Does Eating "Alkaline Foods" Make Your Body Alkaline?
― Whatever You Eat, Your Blood pH Barely Moves by 0.1
"Pickled plums are alkaline, so they're good for you." "Eat too much meat and your body turns acidic." You hear this a lot, but how acidic or alkaline your blood is hardly changes with diet. What changes is your urine. And the real star of acid clearance in your body is something you do with every breath.
A single pickled plum sits in the middle of a lunch box. Many of us were told, "It tastes sour, but it's an alkaline food."
If you think about it, that's odd. How can something that tastes sour be alkaline? And eating it is supposed to tilt your body toward alkaline.
If food really changed your body's acidity, then your insides would be a different thing on a night of grilled meat and on a night of salad alone. What actually happens?
There Are Only Two Reasons Your Body Doesn't Turn Alkaline
This system burns food, dissolves the leftover ash in water, and judges it by that. It has nothing to do with sour taste, or with what goes on inside your body.
A "catcher" dissolved in the blood, the lungs that breathe acid out, and the kidneys that pass acid in urine. Together they pin the blood at about 7.4.
We measure acidity and alkalinity with a number called pH. 7 is neutral; lower means more acidic, higher means more alkaline. Let's take it step by step.
The Pickled Plum Is "Alkaline" Because of Ash
In older nutrition science, food was burned to ash, the ash was dissolved in water, and its nature was checked. What stays in the ash is the minerals that don't burn away.
Vegetables, fruit and pickled plums are rich in potassium and calcium. Their ash turns alkaline in water. That is why they were called "alkaline foods."
Meat, fish and rice, on the other hand, are rich in phosphorus and sulfur. When burned, these become sources of acid. So they are "acid foods." The citric acid that makes a pickled plum sour turns into carbon dioxide and water when burned and vanishes. It leaves nothing in the ash, so it plays no part in the classification.
In other words, this system is a rough guide to whether the final leftovers in your body lean one way or the other. Inside you, too, food is slowly "burned," so the idea isn't completely off the mark. The real question is where those leftovers go.
Blood Can't Leave the Narrow Band in the Figure
Look at Figure 1. It lines up the pH of everyday things on a single ruler. Lemon juice is around 2 and vinegar around 3. Blood, by comparison, sits in a very narrow band between 7.35 and 7.45.
Urine pH, by contrast, moves from about 4.5 to 8 depending on what you eat. On meat-heavy days it leans acidic; on days heavy in vegetables and fruit, it leans alkaline. The leftovers from food end up in urine, not in blood.
Blood stays put because of three defences. The first is the bicarbonate ion dissolved in blood. When acid arrives, it catches it at once and turns it into carbon dioxide. This works in an instant.
The second is the lungs. They breathe out the carbon dioxide that was made. This takes effect within minutes. And the amount the lungs breathe out each day is said to be about 200 times the acid produced by food. The third is the kidneys. They pass the remaining acid into urine over hours to days.
The slider in Figure 1 works the second defence. Breathe out too much and carbon dioxide drops, so the needle moves right. Breathe too little and the needle moves left. Normally the brain adjusts your breathing rate automatically and brings the needle back to the thin band.
Blood pH leaves its band when acid builds up in severe diabetes, or when the kidneys or lungs stop working, for example. In medicine, blood pH is treated not as "the result of your diet" but as "a marker of a bodily emergency." Sorting people into "acidic constitution" and "alkaline constitution" is also said to have no medical basis.
"They make you alkaline, so they're good for you" is wrong, but vegetables and fruit are still good for you. There are plenty of other reasons: fibre, vitamins, potassium and more. A wrong reason doesn't mean you must throw out the conclusion.
Summary
"Alkaline food" is a label based on the nature of the ash left when food is burned. Those leftovers leave as urine before they can change your blood. Blood pH is pinned at about 7.4 by three lines of defence: bicarbonate ions, the lungs and the kidneys. The biggest contributor among them is every breath you take.
What diet changes is the pH of your urine.
What guards the pH of your blood is your breathing.
