“Alkaline diet”, “acidosis”, “acid-base balance”: the subject is in high demand and badly treated online, caught between simplistic claims and excessive dismissal. A rigorous account rests on one essential distinction: what is measured in blood is not what is measured in urine, and what happens in the body is not what the headline sells.
Blood pH, urinary pH: what are we talking about
Blood pH is held between 7.35 and 7.45 by very powerful buffer systems (bicarbonates, phosphates, proteins, breathing, kidneys). Blood almost never acidifies: when it does, that is acute metabolic acidosis, a medical emergency, not a state of tiredness.
Urinary pH varies widely (from 4.5 to 8): the kidney excretes excess acidity, and the urine’s pH reflects the diet’s acid load. This is what test strips measure, and this is what diet actually modifies.
The central confusion of marketing: measuring urinary pH and concluding the “body is acid”. The body is never acid; urine is when the diet produces an acid load to excrete. It is a signal, not a diagnosis.
The PRAL table: quantifying a meal’s acid load
The PRAL table (Potential Renal Acid Load) quantifies the acid load a food presents to the kidney, calculated from its protein, phosphorus, potassium, calcium and magnesium content. Values are expressed in mEq/100g:
- Positive loads (acidifying): concentrated animal proteins (meat, fish, hard cheeses), refined grains, fizzy drinks.
- Negative loads (alkalinising): fruit and vegetables (potassium and organic bases dominate), certain nuts, tubers.
- Neutral or nearly: water, pure fats, certain starches depending on composition.
What makes PRAL valuable: it quantifies what the kidney must excrete, not what the food “does to the blood”. It is a measure of renal load, not a score of a “good” or “bad” food.
The alkalinising lemon paradox
The best-known case: lemon, acid to the taste, is “alkalinising” in the PRAL table. The explanation is simple: what counts for renal load is not the food’s acidity on the plate, but what it leaves after metabolism. Lemon’s citric acid is metabolised into bicarbonate; the lemon alkalinises.
The paradox teaches the rule: PRAL measures the residue after metabolism, not the flavour. A food acid to the taste can alkalinise (lemon, metabolised cider vinegar); a neutral food can acidify (concentrated proteins).
Latent acidosis: what the evidence really says
Latent metabolic acidosis is the most discussed concept: the idea that a chronically acidifying diet (rich in animal protein, poor in fruit and vegetables) produces a subclinical acid load that the body compensates by mobilising bone and muscle buffers, contributing to osteoporosis and age-related muscle loss.
What science documents: chronic acid load is associated in studies with urinary calcium loss, age-related muscle wasting, and possibly long-term bone fragility. What remains debated: the real size of the effect (modest in studies), the relevance of correction in a healthy subject whose kidneys work well, and the “cancer” or “inflammation” extrapolations that exceed the data.
The honest reading: a diet unbalanced over time (excessive animal protein, insufficient fruit and vegetables) produces an acid load that weighs on the kidney and probably on bone; the answer is not an extreme “alkaline” diet but a rebalancing towards more potassium and plants. The article on osteoporosis in TCM shows that the Kidney is the pivot of this question in both traditions.
The 80/20 prescription in practice
The “80 % alkalinising, 20 % acidifying” rule is a useful approximation, not a law. In practice it means: half the plate in vegetables and fruit, animal protein in moderate share, grains not exclusively refined. It is the “acid-base” version of the sensible diet both traditions recommend.
What does not help: daily urine strips that produce anxiety rather than measurement, “alkaline” waters sold at premium prices (bicarbonate water has a real but limited, dose-dependent buffering effect), and the total removal of protein or grains in the name of balance.
| Claim | Verdict | Nuance |
|---|---|---|
FAQ: acid-base balance
Is lemon really alkalinising?
Yes, in terms of renal load: citric acid is metabolised into bicarbonate, so lemon produces an alkaline load. Acid taste does not determine metabolic effect.
Are urine strips useful for anything?
They measure urinary pH, which reflects the diet’s acid load over recent hours. Useful as a rough indicator, useless as daily surveillance that produces anxiety rather than information.
Is latent acidosis proven?
Partially: chronic acid load is associated with urinary calcium loss and age-related muscle wasting in studies. The effect on bone is plausible and documented in some work; the size of the effect and its relevance in the healthy subject remain debated.
Should you follow the alkaline diet?
The 80/20 principle (more fruit and vegetables, moderate animal protein) is a reasonable approximation. A strict alkaline diet has no demonstrated superiority over a balanced plant-rich diet; and its extrapolations (anticancer, “the acid body”) are unfounded.
Acid-base balance, a real subject badly served
Acid-base balance is a real physiological subject (renal load, buffers, PRAL) that has been mistreated by marketing. What holds up: chronic acid load weighs on the kidney, probably on bone, and is reduced by eating more fruit and vegetables. What does not: blood acidosis, alkaline cures, anticancer extrapolations. To locate your terrain: the Yin Shi energy test maps your profile in a few minutes.
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