batch cook.

Wholegrains and resistant starches

Steel-cut oats, wholegrain oats, barley, rye, quinoa, and buckwheat all provide MACs that refined grains lack.

Cooked and cooled potatoes and rice are also worth including. The cooling process increases resistant starch, which behaves like a MAC in the gut.

Vegetables and fruit

Onions, garlic, leeks, asparagus, Jerusalem artichokes, apples with the skin on, green (less ripe) bananas, and berries are all good sources.

The skin of fruit and vegetables often contains a significant proportion of these fibres, so peeling out of habit is worth reconsidering.

Seeds and plant proteins

Chia seeds, flaxseeds, and pumpkin seeds are easy ways to add MACs to meals.

Firm tofu and tempeh are also useful additions, offering both plant protein and compounds that support gut health.

A note on low-FODMAP approaches

If you follow a lower-FODMAP diet for IBS, some higher-fructan foods, such as garlic, onions, and legumes, may not suit you in larger amounts.

That said, there is still plenty of room to nourish your microbiome. Oats, cooked and cooled potatoes or rice, ripe bananas, berries, seeds, and tofu can all contribute MACs without triggering symptoms for many people.

As always, individual response varies. If you are managing IBS or another digestive condition, working with a practitioner to reintroduce and build tolerance to MAC-rich foods gradually is often the most sustainable approach.

How much fibre do you need?

Adults in the UK are advised to aim for around 30g of fibre per day, yet most people fall significantly short. The average intake is closer to 18g.

Focusing on whole, minimally processed plant foods, rather than fibre supplements or fortified products, is one of the most reliable ways to increase both fibre and MAC intake.

In practice, the nuance often comes down to pace and variety rather than volume alone.

Some people need to increase gradually to allow their microbiome to adapt. Others benefit more from expanding the range of plant foods they eat rather than simply increasing quantities of the same few foods.

The starting point is usually simple: eat more whole plants, and eat a wider variety of them.

Easy ways to bring more MACs into your meals

A MAC-focused approach is about adding, not restricting. No foods are off the table. The aim is simply to crowd in more of what your microbiome can use.

Some practical places to start:

• Stir a handful of lentils into your soup or salad

• Top your morning oats with berries and a sprinkle of flaxseeds

• Roast extra potatoes with dinner, cool them overnight, and use them in a salad the next day

• Keep the skin on apples and pears

• Add chickpeas to a pasta dish or curry

• Swap crisps for a small handful of roasted pumpkin seeds

• Swap white rice for quinoa or barley at one meal this week

Small, consistent changes can make a meaningful difference to what your microbiome has to work with.

Working with your gut health as part of a wider picture

If you are experiencing persistent bloating, fatigue, irregular digestion, or hormonal symptoms that don’t quite add up, your microbiome may be one piece of the picture worth exploring.

Nutritional therapy looks at the full context including what you are eating, how your body is responding, and what your individual needs actually are.

If you would like to explore what a personalised, root-cause approach could look like for you, feel free to get in touch or book a free discovery call with me here.

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Disclaimer: This article is educational and informational only. It does not constitute medical advice. Please consult with a healthcare professional before starting any new supplement or making significant dietary changes, particularly if you have existing health conditions or take medications.