The following article presents the claims and investigates the evidence to help people decide whether a low-acid or acid-free diet is the right choice for them.
People should be aware of some background information, regarding how acid and alkaline substances interact with the body, before deciding whether avoiding acidic foods is beneficial or not:
Measuring acidity and alkalinity
A pH scale measures how alkaline or acidic something is.
Measuring the pH values of foods and drinks is how people determine the acidity or alkalinity of them.
The pH values can range from 0 to 14 with distilled water having a pH of 7, or neutral. Other types of water with impurities or minerals may have a slightly different pH value.
Anything below pH7 is acidic while anything above ph7 is alkaline.
pH levels in the body
Different parts of the human body have different pH levels. Within the digestive system, pH values range from extremely acidic to slightly alkaline.
Differences in pH levels within the different organs and body fluids allow them to fulfill their particular function:
|Body part/fluid||Role||pH level|
|Saliva||Eases passage of food through the food pipe and breaks down starch.||6.5–7.5|
|Upper stomach||Begins the predigestion process.||4.0–6.5|
|Lower stomach||Releases hydrochloric acid to break down food and kill bacteria.||1.5–3.5|
|Small intestine||Completes digestion and absorbs nutrients into the bloodstream.||6.0–7.4|
|Large intestine||Absorbs water and eliminates undigested food and fiber.||5.0–8.0|
Human blood should be slightly alkaline with a pH ranging from 7.35 – 7.45.
A pH level in the blood that exceeds these limits in either direction will drastically impair metabolic processes inside the body.
The acid-ash hypothesis
The kidneys play a part in neutralizing acid in the blood.
The acid-ash hypothesis suggests that excessively acidic diets are bad for overall health.
Researchers based the hypothesis on the premise that foods that have been metabolized by the body leave behind a chemical residue known as 'ash.'
When combined with body fluids, this 'ash' can be either acid-forming or alkali-forming, which could cause a reaction in the body.
According to the hypothesis, foods containing acid-forming substances lower the pH level of the blood, causing an accumulation of acid.
The body then compensates for this loss by leaching alkaline minerals, specifically calcium, from the bones and excreting them in the urine.
Supporters of the acid-ash hypothesis claim that regular and prolonged consumption of acid-forming foods increases mineral bone loss, thereby increasing the risk of conditions, such as osteoporosis.
Foods containing acid-forming substances include:
- unsprouted beans
- sunflower and pumpkin seeds
- carbonated drinks
- coffee and other caffeinated drinks
- refined table salt
Foods that promote alkalinity, or 'base-forming' foods, are thought to prevent or counteract the effects of excess acid in the body. These foods include most fruits and vegetables.
Even citrus fruits, which are initially acidic, promote alkalinity once metabolized.
Proponents of the acid-ash hypothesis encourage regular pH testing of the urine as a means of monitoring the pH level of the body.
This information is then used to inform a person's dietary choices.
What does the evidence say?
Knowledge of human physiology and evidence from clinical trials are both helpful in understanding the effects of acidic foods on blood pH and overall health.
Supporters of the acid-ash hypothesis claim that diet affects blood pH level.
However, the body's buffering system tightly regulates blood pH in a process known as acid-base homeostasis.
Examples of buffers include calcium stored in bone, proteins, or other mechanisms by which the body resists pH changes in the bloodstream.
The following two mechanisms are primarily involved in this process:
- Respiratory compensation: Breathing rate increases when acid levels are high. This breaks down the carbonic acid in the blood to water and carbon dioxide or CO2. The process, including the exhalation of the CO2, returns blood pH to normal levels.
- Renal compensation: The kidneys produce bicarbonate ions, which neutralize acid within the blood.
These two mechanisms are so effective at balancing acids and bases that it is almost impossible for a person's diet to have any influence on blood pH.
A blood pH level that falls below pH 7.35 indicates a severe problem with lung or kidney function.
This condition, termed acidosis, causes a buildup of acid in the tissues and fluids and can be fatal if left untreated.
One major prediction of the acid-ash hypothesis is that taking alkalizing salts will directly reduce the acidity of the blood.
This reduction would stop the body's need to leach calcium from the bones, meaning that it would excrete less in the urine. Several studies have investigated this claim by measuring whether alkalizing salts reduce urinary calcium excretion.
According to a 2013 review, initial studies did indeed show that taking the alkalizing salt potassium reduced the amount of calcium in the urine. Researchers then interpreted this as support for the acid-ash hypothesis.
It was later realized, however, that a decrease in the amount of calcium leached from the bones was not responsible for this drop in urinary calcium. Instead, this was because potassium blocks the absorption of excess calcium in the blood.
The lower the calcium levels in the blood, the less calcium available to be filtered out into the urine.
Other clinical trials cited in the review directly investigated whether taking alkalizing salts benefits bone health. Initially, two short studies suggested that these salts may indeed maintain healthy bones and reduce the risk of osteoporosis.
However, more rigorous, longer-term, randomized controlled trials (RCTs) failed to show any benefit of alkalizing salts. As a result, the scientific consensus is that an alkaline diet does not benefit bone health with the initial positive results likely being due to random chance or a placebo effect.
Acidic foods and gastric upset
Acidic foods may trigger acid reflux.
Another reason people may choose to avoid acidic foods is due to concerns that they may cause or aggravate certain digestive disorders, such as acid reflux gastroesophageal reflux disease, otherwise known as GERD.
While acidic fruits and vegetables, such as citrus fruits, can irritate upper gastric disorders, these conditions are also likely to be exacerbated by foods high in fats.
The following foods are known triggers of acid reflux and GERD:
|High-fat foods||Acidic foods|
|Oily and greasy||Lemons|
|Animal fats and lard||Grapefruits|
|Fatty cuts of lamb, pork or beef||Pineapples|
|Creamy sauces or salad dressings||Tomatoes|
Contrary to the acid-ash hypothesis, there is no evidence to suggest that acidic foods are harmful to health. It is simply not possible to alter the pH of the blood through diet alone. A blood pH that is excessively acidic or alkaline indicates a serious underlying medical issue.
Despite this, so-called alkalizing diets consisting mainly of fruits and vegetables are abundant in vitamins, minerals, and antioxidants. People can achieve many benefits for their overall health by increasing their intake of these foods. However, these benefits are not related to alterations in blood pH.
People who are most likely to benefit from a reduced-acid diet are those for whom acidic foods are believed to trigger an upper gastric disorder or symptoms.