TempoLife › Features › Food compounds: acid load, purines, oxalate and histamine › pral
Dietary acid load: PRAL, and what an “alkaline food” really is
PRAL — potential renal acid load — is the one number on this site that is arithmetic rather than chemistry. It is not measured in a laboratory: it is computed from five nutrients you can read off a composition table, and it estimates how much acid or base the kidney is left to deal with after a food is digested.
PRAL estimates a physiological load, and it is not a diagnosis. It cannot tell you whether you have a kidney problem, and no food changes the pH of your blood. If you form kidney stones or have been told your kidney function is reduced, the person who should be setting your diet is a clinician or a renal dietitian.
What PRAL actually measures
Digestion does not treat all foods alike. Sulphur-containing amino acids in protein are oxidised to sulphuric acid; phosphorus arrives largely as phosphate, which is also acidifying. The mineral cations — potassium, calcium and magnesium — arrive with organic anions such as citrate and malate, which the body burns to bicarbonate, and bicarbonate is base. Whatever does not cancel out has to leave through the kidney, and PRAL is an estimate of that leftover.
A positive PRAL means the food leaves net acid to excrete. A negative PRAL means it leaves net base. Zero means the two sides cancel. The number is expressed in milliequivalents per 100 grams of food, and — this is the part most pages get wrong — it says nothing about how acidic the food tastes or what its own pH is. Lemon juice has a pH near 2 and a negative PRAL, because almost all of what it delivers to the body is potassium citrate.
PRAL is also only part of the picture. The full quantity physiologists care about is net endogenous acid production, which adds an estimate of the organic acids you excrete regardless of diet — a term that scales with body size rather than with what you ate. So a PRAL sum for a day is an input to that estimate, not the estimate itself.
The formula, and where the coefficients come from
Thomas Remer and Friedrich Manz published the formula in the Journal of the American Dietetic Association in 1995. For 100 grams of food:
PRAL (mEq/100 g) = 0.4888 x protein (g)
+ 0.0366 x phosphorus (mg)
- 0.0205 x potassium (mg)
- 0.0125 x calcium (mg)
- 0.0263 x magnesium (mg)None of those coefficients is arbitrary. Each mineral term is its intestinal absorption rate multiplied by its ionic charge and divided by its atomic mass, which is why the divalent ions — calcium and magnesium — carry a factor of two in the numerator and why magnesium, being light, ends up with a bigger coefficient than the much more abundant calcium. The protein coefficient is the sulphate yield of average dietary protein. That construction is what makes the formula portable: it works on any composition table that reports those five values.
It also explains the formula’s blind spots. It assumes average absorption, so it will misjudge foods where absorption is unusual — the phosphorus in whole grains is bound as phytate and is absorbed far less completely than the coefficient assumes, so PRAL overstates the acid load of unrefined cereals. It knows nothing about organic acids that are excreted rather than metabolised. And it is per 100 grams of food, so a food with a big PRAL you eat 10 grams of matters less than a mild one you eat 300 grams of.
Work it out
Enter the five values — per 100 g of a food, or as a whole day’s totals — and the formula is applied to them. Composition tables such as USDA FoodData Central publish all five. Nothing is stored or sent anywhere.
JavaScript is off, so use the worked examples below — they are computed on the server with exactly the same formula, and the arithmetic in each row can be checked by hand.
Worked examples
These are illustrative daily nutrient totals, not measurements of any real menu and not a claim about any specific food — they are here so you can see how the five terms trade against each other. A positive total is net acid; a negative total is net base.
| Illustrative day | Protein g | P mg | K mg | Ca mg | Mg mg | PRAL mEq |
|---|---|---|---|---|---|---|
| A day of meat, cheese and refined grain | 110 | 1 800 | 2 400 | 900 | 280 | 51.8 |
| A high-protein day with little plant food | 150 | 2 200 | 2 000 | 700 | 250 | 97.5 |
| A mixed day | 85 | 1 400 | 3 200 | 800 | 330 | 8.5 |
| A vegetable-heavy day | 55 | 1 000 | 4 200 | 900 | 380 | -43.9 |
| A legume-and-greens day | 70 | 1 300 | 4 500 | 950 | 500 | -35.5 |
Read the pattern rather than the totals. Protein and phosphorus push the number up; potassium does most of the pushing down, with magnesium and calcium contributing less than their size in the diet suggests, because their coefficients are small. That is why a day can carry a great deal of protein and still land near zero if enough plant food came with it.
