Alkaline water, "alkalising" supplements and prescriptive eating programs are regular features in health-food shops these days, as part of a movement that promotes an alkaline diet.

Its proponents say too much acidity in the body can lead to poor health, but that following an alkaline diet can prevent and cure conditions including cancer and osteoporosis.

The diet favours vegetables and fruits, along with specialty waters and supplements, over grains and animal products.

So is the alkaline diet healthy? Well yes, but not for the reasons its fans say.

What does alkaline mean again?

Cast your mind back to high school chemistry and learning about pH, or how acidic or alkaline (also sometimes called basic) a substance is.

A pH of 7 is neutral. Water usually has a pH of around 7.

As that pH gets lower, we're moving into substances that are more and more acidic: for example, orange juice has a pH of about 4 and battery acid has a pH of 0.3.

And as pH gets higher than 7, we're talking about things that are more alkaline or basic: bicarb soda has a pH of about 8.4 and bleach has a pH of about 12.

The acid ash hypothesis

The alkaline diet promotes eating plenty of vegetables and fruits, which is good for your health — but for different reasons than proponents of the diet claim. ( ABC News: Sally Brooks )

The idea that an alkaline diet was good for you came from a theory called the acid ash hypothesis about 100 years ago.

The theory was that protein and grains in the diet produced a high acid load in the body and, among other things, released calcium from the bones, causing osteoporosis.

When we're talking about acid ash or alkaline ash here, we're talking about the residue left over if the food is burnt in laboratory conditions — the residue these foods are supposedly leaving in your body after your body has metabolised the food.

So, for the purposes of this diet, some foods that we would think of as being acidic, like citrus, are actually considered alkaline-producing.

Followers of the diet sometimes measure the pH of a sample of their saliva or urine to work out if their body is alkaline or acidic.

But what does this really mean? According to Clare Collins, professor of nutrition and dietetics at the University of Newcastle, testing these fluids doesn't give your "overall pH", as the pH in many different parts of your body varies.

Consider the pH journey of a piece of food travelling through your digestive tract: it goes from a fairly neutral pH in your saliva, to a strongly acidic environment in your stomach, to being buffered with alkalising substances in your small intestine so digestion can continue to take place there.

In contrast to the extreme changes in your digestive tract, your blood's pH is tightly controlled between 7.35 and 7.45 — slightly alkaline. This is crucial, Professor Collins said.

"Otherwise you drop dead," she said.

"What would happen is a lot of enzymes wouldn't work or minerals would crystallise into your blood and it's not consistent with being alive."

One of the ways your body keeps your blood pH within this slim margin is by excreting acid or alkaline in your urine, but Professor Collins said proponents of the alkaline diet read too much into this.

Evidence doesn't stack up

While the original theory posited that an acid-producing diet would leach calcium from your bones, a recent meta-analysis found this wasn't the case.

An alkaline diet has also been hailed as a prevention or cure for cancer, but recent reviews have found no evidence the acidity or alkalinity of food affects cancer risk.

Professor Collins said since the acid ash hypothesis was put forward about a century ago, we have made huge leaps in our understanding of nutrition and how our bodies work.

"You know, 100 years ago we still hadn't even discovered most of the vitamins. It was still when we didn't know anything really about micronutrients or anything like that," she said.

Health benefits of an alkaline diet

Is the alkaline diet all bad? No, said Professor Collins.

"If you have to say something positive about the alkaline diet you'd say they're generally pushing more vegetables and fruit, and that's positive," she said.

But shelling out for specialty waters, supplements and programs to alkalise your body is unlikely to help you avoid complex health conditions, because diet is not going to change the pH of your blood — and that's a good thing.

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"In terms of beating a cancer, for example, the main food for a cancer is nutrients delivered via the blood supply and you cannot change your blood pH … otherwise you're dead," Professor Collins said.

She recommended people who were looking to improve their health to skip fad diet products and focus on evidence-based healthy eating.

"If you really want to invest in your health, the one thing you can do is that money that you would have spent on alkaline waters or alkaline diet books, go and spend that on vegetables and fruit."