For acid actually dissolving things outside the body, see "Teeth Are the Hardest Part of the Body, So Why Do Cavities Dissolve Them?". For food and the body's energy, see "Does Drinking Cold Water Help You Lose Weight?".
- Chop purple cabbage finely and soak it in hot water. When the water turns purple, strain it and let it cool.
- Divide the juice among clear cups and add a little lemon juice, vinegar, or baking soda dissolved in water to each. Acid turns it reddish; alkaline turns it greenish.
- Next, crush a little pickled plum and add it. Before burning, a pickled plum shows a properly acidic colour. You can see that the name "alkaline food" is something separate from its taste or look.
Take care that baking soda doesn't get into your mouth or eyes, and do this with an adult. Pour the used juice away; don't drink it.
For Those Who Want More ― Terms, Formulas, and Links to TextbooksShows which level each part belongs to, from middle school science to university courses
- Middle schoolCovered in middle school science
- High schoolCovered in high school chemistry or biology
- High school+Advanced high school material, or textbook sidebar content
- UniversityUniversity-level specialist courses (physiology, biochemistry) not taught in high school
- ResearchNot yet settled even at university; what researchers are investigating now
Middle schoolTerms: This Phenomenon Has Names
- pH (hydrogen ion exponent): A number for how acidic or alkaline something is. 7 is neutral, and each drop of 1 makes the acid (the concentration of hydrogen ions) ten times stronger.
- Buffering: The action that keeps pH almost unchanged when a little acid or alkali is added. In blood, the bicarbonate ion is the main player.
- Acid–base balance: The name for the whole system, including the lungs and kidneys, that keeps the body's pH within a narrow range.
Middle schoolHigh schoolChecking with a Formula: Why Is Blood pH 7.40?
Blood pH is set only by the ratio between the concentration of bicarbonate ions and the concentration of dissolved carbon dioxide. Work out that ratio and the value 7.40 comes straight out.
| In symbols | pH ＝ 6.1 ＋ log₁₀（ [HCO₃⁻] ÷（ 0.03 × P ）） |
| In words | Blood pH ＝ 6.1 ＋ the common logarithm of (bicarbonate ion concentration ÷ dissolved carbon dioxide concentration) |
| Where it comes from | The chemical equilibrium "carbon dioxide ＋ water ⇄ bicarbonate ion ＋ hydrogen ion" (law of mass action), rewritten using logarithms. It is called the Henderson–Hasselbalch equation. |
| Symbol | Meaning and unit |
|---|---|
| [HCO₃⁻] | Concentration of bicarbonate ions in blood (millimoles per litre) |
| P | Partial pressure of carbon dioxide dissolved in blood (millimetres of mercury). This is the slider value in Figure 1 |
| 0.03 | Factor converting partial pressure to concentration (millimoles per litre per millimetre of mercury) |
| 6.1 | A constant expressing the strength of this reaction's equilibrium (value at body temperature) |
| Bicarbonate ion concentration in blood (normal) | 24 millimoles per litre |
| Partial pressure of carbon dioxide dissolved in blood (normal) | 40 millimetres of mercury |
| Acid made each day from food and so on that cannot be breathed out | About 70 millimoles (said to be about 1 millimole per kilogram of body weight) |
| Carbon dioxide the lungs breathe out each day | About 15000 millimoles, it is said |
| Body water of a person weighing 70 kilograms | About 42 litres |
| Concentration of dissolved carbon dioxide | 0.03 × 40 ＝ 1.2 |
| Ratio to bicarbonate ion | 24 ÷ 1.2 ＝ 20 |
| Common logarithm of 20 | Said to be about 1.30 |
| Blood pH | 6.1 ＋ 1.30 ＝ 7.40 |
| How many times the food acid the lungs clear | 15000 ÷ 70 ≒ 214 |
| If there were no catcher and no lungs, and a day's acid dissolved in the body's water (per litre) | 70 ÷ 42 ≒ 1.67 |
| Converting hydrogen ions of 1.67 millimoles per litre to pH | Comes to about 2.8 |
Without the defences, a single day's food would bring the body's water close to the pH of vinegar. In reality, the catcher, lungs and kidneys clear this away and blood stays at 7.40. The lungs breathe out about 200 times as much carbon dioxide each day as the acid from food. That is why differences in food fall within the margin of error. If you cut carbon dioxide to 20 with the Figure 1 slider, pH rises to 7.70. Breathing is a far stronger dial than diet.
