It was one of the most popular fad diets of recent decades – rocketing to fame in 2013 after a single tweet from Victoria Beckham.

‘Love this healthy eating cook book!!’ gushed the former Spice Girl turned fashion designer, alongside a picture of recipe book Honestly Healthy.

Beckham was, at the time, almost as famous for her slender frame and rigid eating habits as her pop or fashion careers – and within a year, the book, which outlined ‘how to eat the alkaline way’, was a number one bestseller with a sequel in the works.

The alkaline diet quickly gained a celebrity following, including actress and wellness guru Gwyneth Paltrow, Friends star Jennifer Aniston and supermodel Elle Macpherson.

The approach divided foods into either acidic or alkaline.

Those that ‘produce acid’ when digested, such as meat, poultry, dairy, eggs, processed foods and alcohol, are deemed to raise the blood’s naturally slightly alkaline pH level and unbalance the body. This, in turn, can lead to a range of problems from low energy and poor memory, to headaches, muscle pain and insomnia.

So-called alkaline foods, meanwhile – fresh fruit, vegetables, nuts, legumes and wholegrains – help to restore the body’s pH balance, improving health problems and even slowing the ageing process.

By cutting out all acidic foods, alkaline diet advocates claimed, dieters could prevent everything from tiredness, bloating and spots to serious illnesses such as heart disease, bone-thinning condition osteoporosis, arthritis and even cancer.

Victoria Beckham, who was famed for her slender frame and rigid eating habits, pictured in 2003

Gwyneth Paltrow is said to be a fan of the alkaline diet, which aims to cut out acidic foods

Supermodel Elle Macpherson was also a fan of the diet, which helps to restore the body’s pH balance

But just as quickly as it became a worldwide phenomenon, the diet fell from grace, as experts cast doubt on the science behind it.

High-profile medics refuted the idea that blood pH could be changed by diet – or that the diet could prevent cancer. Any benefits, they countered, were due to the fact that ‘acidic’ foods to avoid – including caffeine, sugar and anything processed – were often less healthy, while ‘alkaline’ foods included fresh fruit and veg.

In 2017, the ‘naturopathic doctor’ credited with inventing the diet, Robert Young, was jailed for practising medicine without a licence – and allegedly defrauding terminally ill cancer patients by ‘treating’ them with infusions of baking soda.

Now, more than a decade after its shady heyday, pioneering research is bringing the alkaline diet back into the spotlight. Recent studies suggest that it may reduce the risk of conditions including osteoporosis, type 2 diabetes and kidney disease.

Unlike the vague, detox-like claims of the past, the modern approach is based on a measurable concept, called potential renal acid load, or PRAL.

This estimates the amount of acid the kidneys must excrete after food is metabolised. That acid comes from the breakdown of certain nutrients – particularly protein and phosphorus – which generate compounds such as sulphuric and phosphoric acid as waste.

Foods with a negative PRAL – including fruits, vegetables and legumes – are considered alkalising, as they produce fewer of these acidic by-products. In contrast, meat, dairy and refined grains typically have a positive PRAL, meaning they generate more acid and so are classed as acid-forming.

It’s the long-term build-up of these by-products, researchers believe, that may contribute to chronic disease – not any meaningful shift in the body’s pH level.

And while some experts warn the claims may still be overstated, others believe this evidence-based version of the diet could be part of the answer to the obesity crisis.

‘The more alkaline foods you eat, the easier it is for the body to lose weight,’ says Washington DC-based endocrinologist Dr Hana Kahleova. ‘We know that GLP-1 medications [such as Mounjaro] are an effective way to lose weight, but they’re also expensive, have side effects and are causing people to lose a lot of muscle. Some people will even end up worse off than when they started.

‘Instead, more people need to start making changes to their diet. Low-fat, vegan foods can help reduce the risk of diabetes, lower stress levels and help shed excess fat, without muscle-loss.’

A study of alkaline eating, led by Dr Kahleova alongside researchers from the University of Utah and George Washington University, found it helped participants lose nearly a pound a week.

The trial – which compared an alkaline-heavy, low-fat vegan diet with the Mediterranean diet, which includes meat, fish and olive oil – saw 62 overweight adults try both diets for four months each.

There was no calorie restriction on either diet – they could eat until they were full.

While on the alkaline diet, participants lost, on average, 13.2lb over 16 weeks. The Mediterranean diet, on the other hand, resulted in no weight loss at all.

The results, says Dr Kahleova, could be a game changer for those struggling with obesity – 16.8 million people in the UK.

She says: ‘On the Mediterranean diet, participants saw their blood pressure go down. But on the vegan diet their blood pressure reduced, their insulin sensitivity and blood lipids improved and they lost fat while gaining muscle.’

This occurs, claims Dr Kahleova, because eating acidic foods triggers the release of stress hormone cortisol, which is needed to process and digest the meal, and the body has to work harder to regulate it – causing cortisol to spike even further. High levels of cortisol have been shown to disrupt metabolism, increase appetite and promote fat storage, particularly in the stomach.

‘The body has an amazing capacity to correct any fluctuations in pH – but we’ve only started to realise how much stress this can put on the body,’ says Dr Kahleova. ‘The more alkaline foods you consume, the less stress hormones are being released and the easier it becomes to lose weight.’

However, Dr Susan Lanham-New, Professor of Human Nutrition at the University of Surrey, claims the weight loss benefits of alkaline eating shown in the trial may be overstated.

‘The study showed that changes in the dietary acid load was associated with changes in body weight,’ she explains. ‘But adjusting for energy intake lowers that statistical significance. Those on the vegan diet were simply eating less.’

Despite this, says Dr Kahleova, even when a difference in calories was factored in, higher consumption of alkalising foods led to more weight loss. ‘A reduction in dietary acid load was still associated with weight loss, independent of the calories consumed,’ she insists.

A 2017 study by the University of Sao Paulo in Brazil found eating an excessive amount of acidic food – and not enough alkaline food – can cause a condition called low-grade metabolic acidosis, that causes inflammation and, eventually, kidney damage. Another paper, published in the International Journal of Cardiology Cardiovascular Risk and Prevention this year, linked a more acidic diet to a type of arterial thickening that causes heart disease.

Too much acidic byproduct in the body has even been shown to raise the likelihood of osteoporosis – the bone thinning condition.

In general, Professor Lanham-New adds, dietary acid load is a concept that has the potential to be very exciting.

‘What we have seen so far is that dietary alkalinity has been shown to have positive effects on a range of health outcomes. We just need more research into it.’

But a vegan diet – or one with a reduced amount of dairy and animal products – can come with its own challenges, says registered nutritional therapist Grace Kingswell.

‘It’s no surprise that when people go vegan they start to immediately feel better, because all of a sudden they’ve cut out processed food and are eating far more fruit and vegetables,’ she explains.

‘But we know that long term, there’s a real risk of nutrient deficiencies when following a vegan diet.’

And just as essential is protein – which is fundamental to our muscles and cells, as well as blood sugar, inflammation and insulin regulation.

‘Unless you do a vegan diet very well, it’s really hard not to be nutrient deficient,’ warned Ms Kingswell.

But reducing your acidic load doesn’t require an all-or-nothing mindset. ‘There’s no need to obsess over it,’ insists Dr Kahleova. ‘The aim isn’t to only eat alkaline foods – it’s more like a shift towards more alkaline eating that we need.’

And after all, even Gwyneth Paltrow admits that she (occasionally) eats pasta now.