Love your guts: Prebiotics and Probiotics

I’m interested in what diet does to the brain, but naturally I think about how we digest the foods we eat, and how they influence our physiology.

The digestive system is the engine of the body. It’s key responsibility is for breaking the foods (fuel) we consume down into glucose (which provides the energy to power cells) and the key nutrients for the biochemical reactions that happen within our organs. But we are not the only ones getting fed when we eat a meal. Our intestines are home to an entire ecosystem of organisms that influence how our body processes food, poses an immune response to pathogens and even the moods and emotions we experience.

The gut microbiome

Your gut contains trillions of microbes (including single celled organisms including bacteria, archaea and fungi) that interact with our body in the same way that humans live on Earth. These microbes together form a “microbiome”, which can be defined as the full collection of genes of all the microbes in a community. The human microbiome (all of our microbes’ genes) can be considered a counterpart to the human genome (all of our genes). Amazingly, the genes in the microbiome outnumber the human genome by more than 100 times.


Microbes interact within their communities, and they respond to their surrounding environment. In the case of the gut microbiome, this means the foods that are digested. Our microbial populations shift when their environment changes, so eating an unhealthy diet full of sugar and saturated fat encourages different microbes to grow in comparison to a healthy diet.

I think of it like the different soils that plants grow in – certain plants grow well in a nutrient rich compost, but they won’t survive in sandy soil near a beach!

How can we change our gut microbiome?

We can change the different populations of microbes through environmental changes, which can be achieved through either consuming probiotics or prebiotics.

Probiotics are a food or dietary supplement containing  certain microorganisms (such as Lactobacillus or Bifidobacterium) that when consumed maintains or restores beneficial bacteria to the digestive tract. Probiotics also describe a substance that stimulates the growth of microorganisms. 

Probiotic-rich foods include live yoghurt, kefir, miso, tempeh, kombucha, kimchi, buttermilk, sauerkraut and other fermented vegetables. Adding these foods to meals can really boost probiotic benefits. Fermented foods not only contain still-living microbes that aid gut health, but these foods are also prebiotic, as the microbes have already processed these foods for us, making them easier to digest and often unlocking nutrients that our own body could not otherwise have accessed.

The difficulty with probiotics is ensuring that when they are ingested as a food supplement they land in a vat of gastric acid in your stomach. Gastric acid is an important element of digestion because it 1) breaks down protein into amino acids, 2) provides the appropriate pH for digestive enzymes like pepsin to work, and 3) it kills many harmful microorganisms that might have been ingested along with the food. The problem when you ingest probiotics is point 3 – gastric acid can kill the good bacteria.


(Showing my age here, but the this always reminds me of the bit in Who Framed Roger Rabbit where Judge Doom murders the cartoon shoe, which thoroughly upset me as a child, and clearly haunts me to this day.)


This is where faecal microbiome transplants (FMTs) can come in. Faecal matter, (or stool, or poop, or whatever you want to call it), is collected from a donor, mixed with a saline or other solution, strained, and placed in a patient’s lower intestine / bowel by colonoscopy, endoscopy, sigmoidoscopy (these all basically involve a squirty tube), or enema, so bypassing the stomach! However, researchers have been investigating new ways of controlling the “dose” of probiotics that can be ingested orally, so they survive into the gut.

Pills containing frozen faecal matter have been used to combat life threatening infections with Clostridium difficile (C. diff),  an antibiotic resistant bacteria that causes debilitating diarrhoea. Doctors have recently discovered that a faecal transplant will restore good gut bacteria that combats the C. diff.

Not feeling the FMT? There are other ways to help out your gut bugs

In order for the good bacteria to survive in the gut, you need to feed them ‘prebiotic’ foods. Prebiotics are non-digestible food fibres that enable beneficial bacteria to stick to the intestinal wall and also helps to stimulate their growth. I think of them as the nutrient rich fertiliser that helps plants grow.

Prebiotic-rich foods include bananas, onions, asparagus, soy beans, Jerusalem artichokes, whole oats, wheat, barley, garlic, flaxseeds, legumes, tomatoes and leafy green vegetables (spinach, kale etc).

As with most foods, the composition of prebiotic fibre foods changes when they are cooked. Heating anything changes its composition causing certain parts to break down and be lost, thus altering its nutritional parameters. In the case of prebiotics, if you cook them, you lose some of that precious prebiotic fibre. Salads are a great way of getting prebiotics into your diet… see below for my go to gut bug friendly salad recipe…


How do your gut bugs influence your health?

Your gut flora plays an important  part in metabolising the foods that you eat, particularly fibre, and form short chain fatty acids (SCFAs). Recent evidence suggests that SCFAs made by the gut microbiota may regulate metabolic processes, including insulin sensitivity. This is particularly important to understand as so many people nowadays are overweight or obese, and this is a key factor in the development of type 2 diabetes. Harnessing gut bacteria may help restore insulin sensitivity in those who have developed resistance and halt the development of diabetes.

The results of evidence-based analyses from experimental studies have shown the clinical potential of probiotics against many diseases and disorders, including irritable bowel symptoms, suppressing diarrhoea and prevention of inflammatory bowel disease. More recently, research has focussed on “psychobiotics” – using probiotics or prebiotics to shift the gut microbiome composition to help with mood and cognitive control, this is opening up a whole new field of therapies for people suffering from mental and physical disorders.


Gut flora friendly Green lentil Greek salad


(Serves 2-3 people, or me for 3-4 days of work lunches)


1 tin of ready cooked green lentils, drained and rinsed

1 red onion

10 cherry tomatoes

Half a continental cucumber

Half a block of Goat or Sheep milk feta, crumbled

1/4 cup pepitas, I also threw in some sesame and sunflower seeds

1/3 cup walnuts

Chopped fresh mint leaves (1-2 handfuls)



Juice of 1 lemon

1 clove of garlic (crushed)

1 tablespoon Apple Cider Vinegar

2 tablespoons avocado oil

1 teaspoon dried sumac* (or paprika / sprinkle of chili if you can’t find sumac)

Salt / Black pepper to taste



Cut up all the salad ingredients and mix together in a big bowl.

Mix up dressing. Pour over salad and mix about. Voila. Eat with spoon straight from bowl or serve on a plate like a civilised person.

(and hey, feel free to mix it up with other things, add rocket, baby kale, capsicums, celery… most of my meals consist of whatever I can find in my fridge).

*I’m obsessed with sumac. It has an amazing fresh tangy lemony flavour and is often used in Lebanese, Syrian, Turkish and Iranian cuisines. It pairs well with fish, chicken, beef, lentils, eggplant, chickpeas, rice…





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