High schoolHigh school+Why Does the Ratio Alone Decide It?
High schoolA solution mixing a weak acid with its salt works as a buffer. When acid is added, the salt side takes it up; when alkali is added, the acid side does. In blood, carbonic acid and the bicarbonate ion form this pair.
High school+Blood's buffer is especially strong because it is an "open system," in which carbonic acid can escape through the lungs as carbon dioxide. In a buffer in a closed container, carbonic acid would build up with each catch and pH would drift. In blood, the build-up can be breathed away, so the buffering power multiplies several times over.
UniversityThe Kidneys' Work and "Acid–Base Balance"
In physiology, the body's acids are divided into "volatile acid" (which can leave through the lungs as carbon dioxide) and "non-volatile acid" (such as sulfuric and phosphoric acid, which only the kidneys can remove). What varies with diet is mainly non-volatile acid. The kidneys pass it into urine as ammonium ions and titratable acid, and make new bicarbonate ions to replace what was used. The acid load of each food is estimated with an index called "potential renal acid load (PRAL)." It has been reported to match urine pH well. Blood disorders are classified as respiratory or metabolic acidosis and alkalosis.
📖 Derivation of the formula and further reading: Henderson–Hasselbalch equation (Wikipedia) ／ Acid–base balance (Wikipedia)
ResearchWhat We Still Don't Clearly Know
- Does the acid load of diet affect bone? There was a theory that an acid-heavy diet dissolves calcium from bone. But an analysis pooling studies of calcium balance did not support it. The long-term effect is still debated.
- Kidney disease and dietary acid. In people whose kidney function has declined, research is looking at whether eating more vegetables and fruit, and so lowering the acid load, slows progression. This is a separate matter from changing the blood pH of healthy people.
- pH differences between places in the body. Even if blood is constant, pH differs greatly in the stomach, inside cells, and in tissue around inflammation or cancer. What this means is still being researched. But there is no evidence for the idea that "making your blood alkaline with food prevents disease."
So what this article says is also "an explanation within what is known today." Still, that the blood pH of healthy people does not change with diet has been confirmed by many years of measurement.
Links to Textbooks (by Level)
| Level | Subject and unit | Where in this article |
|---|---|---|
| Middle school | Science: "Acids, alkalis and ions" | The pH ruler, the purple cabbage experiment |
| High school | Chemistry: "Acids and bases", "Buffer solutions"; Biology: "Maintaining the internal environment" | The three lines of defence, formula ⓪ |
| High school+ | Advanced chemistry: "Ionization equilibrium" | Buffering in an open system |
| University | Physiology: "Acid–base balance", renal physiology | Non-volatile acid, potential renal acid load |
| Research | Nutrition science, nephrology | Long-term effects on bone and kidneys |
| ― | Links to daily life | Seeing through food advertising claims, what health-check numbers mean |
- Remer, T., Manz, F. (1995) Potential renal acid load of foods and its influence on urine pH. Journal of the American Dietetic Association, 95(7)
- Fenton, T. R. et al. (2009) Meta-analysis of the effect of the acid-ash hypothesis of osteoporosis on calcium balance. Journal of Bone and Mineral Research, 24(11)
- Wikipedia, "Acid–base balance" (酸塩基平衡, Japanese)
- J. E. Hall, Guyton and Hall Textbook of Medical Physiology (Japanese edition: ガイトン生理学, Elsevier Japan), chapter on acid–base balance
*This article is a general-audience science explanation. The figures given are guides and approximations to help you understand how things work. For dietary decisions that involve your health or an existing condition, please consult a doctor or a registered dietitian.