Alkaline waters, alkalising powders, alkalising-diet regimens … alkalising fans claim it helps restore balance and assists with digestion, weight loss and wellbeing. But what does it actually mean, and do you need to alkalise?

What does alkaline even mean?
You might remember the pH scale from high-school chemistry. It describes how acidic or alkaline things are. The lower the pH, the more acidic; the higher the pH, the more alkaline. We all know acid burns, so it’s easy to think acid is bad and alkaline is good. This is the simple logic used to sell alkalising products. But alkaline products can burn too: drain cleaner, oven cleaner, dishwasher tablets and bleach are all very alkaline products. The pH of foods varies greatly. Acidic foods and drinks taste sour, and alkaline foods have a soapy mouthfeel.

Acid and alkali in digestion
If you’ve bought a test kit that checks the pH of your saliva, don’t worry if it returns an acidic result. Saliva is meant to be acidic, which helps us break down the food we eat. It’s also worth noting the pH in your mouth isn’t linked to the pH of your blood. The stomach digests food by using stomach acid. The pH of stomach acid is between 1.5 and 3.5. Given straight lemon juice has a pH of 2.5, most things you eat and drink are much less acidic than what’s already in there. Secretions from your pancreas neutralize the stomach acids, and the food becomes alkaline as it moves along your digestive tract.

What about your blood?
The pH of blood is about 7.4, which is ever so slightly alkaline. Blood contains natural buffers that keep the pH within a strict range and have clever chemical properties that allow you to add lots of acid or alkali without changing the pH at all. With the help of our kidneys and lungs, our blood maintains its pH regardless of what we eat. This means there’s no point testing the pH of your urine: it might be acidic, but that’s not a sign of a poor diet. It is true that if your blood becomes acidic, you’re in trouble, but then we’re talking emergency-room trouble, not change-your-diet trouble. And what the alkalising-diet salespeople won’t tell you is that if your blood gets too alkaline, that’s just as bad for your biology.

Is there any point to alkalising?
Most lists of acidic and alkaline foods you will find in alkalising diets are incorrect when it comes to stating the actual pH level. Many of them wildly overgeneralise, grouping all meats and alcohol as acidic, and all fruits and veggies as alkaline (even acidic ones such as lemons and tomatoes). The generous view is that these diets do encourage healthier eating. But there is definitely no evidence that an alkalising product will do your body any benefit.

Dr Emma Beckett is Broadsheet’s nutrition columnist. A molecular nutritionist with a PhD in Food Science, she is a postdoctoral fellow in the School of Medicine and Public Health at the University of Newcastle. In a world of mixed messages she can be found busting nutrition myths and empowering people to critically assess nutrition information. She tweets at @synapse101.