When Russian physician Dr Elie Metchnikoff first suggested, in 1905, that drinking fermented milk was the key to long life, he sparked a craze that rapidly spread across Europe.

And more than a century later, the trend is back in the form of kefir – a tangy, yogurty, slightly fizzy drink. For some, it smells like milk that has gone off.

Made by adding kefir grains – small clusters of beneficial bacteria and yeast – to milk to make it ferment, it is a rich source of protein and calcium.

Britons now consume £40million worth of it every year, and experts believe this will keep growing, with brands making bold claims their products can improve the immune system, strengthen bones and enhance brain power. Studies have even suggested kefir can improve cardiovascular health, digestion and aid weight loss.

So should we all be drinking kefir? Should it be a fridge staple, or is it just the latest wellness fad?

I decided to find out.

The challenge was simple: every morning for a month I would drink a 250ml bottle of Biotiful kefir, which is available in most supermarkets.

Before starting, I undertook a series of tests. The Functional Gut Clinic in London screened my gut microbiome – the trillions of bacteria, fungi and other microbes that live in the digestive system and play a crucial role in health. I also had my cholesterol level tested and weight recorded.

My results did not make for great reading.

The test revealed I had low gut microbial diversity – meaning there were fewer different types of beneficial microbes living in my digestive system than is considered ideal. This may be linked to a greater risk of health problems, including bowel cancer.

I did have some symptoms to suggest this might be the case. Nearly every morning after a hot shower I would suffer from acid reflux, along with a regular feeling of bloating.

My cholesterol test also revealed that, at the age of just 26, I had slightly elevated ‘bad’ cholesterol. Known as LDL, it is responsible for plaque build-up in arteries, increasing the risk of heart disease and stroke.

My concerning cholesterol level was perhaps less surprising when I stepped on the scales. Weighing in at just over 14 stone, and standing at 6ft, it put my BMI at 27 – firmly in the overweight category.

So I was quite sceptical that making one change to my lifestyle – drinking one glass of kefir a day – could make much difference.

The first impact came two weeks in. After a year of suffering from painful acid reflux in the mornings, it just disappeared.

‘While there is no specific research on kefir and acid reflux, this finding doesn’t really surprise me,’ says Dr Federica Amati, a nutritionist at Imperial College London and part of the team behind gut health app Zoe. ‘This is because our gastrointestinal tract is one tube, so when you are improving the diversity in the colon, which kefir may help to do, that is going to have an impact further up the tract – in this case in the form of acid reflux.’

Another surprising finding came when I stepped back on the scales after four weeks of drinking kefir – I had lost almost 9lb, edging me closer to an average BMI.

Looking back, I think that benefit came from less snacking. Previously, I’d often skip breakfast. But my glass of kefir came in at around 150 calories, with 8g protein and 8g of fat – roughly the equivalent of one-and-a-half eggs and surprisingly filling. It led to me having a later lunch, and meant I didn’t feel the need for my usual mid-afternoon and evening snacks.

Some research suggests the way fermented foods interact with the gut microbiome can affect appetite regulation hormones such as GLP-1 – the same one that weight-loss jabs mimic. However, the evidence for this is still emerging.

The results from my gut microbiome follow-up test, though, show that the kefir certainly had an effect over the month.

Its diversity rose from 134 to 147 species, and the missing ‘good’ bacteria levels also dropped.

There is some research suggesting that kefir can reduce cholesterol – but after a month there was no change in my reading. While a few studies show small drops in LDL from drinking kefir, the evidence is mixed and the impact is modest at best.

It’s also worth noting that full-fat kefir contains saturated fat, which can push cholesterol in the wrong direction. There are low-fat versions available.

Overall, my experience was very positive. I lost weight, found a solution to my acid reflux and improved my gut microbiome – something which is linked to positive long-term health outcomes.

Experts say that while you do not have to down a glass of kefir every morning, there are benefits to increasing our intake of fermented foods.

‘We should aim for three fermented food servings every day,’ says Dr Amati. ‘250ml of kefir is actually equivalent to three servings, but you can also get it from adding miso into your sauces when cooking, having kimchi with your lunch or adding a spoonful of kefir into your breakfast.’

I can’t say I much fancy eating miso and kimchi every day, but I do think I’ll be sticking to my daily glass of kefir. It’s quite addictive.

When opting for kefir, Dr Amati suggests checking the label to avoid brands with added sugars and, where possible, choosing a plain product before adding your own flavourings.

‘We have heard a lot about increasing fibre intake recently, but increasing our intake of fermented foods is just as important and can have a great effect on our overall health,’ she adds.