Acid and Alkaline Foods – What Are They?

by Paul Rogers on June 23, 2009

We all know that citrus fruits and pineapple are acid foods: right? Coffee and tea are acid forming: right?


Much of what you may have read from the alternative health movement over many years is just that. Wrong!

Here’s how it works. The body very carefully maintains a crucial balance of acidity and alkalinity in the body. This is called acid-base balance. For those that know about pH range  of 0-14, that number varies for different body fluids. Seven is neutral, below 7.0 is acid and above is alkaline. For example, saliva is usually around 7.7, on the alkaline side, whereas gastric juice (hydrochloric acid etc) is quite acid at 1.6. Arterial blood is about 7.45 and venous blood (in the veins) is slightly more acid at 7.35.

The body keeps a tight reign on blood acidity with a buffering system, usually balancing acid foods and metabolic outcomes with bicarbonate and carbonic acid and other alkaline salts or with acid excretion in the urine. The kidney is the crucial organ of pH control. All foods are assessed by the kidney (or at least their digestive outcomes), at which time the kidney works some sophisticated chemistry to maintain the body’s acid-base balance. The totality of this is called the ‘potential renal acid load’ (PRAL) and involves net acid excretion (NEA) — apologies for the additional acronyms.

Consequences of acid and base irregularities

If homeostatic mechanisms cannot control plummeting acidity, as is the case in ketoacidosis, which can occur in diabetics with poorly controlled blood sugar, death can result. This is called ‘metabolic acidosis’. Metabolic alkalosis can also occur. Both are life threatening if not quickly corrected. It is a complex process with calcium, magnesium, phosphorus and potassium and sodium and perhaps vitamins K and D involved in regulatory control, particularly in relation to bone which supplies calcium as part of the buffering mechanism. 

So what about food? A word of caution first. The most quoted studies are from the team of Remer and Manz, eg, (Remer T, Manz F.  Potential renal acid load of foods and its influence on urine pH. J Am Diet Assoc 1995;95:791-797.) It would be useful for the research base to be wider. However, for a list of PRAL values from the Remer study see Loren Cordain’s Paleo Diet site. (I don’t agree with other aspects of Paleo dieting.) 

Acid and alkaline foods

In general, meat, cheese and cereals are acid forming, vegetables and fruit are alkaline and milk and yogurt are around neutral. Cheese tends to be acidic — perhaps partly because of the sodium chloride in cheeses. Certain grains and cereals are also quite acid. Drinks, including coffee, tea, soft drinks, juices, beer, wine etc tend to be close to neutral to alkaline — somewhat surprisingly perhaps. Note the strong alkalinity of spinach and raisins and the strong acidity of hard cheeses. In general, you can see the tendency for protein and cereal foods to be acidic, which is perhaps a caution against acceptance of high-protein or high-cereal diets without an alkaline balance — for the following reasons.

Potential importance of PRAL for health

One of the mechanisms the body uses to buffer excess acidity is to use bone calcium as calcium carbonate, an alkaline chemical buffer. Some of this calcium can get excreted in the urine. So not only is calcium intake important for bone health, but calcium excretion is also part of the equation. Too much acid food (protein) and too little alkaline food (fruit and vegetables) could lead to bone loss and osteoporosis. There exists some epidemiological evidence in support of this. A net acid diet may also cause other health afflictions.

The bottom line here is that we need to ensure an adequate intake of fruit and vegetables, perhaps more than the ‘two and five’ servings in healthy eating guidelines. But at least that’s a start. However, don’t get confused by some of the pronouncements of the natural health movement that seemed to take acid and alkaline values from thin air and pronounce them as fact. Acid-base balance is an important concept in human health and deserves further study.

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