Where acid load has real evidence, and where it does not
The strongest practical use is in kidney stones. Urine pH is a genuine lever there: uric-acid stones form in persistently acid urine and calcium-phosphate stones in persistently alkaline urine, and clinicians deliberately move urine pH with alkali citrate when the stone type calls for it. Diet moves urine pH in the same direction PRAL predicts, which is why a stone clinic may talk about fruit and vegetable intake in these terms.
The second is chronic kidney disease, where the failing kidney is less able to excrete an acid load and dietary base has been studied as a way to slow the drift into metabolic acidosis. That is an area of active research and of prescribed treatment, not of self-management from a table.
The weakest use is the one the term is famous for. The “acid-ash” hypothesis of osteoporosis — that dietary acid leaches calcium from bone — was tested directly: a systematic review and meta-analysis led by Tanis Fenton, published in Nutrition Journal in 2011, applied causal criteria to the evidence and did not find the association that the hypothesis requires. A later systematic review by the same group, in BMJ Open in 2016, looked for evidence that an alkaline diet or alkaline water affects cancer and found none worth acting on.
Urine pH is not blood pH
Arterial blood pH is held between roughly 7.35 and 7.45 by two systems that dwarf anything on a plate: the lungs, which blow off carbon dioxide within minutes, and the kidney, which adjusts bicarbonate and acid excretion over hours. Food does not move it. If your blood pH were genuinely shifting because of dinner, you would be in hospital, not on a diet plan.
What food does move, reliably and within hours, is the pH of your urine — which is exactly what you would expect from an organ whose job is to excrete the surplus. Every claim that an “alkaline diet” has alkalinised the body, evidenced by a urine test strip, is really a demonstration that the kidney did its job.
None of which makes the underlying advice bad. A diet with a strongly negative PRAL is, almost by construction, a diet with a lot of vegetables, fruit and potatoes in it, and that is worth eating for reasons that have nothing to do with acid. The mechanism is wrong; the shopping list is fine.
Acid load (PRAL) by food group
Bands, not decimals. Where a published quantity for a specific food could not be verified against a document, this surface prints the band rather than a number — and every food below links to its own nutrition page so you can see what else is in it.
| Food group | Typical PRAL band | Examples |
|---|---|---|
| Hard and processed cheese | The highest single food group on the published table, roughly +15 to +30 mEq per 100 g. High protein and very high phosphorus, and the calcium does not offset it. | Cheddar cheese, Parmesan, Gouda cheese, Blue cheese |
| Meat, fish, poultry and egg | Around +7 to +12 mEq per 100 g. Almost all of it is the protein and phosphorus terms; there is little potassium to push back. | Ground beef, Chicken breast, Salmon, Tuna, Egg (hard-boiled), Ham |
| Grains, bread and pasta | Roughly +2 to +12 mEq per 100 g, higher for wholegrain products on paper — although that is one place the formula overstates, because cereal phosphorus is bound as phytate and poorly absorbed. | White bread, Rye bread, White rice, Brown rice, Oats, Pasta |
| Milk, yoghurt and legumes | Close to neutral, usually between 0 and +4 mEq per 100 g. Legumes carry a lot of potassium and magnesium, which cancels much of their protein and phosphorus. | Whole milk, Yogurt, Lentils, Chickpeas, Peas |
| Fats, oils and sugar | Effectively zero. There is almost no protein and almost no mineral content, so every term in the formula is near nothing. | Butter, Olive oil, Sugar, Honey |
| Vegetables and fruit | Negative, typically -1 to -6 mEq per 100 g. This is the potassium term doing the work. | Carrot, Tomato, Cucumber, Apple, Banana, Orange, Potato (boiled) |
| Dried fruit and high-potassium leaves | The most strongly base-forming foods on the table, down to roughly -20 mEq per 100 g for dried fruit, where the water has gone and the potassium has not. | Raisins, Dates, Spinach, Beet greens, Swiss chard |
What changes the number
- Drying concentrates it. Removing water from grapes does not change the potassium, so raisins are several times more base-forming per 100 g than grapes.
