The alkaline diet: what it can and can't do, and where bicarb fits

"Alkaline diet" is one of those phrases that sounds scientific and means 2 very different things depending on who's saying it. The version sold online promises to "alkalise your body" and keep disease away. The version in the nutrition research is a modest, useful idea about how much acid your kidneys have to clear each day. This article separates the two, so you can keep what's worth keeping.

What an alkaline diet is

The alkaline diet (also called the acid-alkaline or alkaline ash diet) sorts foods by the residue they leave once your body has burned them for energy. Meat, fish, eggs, cheese and grains leave an acid residue, mostly from their protein and phosphorus. Fruit, vegetables and legumes leave an alkaline residue, mostly from their potassium, magnesium and calcium. Fats and sugars are close to neutral. The diet's rule is simple: eat more from the second group, less from the first.

Nutrition scientists measure this with a number called PRAL (potential renal acid load), worked out in 1995 from the protein and mineral content of a food. A 100 g serving of hard cheese scores around +19; spinach scores around −14. The sign tells you which way the food pushes, and the size tells you how hard. It's a real, published measure, and it's the only part of the alkaline diet that holds up on its own.

What it can't do: change your blood pH

This is the claim the whole diet usually hangs on, and it isn't true. Your blood sits between pH 7.35 and 7.45, and your lungs and kidneys hold it there minute by minute, whatever you ate for lunch. If blood pH drifted with diet, the drift would be a medical emergency, not a wellness goal. The words you'll see thrown around, acidosis and alkalosis, describe serious illness (uncontrolled diabetes, kidney failure, poisoning), not what happens after a steak. No diet treats them, and nobody healthy has them.

What diet does change is the pH of your urine, because that's where the kidneys send the acid they've cleared. A high-PRAL meal makes urine more acidic; a plate of vegetables makes it more alkaline. That's why the pH strips sold with alkaline-diet books appear to "work": they measure the kidneys doing their job, not your body changing state.

What the research says

Bones. The oldest argument for the diet is that an acid load leaches calcium from bone. A 2011 systematic review and meta-analysis tested that idea against the standard criteria for causation and found no evidence that a diet's acid load causes bone loss, or that an alkaline diet protects bone. Urine calcium goes up on a high-protein diet, but the calcium is being absorbed from food, not pulled from the skeleton.

Cancer. A 2016 systematic review searched for any study linking dietary acid load or alkaline water to cancer, in either direction, and found almost nothing: 1 study on bladder cancer, which found no link, and no study at all on alkaline water. The "cancer can't survive in an alkaline body" line has no study behind it. We say the same thing in our bicarbonate of soda guide, because the myth travels with that product too.

Where it may help. A 2012 review of the evidence is fair about the upsides, and they're all explained by what the diet makes you eat rather than by pH. More fruit and vegetables means more potassium and magnesium, which most adults under-consume; a lower acid load eases the work the kidneys do, which matters more as kidney function declines with age; and the diet is, in practice, a vegetable-heavy, lower-processed-food way of eating. Those are good things. None of them require a pH strip.

The honest version of the diet

• Fill half the plate with vegetables, and eat fruit. That is the whole diet, and it's good advice for everyone.
• Keep the protein. The strict version cuts meat, fish, eggs and dairy as "acid-forming", and that's where it goes wrong, especially for women over 40, who need more protein to hold on to muscle, not less. Balance the acid load with vegetables instead of dropping the protein.
• Skip the pH strips and the alkaline water. Urine pH tells you what you ate; it doesn't tell you anything about your health.
• Ignore any version that promises to treat a condition. Diet doesn't move blood pH, so no claim built on moving it can be true.

Where bicarbonate of soda fits

Bicarb is often sold as the shortcut, the thing that "alkalises" you without the vegetables. It doesn't, for the same reason the diet doesn't. What bicarb does is change pH where it lands: it neutralises acid in the stomach on contact, which is why ¼ teaspoon dissolved in a glass of water settles occasional heartburn, and it raises urine pH briefly. Both are local, temporary effects, and both are useful. "Alkalising the body" isn't one of them.

It's a sodium salt, so it counts towards salt intake. Keep it occasional, dissolve it fully, and take it at least 2 hours away from prescription medicines. Skip it altogether if you have kidney or liver disease or high blood pressure, are pregnant or breastfeeding, or for a child under 5, unless a doctor or pharmacist says otherwise. It's a household helper, not a treatment.

Does collagen fit an alkaline diet?

Collagen is a protein, so on paper it's acid-forming, like any protein. In practice a 10 g to 15 g scoop is a small acid load that a serving of vegetables more than balances, and cutting protein to chase a pH number is the mistake we'd most like you to avoid. If you're eating the vegetable-heavy version of the diet, a daily scoop of collagen sits in it comfortably.

Frequently asked questions

Which foods are alkaline?
By PRAL: most vegetables (spinach, kale, celery, carrots), potatoes, most fruit (bananas, apricots, raisins especially), and legumes. Lemons taste acidic but leave an alkaline residue, because their citric acid is burned off and their potassium remains.

Which foods are acid-forming?
Cheese (the highest), meat, fish, eggs, grains and most processed foods. Acid-forming isn't the same as bad: the list is also the list of your best protein sources.

Can I test my pH at home?
You can test your urine or saliva, and the result will change through the day with meals. Neither is a measure of blood pH or of health. We'd skip it.

Does alkaline water do anything?
It raises urine pH slightly and is otherwise water. The 2016 review found no evidence it affects cancer risk, and there's no good evidence it affects anything else.

Is the alkaline diet safe?
The vegetable-heavy version is. The strict version that removes animal protein and dairy risks low protein and calcium intake, which is a worse trade than the acid load it's trying to avoid.

The sources

• Remer T, Manz F (1995). Potential renal acid load of foods and its influence on urine pH. Journal of the American Dietetic Association 95:791–797. The PRAL method. View →
• Fenton TR, Tough SC, Lyon AW, Eliasziw M, Hanley DA (2011). Causal assessment of dietary acid load and bone disease: a systematic review and meta-analysis applying Hill's epidemiologic criteria for causality. Nutrition Journal 10:41. View →
• Fenton TR, Huang T (2016). Systematic review of the association between dietary acid load, alkaline water and cancer. BMJ Open 6:e010438. View →
• Schwalfenberg GK (2012). The alkaline diet: is there evidence that an alkaline pH diet benefits health? Journal of Environmental and Public Health 2012:727630. View →

Find out more

• Bicarbonate of soda: facts, health benefits and safe uses
• What to drink for acid reflux: 5 drinks that help, and what to avoid
• Bicarbonate of soda and athletic performance
• The benefits of collagen for a keto diet

This article is general information, not medical advice. If you have kidney disease, or have been told your blood chemistry is out of range, your diet is a conversation for your doctor or a registered dietitian.

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