Image: Rosie Huntington-Whiteley’s Instagram

Haven’t heard of the alkalising diet yet? Well, this could be the year it reaches paleo-level notoriety.

One of the diet’s most well-known advocates, Natasha Corrett, is about to launch a new book of alkalising cleansing plans – just in time for the arrival of 2015. Nothing says ‘new year’ like the sudden urge of people everywhere to embark on eating regimens to detox them of all the excesses (and guilt, apparently) of the silly season.

Corrett - who happens to be Sienna Miller's stepsister - is the woman behind Honestly Healthy, which bills itself as the leading alkaline lifestyle guide. The Honestly Healthy bookbooks and blog have found some famous fans in Victoria Beckham and Made in Chelsea star Millie MacIntosh.

So how does alkalising work, and does it actually do anything? We asked Dr Joanna McMillan to give us a rundown...

Dr Joanna McMillan writes: It’s the diet that many celebrities, including Elle McPherson and Victoria Beckham, are touting as their secret to feeling and looking so fabulous.

It’s called the alkaline diet - or sometimes the alkaline-ash diet or alkaline-acid diet - and proponents claim it can help you to lose weight, have more energy, reduce your risk of many chronic diseases including cancer and have stronger bones. Sounds pretty awesome, doesn’t it?

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The theory is that the typical modern Western diet is affecting the acid load on the body, with knock-on negative health effects. Foods that we eat are either acid forming or alkaline forming, with some being more or less neutral. The diet advises limiting the consumption of acid-forming foods, including meat, dairy, highly processed foods, alcohol, grain products and refined sugar (although this is actually not acid or base producing).

Instead, the recommendation is to eat plenty foods that are alkaline forming, predominantly fruits and veggies.

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So the question is, does it work?

Unsurprisingly lots of people will lose weight by following that advice, but not because of any effect of body acidity. Clearly cutting out processed foods, added sugar and alcohol while packing your diet with veggies and fruit is a positive health move and will more than likely reduce your kilojoule intake and boost your nutrient intake. For these changes, this diet certainly gets my tick of approval.

However, there is more to this than meets the eye. Acidity and alkalinity is measured using pH on a scale of 1-14. Neutral is a pH of 7, below 7 indicates acidity and above 7 alkaline. Our blood is slightly alkaline and although it can vary very slightly, the body has sensitive systems to ensure it stays in the range of 7.35-7.45. I have heard people dramatically announce that their bodies are acidic, having had their blood tested. This is just not possible – they would be severely ill or dead! In contrast, other parts of your body are acidic, purposefully.

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Your stomach has a pH of about 3.5, sometimes even lower, and this is necessary to start the process of breaking down the foods that you eat. It also helps to kill off any micro-organisms that may make us sick. Your skin is also slightly acidic as this also helps control the types of micro-organisms that live on our skin and is an important factor in the skin’s barrier functions. Where the pH can vary is in urine. Your kidneys play an important role in balancing the acidity in the body and what you eat certainly affects this.

There is some evidence to show that eating in a way that assists the acid-base balance in your body does have health benefits. It reduces the risk of kidney stones and other kidney problems, may be important in bone health (although a 2011 review found no association between dietary acid load and osteoporosis), may help to reduce muscle wastage with age, may improve heart health and brain function, and has even been associated with less lower back pain. There seems little doubt that understanding more about our acid-base balance is crucial to health. But the question is how to do this.

We have a tendency to oversimplify things in the diet world and we love to label foods as good or bad. In the case of the alkaline diet the acid-forming foods are suddenly the bad guys, while the alkaline-forming foods the good. This puts meat as a bad guy – but also fish, a food that is almost always thought of in favourable terms, at least from a nutritional perspective. Also despite the fact that meat is acid-forming, a high meat intake is not associated with poor bone health. In fact quite the opposite is true. So the picture is more complex than this diet suggests.

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So what can we take home from the science? The first thing is that there is little doubt typical modern diets with a high animal food intake, not nearly enough plant food, and roughly a third of kilojoules coming from non-core discretionary foods (think cakes, biscuits, chips, soft drinks, junk food and so on) are not good for us. They are also acid-producing and this may explain some of the negative health effects.

But the acid-forming potential of a food is not a sole criteria for judgment as good or bad! It is the balance of foods in our diet that is important, and there are other characteristics of the food that have to be taken into account. Meat may be acid-forming, but it also delivers protein, iron, zinc and a wealth of other nutrients. The problem is not meat, but the lack of plant food consumed alongside.

So instead, think of increasing the alkaline-promoting foods, chiefly veggies (especially leafy greens), fruit, nuts and consider using legumes more often as these are neutral. These foods have many benefits and most of us are not eating enough. Cut down on how much salt is in your diet as this is acid-producing, and drink more water to help your kidneys work effectively. But there is no need to take expensive supplements claiming to be alkaline, or to follow an entirely vegetarian diet unless you want to for ethical reasons.

The bottom line is that yes acid-base balance is important, but put it alongside everything else we know about nutrition and healthy eating. If you’re eating a whole food diet with plenty of plant food, you can rest assured you are on the right track.

Have you ever tried alkalising?