Bacteria: how quickly can it change?

The gut is not just where food is digested. It is a living ecosystem thriving with bacteria. Literally trillions of them.

By weight, they are between 1-3% but they are more abundant than human cells.

You are home to trillions of bacteria, viruses, and fungi known as the gut microbiome. Their role is to support digestion, immunity, and metabolism.

Remarkably, the bacteria can change. Sometimes in a day.

In a landmark study published in Nature [link here], researchers asked volunteers to switch between two extreme diets. One plant based and another animal based.

Within 24 hours, the participants’ gut bacteria shifted dramatically. Fiber-loving species such as Roseburia declined, while bile-tolerant microbes like Bilophila wadsworthias surged.

When they returned to their normal diets, the microbiome reverted to baseline within two days.

Another study found that people who habitually consumed high-fat, high-protein diets had a very different microbiome from those on high-fiber, plant-rich diets.

In simple terms, if you ate the same thing, the flora of your microbiome remained largely stable.

This flexibility is both a strength and a vulnerability.

The gut adapts quickly to new foods, climates, and lifestyles. Yet short-term extremes such as crash diets, fasting, or high-sugar intake can disrupt microbial balance.

Fortunately, the change is reversible once a balanced, fiber-rich diet is reintroduced.

So what does this mean for you?

In simple terms, your gut is a mirror of your habits. It responds within hours but builds strength over months and years. So what is it responding to?

In short everything.

Prebiotics, fibre that the bacteria will use to multiply. Probiotics, which contain the genus of bacteria themselves. But also alcohol or antibiotics, which can wipe out the bacteria quickly.

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