The alkaline diet is based on the idea that replacing acid-forming foods with alkaline foods can improve health.

Proponents of this diet even claim that it can help fight serious diseases like cancer.

This article examines the science behind the alkaline diet.

Alkaline Diet Myth

The alkaline diet is also known as the acid-alkaline diet or alkaline ash diet.

Its premise is that your diet can alter the pH value — or the measurement of acidity or alkalinity — of your body.

Your metabolism — the conversion of food into energy — is sometimes compared to fire. Each involves a chemical reaction that breaks down a solid mass.

However, the chemical reactions in your body happen in a slow and controlled manner.

When things burn, an ash residue is left behind. Similarly, the foods you eat also leave an “ash” residue known as metabolic waste.

As it turns out, this metabolic waste can be alkaline — also called neutral — or acidic. Proponents of this diet claim that metabolic waste can directly affect your body’s acidity.

In other words, if you eat foods that leave acidic ash, it makes your blood more acidic. If you eat foods that leave alkaline ash, it makes your blood more alkaline.

According to the acid-ash hypothesis, acidic ash is thought to make you vulnerable to illness and disease, whereas alkaline ash is considered protective.

By choosing more alkaline foods, you should be able to "alkalize" your body and improve health.

Food components that leave an acidic ash include protein, phosphate and sulfur, while alkaline components include calcium, magnesium and potassium (1, 2).

Certain food groups are considered acidic, alkaline or neutral:

  • Acidic: Meat, poultry, fish, dairy, eggs, grains and alcohol.
  • Neutral: Natural fats, starches and sugars.
  • Alkaline: Fruits, nuts, legumes and vegetables.
Summary According to proponents of the alkaline diet, the metabolic waste — or ash — left from the burning of foods can directly affect the acidity or alkalinity of your body.

When discussing the alkaline diet, it’s important to understand pH.

Put simply, pH is a measurement of how acidic or alkaline something is.

The pH value ranges from 0 to 14:

  • Acidic: 0.0–6.9
  • Neutral: 7.0
  • Alkaline (or basic): 7.1–14.0

Many proponents of this diet suggest that people monitor the pH of their urine to ensure that it is alkaline (over 7) and not acidic (below 7).

However, it's important to note that pH varies greatly within your body. While some parts are acidic, others are alkaline — there is no set level.

Your stomach is loaded with hydrochloric acid, giving it a pH of 2–3.5 — that’s highly acidic. This acidity is necessary to break down food.

On the other hand, human blood is always slightly alkaline, with a pH of 7.36–7.44 (3).

When your blood pH falls out of the normal range, it can be fatal if left untreated (4).

However, this only happens during certain disease states — such as ketoacidosis caused by diabetes, starvation or alcohol intake — and has little to do with your diet (5, 6, 7).

Summary The pH value measures a substance’s acidity or alkalinity. For example, stomach acid is highly acidic, while blood is slightly alkaline.

It is critical for your health that the pH of your blood remains constant.

If it were to fall outside of the normal range, your cells would stop working and you would die very quickly if untreated.

For this reason, your body has many effective ways to closely regulate its pH balance. This is known as acid-base homeostasis.

In fact, it’s near impossible for food to change the pH value of blood in healthy people, although tiny fluctuations can occur within the normal range.

However, food can definitely change the pH value of your urine — though the effect is somewhat variable (1, 8).

Excreting acids in your urine is one of the main ways your body regulates blood pH.

If you eat a large steak, your urine will be more acidic several hours later as your body removes the metabolic waste from your system.

Therefore, urine pH is a poor indicator of overall body pH and general health. It can also be influenced by other factors than diet.

Summary Your body tightly regulates blood pH levels. In healthy people, diet doesn’t significantly affect blood pH — but it can change urine pH.

Osteoporosis is a progressive bone disease characterized by a decrease in bone mineral content.

It is particularly common among postmenopausal women and can drastically increase your risk of fractures.

Many alkaline-diet proponents believe that, in order to maintain a constant blood pH, your body takes alkaline minerals such as calcium from your bones to buffer the acids from the acid-forming foods you eat.

According to this theory, acid-forming diets, such as the standard Western diet, will cause a loss in bone mineral density. This theory is known as the "acid-ash hypothesis of osteoporosis."

However, this theory ignores the function of your kidneys, which are fundamental to removing acids and regulating body pH.

The kidneys produce bicarbonate ions that neutralize acids in your blood, enabling your body to closely manage blood pH (9).

Your respiratory system is also involved in controlling blood pH. When bicarbonate ions from your kidneys bind to acids in your blood, they form carbon dioxide, which you breathe out, and water, which you pee out.

The acid-ash hypothesis also ignores one of the main drivers of osteoporosis — a loss in the protein collagen from bone (10, 11).

Ironically, this loss of collagen is strongly linked with low levels of two acids — orthosilicic acid and ascorbic acid, or vitamin C — in your diet (12).

Keep in mind that scientific evidence linking dietary acid to bone density or fracture risk is mixed. While many observational studies have found no association, others have detected a significant link (13, 14, 15, 16, 17).

Clinical trials — which tend to be more accurate — have concluded that acid-forming diets have no impact on calcium levels in your body (9, 18, 19).

If anything, these diets improve bone health by increasing calcium retention and activating the IGF-1 hormone, which stimulates repair of muscle and bone (20, 21).

As such, a high-protein, acid-forming diet is likely linked to better bone health — not worse.

Summary Although evidence is mixed, most research does not support the theory that acid-forming diets harm your bones. Protein, an acidic nutrient, even seems to be beneficial.

Many people argue that cancer only grows in an acidic environment and can be treated or even cured with an alkaline diet.

However, comprehensive reviews on the relationship between diet-induced acidosis — or increased blood acidity caused by diet — and cancer conclude that there is no direct link (22, 23).

Firstly, food doesn’t significantly influence blood pH (8, 24).

Second, even if you assume that food could dramatically alter the pH value of blood or other tissues, cancer cells are not restricted to acidic environments.

In fact, cancer grows in normal body tissue, which has a slightly alkaline pH of 7.4. Many experiments have successfully grown cancer cells in an alkaline environment (25).

And while tumors grow faster in acidic environments, the tumors create this acidity themselves. It is not the acidic environment that creates the cancer, but the cancer that creates the acidic environment (26).

Summary There is no link between an acid-forming diet and cancer. Cancer cells also grow in alkaline environments.

Examining the acid-alkaline theory from both an evolutionary and scientific perspective reveals discrepancies.

One study which estimated that 87% of pre-agricultural humans ate alkaline diets formed the central argument behind the modern alkaline diet (27).

More recent research approximates that half of pre-agricultural humans ate net alkaline-forming diets, while the other half ate net acid-forming diets (28).

Keep in mind that our remote ancestors lived in vastly different climates with access to diverse foods. In fact, acid-forming diets were more common as people moved further north of the equator, away from the tropics (29).

Despite the fact that around half of hunter-gatherers were eating a net acid-forming diet, modern diseases are believed to have been much less common (30).

Summary Current studies suggest that about half of ancestral diets were acid-forming, especially among people who lived far from the equator.

The alkaline diet is quite healthy, encouraging high consumption of fruits, vegetables and healthy plant foods while restricting processed junk foods.

However, the notion that the diet boosts health because of its alkalizing effects is suspect. These claims haven’t been proven by any reliable human studies.

Some studies do suggest positive effects in a very small subset of the population — an alkalizing diet, low in protein, may benefit people with chronic kidney disease (31).

In general, the alkaline diet is healthy because it is based on whole and unprocessed foods. Its benefits have nothing to do with pH levels.