The trendiness of dietary acid and alkaline balance — along with diets that promote "alkaline" vegetables and fruits and discourage "acidic" meats, dairy and processed foods — has waxed and waned for decades. But what does dietary alkalinity mean? And is there enough evidence to support its use for overall health, and bone health in particular?
Confusion and interest about food's influence on acid-alkaline balance abound, due in part to popularity among celebrities and non-scientific health enthusiasts who tout versions of an "Alkaline Diet" as a cure-all for a wide range of maladies, from cancer to heart disease. While these types of claims typically are red flags within the health and medical community, as well they should be, there is some research backing health benefits of this eating pattern — making it worth clarifying what an alkaline eating pattern entails and the science surrounding this popular diet.
Before sifting through the evidence, two common points of confusion should be addressed: First, the acid-alkaline balance in question is not that of the blood. The pH of human blood is strictly maintained at about 7.4 (7.35 to 7.45). The lungs and kidneys spare nothing to keep the pH tightly controlled since the consequences of blood pH changes would be life-threatening. It's more about what the body has to do in order to keep the blood's pH where it needs to be. The basic premise of an alkaline diet is that what a person eats influences how much compensating the body has to do in response to that meal.
The second point is the concept of a food being "acidic" or "alkaline" in composition, on its own, versus its potential effect on the body. Lemon juice and tomatoes, for example, are acidic. But when ingested, they promote alkalinity. The pH of the actual food does not dictate the net effect on the body. Rather, it's the "potential renal acid load," or PRAL, of a food — a value that measures acid excretion in the urine — that determines where it fits within the context of acid-alkaline balance.
Diet and pH
Foods can be categorized by their PRAL as acid-forming or alkaline-forming. Foods that have a negative acid load are considered alkaline (mainly fruits and vegetables), while grains, meats, dairy foods, fish, soda and beer are acid forming, meaning they have a positive acid load.
Eating patterns matter more than specific foods, and healthy acid-forming foods — such as poultry, whole grains and eggs — are not necessarily detrimental but may need to be combined with alkaline-forming foods to make the net effect of the meal either neutral or alkaline. This can be achieved by the time-honored recommendation to eat more fruits and vegetables.
Different systems of the body maintain different pH levels, with some more tightly regulated than others. The pH of each system has a specific function, and certain pH levels may improve certain functions. For example, the stomach maintains a very acidic pH at 1.2 to 3.0 so that it can break down dietary protein and kill ingested pathogens. Fluid inside cells can range between slightly acidic and slightly alkaline, but an alkaline environment within cells (achievable through an alkaline-promoting diet), improves cellular function.
To maintain pH balance, some systems may have to support others. For example, bone matrix contains an alkaline reservoir in the form of calcium and magnesium. Blood contains its own buffering mechanisms, but if there is a very "acidic" dietary load over time and those mechanisms become insufficient, the kidneys may signal the release of buffering minerals from the bone matrix to maintain the blood's pH.
A high-protein eating pattern increases total acid load, but does not result in a change in blood chemistry or pH. That's because the kidneys send in mineral buffers, such as calcium phosphate and calcium carbonate, to keep the blood pH at a safe 7.4. However, that same diet also changes urinary chemistry. Urinary magnesium, citrate and pH all decrease, while urinary calcium, uric acid and phosphate increase. Excess protein plus a high PRAL diet may decrease bone density over time if not properly balanced by foods or supplements that are rich in alkali, such as potassium, magnesium and bicarbonate. If there are adequate buffering minerals available, then the body can handle a high acid load from food. But if the diet doesn't supply enough of these minerals or they become depleted from acidic eating patterns over time, then the body has to pull mineral buffers from their storage depot — the bone matrix. Over time, borrowing these buffers can demineralize bone and increase the risk for kidney stones. This may explain the association between diets high in alkali-rich fruits and vegetables with better bone mineral density.
Effects of Acidic and Alkaline Diet Patterns
Eating patterns high in animal protein, dairy foods and refined grains and low in fruits and vegetables are acid-forming diets with high PRAL. Most Western diets are very low in potassium and magnesium, but high in sodium and chloride. Over time, this can result in diet-induced, low-grade, chronic metabolic acidosis.
In addition, consuming large amounts of sodium may exacerbate dietary metabolic acidosis. High-sodium diets increase conditions that are linked to osteoporosis in women, while dietary potassium balances these effects. Although Western diets are associated with osteoporosis, urinary pH does not appear to predict bone fractures or bone mineral densities in some studies. Other consequences of high-PRAL diet include increased risk for muscle wasting and kidney stone formation — and possibly osteoporosis, although these data are conflicting, so more research is needed to confirm a definitive link.
Alkaline diets — high in potassium, magnesium and bicarbonate from fruits and vegetables — have been found to have a range of potential benefits. Aside from high fruit and vegetable intake being linked with lower rates of osteoporosis, cardiovascular disease, cancer and other chronic diseases, alkaline diets in particular may offer specific benefits.
While the data linking alkaline eating patterns and osteoporosis prevention are mixed, there do seem to be some benefits of alkaline diets in terms of fracture prevention. Alkaline diets have been found to preserve muscle mass in seniors and women, possibly by supplying adequate dietary potassium and magnesium. Since muscle mass is important to fall and fracture prevention, these findings may be significant to bone health. While an alkaline diet increases urine pH, which some assert may reduce urine calcium loss, the effects on bone health remain inconclusive.
An alkaline diet also has been shown to reduce the risk of other chronic diseases, including stroke and hypertension, as well as improve memory and cognitive function. The pH within each cell of the body is responsive to PRAL, and an alkaline pH within the cell improves enzymatic function, which may have health benefits. Furthermore, animal and cell studies indicate that a lower (acidic) pH within cells may support cancer proliferation, but there are no clinical data yet to support those studies.
In light of emerging and ongoing research, and in spite of red flags and confusing celebrity endorsements, nutrition experts may consider keeping acid-alkaline balance on their radars. At its core, an alkaline diet is simply higher in fruits and vegetables, which in itself supports a host of health benefits. And while some versions of alkaline diets recommend avoiding "acidic" foods, such as meats, dairy and grains, a better strategy may be to focus on adequate protein intake — and then simply enjoying an overall healthful eating pattern.