- Portion size beats the per-100 g figure. A 30 g piece of cheese and a 300 g plate of vegetables are not comparable on a per-100 g table.
- Boiling leaches potassium into the water, so boiled-and-drained vegetables are less base-forming than the same vegetable steamed or roasted.
- The formula does not know about phytate. Wholegrain foods score more acid than they behave, because the phosphorus coefficient assumes absorption that does not happen.
- Mineral water can carry real bicarbonate. Some naturally carbonated waters are a meaningful base source; most tap water is not.
Where else these foods are flagged
Foods raised on this page that also sit in a raised band on at least two of the other fifteen. Reassuring bands are not counted, so a row here means the food genuinely collects labels rather than merely getting mentioned a lot.
It is worth reading before removing anything. The lists on this surface were each written for a different person with a different problem, and they converge on the same small group of nutrient-dense foods. Satisfying all of them at once produces a diet nobody designed and nobody has tested.
Talking to someone who can actually test
Nothing on this surface is personal medical advice. If a food genuinely makes you ill, if you have had a kidney stone or a gout attack, or if you are considering removing a food group from your diet, the person to talk to is your doctor, pharmacist or a registered dietitian — they can test for the things that can be tested for, which a table cannot.
Sources: Remer T, Manz F. Potential renal acid load of foods and its influence on urine pH. Journal of the American Dietetic Association, 1995 — the source of the formula and its coefficients. · Fenton TR et al. Causal assessment of dietary acid load and bone disease. Nutrition Journal, 2011 — systematic review and meta-analysis. · Fenton TR, Huang T. Systematic review of the association between dietary acid load, alkaline water and cancer. BMJ Open, 2016. · USDA FoodData Central — the composition tables a PRAL calculation is run on. · Food names and links: the site food database. Checked 2026-09-06.
Frequently asked questions
Is PRAL measured or calculated?
Calculated. There is no laboratory assay for PRAL — it is the Remer-Manz formula applied to five nutrient values from a composition table. That is why two websites can publish different PRAL figures for the same food: they used different composition data, not different measurements.
Can food change the pH of my blood?
No. Blood pH is held between about 7.35 and 7.45 by breathing and by the kidney. Food changes the pH of your urine, which is the kidney doing its job of excreting the surplus — it is not evidence that the body has been alkalinised.
Does an alkaline diet prevent cancer or osteoporosis?
The systematic reviews that looked for it did not find it. A 2011 meta-analysis in Nutrition Journal did not support the acid-ash hypothesis of osteoporosis, and a 2016 systematic review in BMJ Open found no usable evidence that alkaline diets or alkaline water affect cancer. Eating more vegetables is still worth doing, for other reasons.
Is lemon juice alkalising?
Its PRAL is negative, yes — because what reaches the bloodstream is potassium citrate, and citrate is burned to bicarbonate. That is entirely compatible with lemon juice having a pH around 2 in the glass, and with it eroding tooth enamel on the way past.
So when does acid load actually matter?
In kidney stone management, where urine pH determines which crystals form, and in chronic kidney disease, where the kidney has less capacity to excrete an acid load. Both are clinical situations with clinical management, not reasons to buy alkaline water.
Other compound pages
Food pH
Home canners and picklers, anyone with dental erosion, anyone confused by “alkaline foods”
Histamine
People investigating flushing and headache after meals; anyone who has had a reaction to fish
Tyramine
People prescribed a monoamine oxidase inhibitor; people told tyramine triggers their migraine
Salicylates
People considering an elimination diet; people told salicylates cause their asthma or hives
Nickel
People with confirmed nickel contact allergy and unexplained dermatitis
Purines
People with gout or hyperuricaemia, and people with uric-acid kidney stones
Track the plate, not the pH strip
The vegetables-and-potatoes pattern that gives a diet a negative PRAL is worth eating on its own merits. TempoLife logs what actually landed on the plate, day after day.