If I were to devise a diet plan that was built around a fallacy, or worse still, a biochemical impossibility, but the foods that were recommended were basically good, would that be dishonest? I grapple with this question when I read about the so-called “acid alkaline” diet. It is based on the theory that disease thrives in an acid environment but eating certain foods promote alkalinity.

The measure for levels of acid or alkalinity is known as “potential hydrogen”, or ph for short, and ranges from 0 to 14. A low ph is acidic, higher is alkaline. In the stomach, food is exposed to a strong acid with a ph of between 1 and 3. This works with enzymes to break down foods into a substance called chyme which is passed through a valve into the intestines.

Chyme has a low ph, having been lolling around in stomach acid for several hours but if it remained acidic it would, in simple terms, burn a hole in the intestines’ soft tissue. Were this true then we would die the first time we ate. Instead the body cleverly neutralises the acid by combining it with sodium bicarbonate, and the ph is instantly changed to around 5 in that part of the intestines (the majority of the body has a ph of 7.3 but some parts are naturally a little more acid).

What we eat has no meaningful bearing on the body’s ph, and we can’t change the ph. The body has built-in mechanisms to prevent it becoming too much of anything and levels of ph are tightly regulated.

That we can change our ph for anything longer than a milisecond is a misconception. Yet the foods that supporters claim can do this are generally good. They include lean protein, vegetables, nuts, seeds and some grains while acidic foods include the usual suspects — that’s refined anything, alcohol, saturated fats, sugars, dairy etc.

This is where I am conflicted as I think that a good diet has a multitude of benefits but selling it based on a myth undermines the plan’s integrity. You would question the professionalism of an engineer who suggested you put high octane petrol in your car because the elves that pedal the wheels prefer it to the regular stuff, and so I feel that the supporters of the alkaline plans shoot themselves in the foot.
Eat well, of course, but it’s better to understand why rather than do so because of some flawed theory.

Ian Marber is one of the UK’s most highly regarded nutrition experts and the author of 11 books. www.ianmarber.com