Don't think dairy when it comes to building strong bones, say proponents of the low-acid diet. Focus instead on a diet rich in fruits and vegetables, these nutrition scientists say.
The low-acid diet (also called the alkaline acid diet) has been the subject of scientific debate of late, as doctors and researchers question what, precisely, it takes to keep bones strong. The thinking behind the diet goes like this: Blood is slightly alkaline, with a pH just above 7. If the diet is rich in acids, the body tries to restore alkalinity by eliminating minerals, including potassium, magnesium and calcium, which the blood essentially pulls from the bones.
Following a low-acid diet doesn't mean avoiding vinegar and citrus fruits. On the contrary, it means not overloading on proteins, which are made up of amino acids that, as the name suggests, are acidic in nature. Low-acid diet adherents point out that because humans did not evolve on a diet heavy in meat and dairy, the modern Western diet, which is rich in animal proteins, may be contributing to illness and disease -- including osteoporosis.
That's the theory. What about the science?
"There's good evidence to suggest that pH in diet can make a difference" when it comes to bone health, says Dr. Bart Clarke, an endocrinologist specializing in osteoporosis and an associate professor of medicine at the Mayo Clinic in Rochester, Minn.
However, Clarke adds, the difference is small.
The theory was first proposed in the 1960s by scientists who hypothesized that human bones might be giving up calcium to compensate for pH imbalances caused by protein-rich Western diets. Evidence to support the theory accumulated in the 1980s, as scientists began to show that in rats, even small shifts in pH (such as a drop to pH 7.15 from 7.25) increased bone loss up to six-fold.
In test tube studies, acidity was shown to ramp up the activity of osteoclasts, cells that promote bone loss, and dampen that of osteoblasts, cells involved in bone formation.
Small studies in humans soon began to suggest that the body excreted more calcium, or less, depending on the acidity of the diet.
In a 1994 study by scientists at UC San Francisco that was published in the New England Journal of Medicine, calcium levels in urine decreased in 18 post-menopausal women who took potassium bicarbonate daily to neutralize the acid in their diets. The authors concluded that taking the potassium bicarbonate reduced the women's bone loss and increased bone formation.
And in a 2001 study conducted by Swiss researchers and published in the journal Osteoporosis International, four people placed on a high-acid diet excreted 74% more urinary calcium than four others placed on an alkaline diet (which included lots of mineral water).
Larger populations also have provided support for a low-acid diet. In the late 1990s, British researchers conducting a long-term osteoporosis study in Scotland found that bone health appeared better in participants who consumed more fruits and vegetables. The researchers conducted a smaller study on just over 1,000 of the women, and found -- reporting in the American Journal of Clinical Nutrition in 2004 -- that those who consumed diets lower in protein and higher in potassium had greater bone mineral density in the spine and hip, and greater bone mass in the forearm.
The difference was small but significant: Lower dietary acidity reduced bone loss by 2% to 4%.
In a follow-up study of more than 3,000 of the women in the Scottish study, the same researchers found that bone mineral density was 2% higher in women who consumed the lowest-acid diets compared with those who ate the highest-acid diets. Among pre-menopausal women, hip bone mineral density was 8% higher and lumbar spine density 6% higher in those who consumed the most potassium compared with those who consumed the least.
"The Western diet . . . generates a large amount of acid. Without sufficient alkaline-forming foods in the diet, bone health may be compromised," study author Helen Macdonald and colleagues wrote in the American Journal of Clinical Nutrition in 2005. "It would appear that even subtle chronic acidosis could be sufficient to cause considerable bone loss over time."
The study authors concluded that the greater bone density in women on the lowest-acid diets could translate into a 30% reduced risk of fracture in old age.
But such a dramatic risk reduction has yet to be proven. And any shifts in blood pH brought about by dietary changes are likely to be small, says Clarke, who doesn't advocate drastic measures to achieve a low-acid diet.
Regulating the pH of the blood is primarily an issue for people with existing bone loss who go on extreme diets, he says. "If we knew someone with osteoporosis was following a high protein diet for weight loss, we'd have concerns about that."
The primary virtue of a low-acid diet may be its emphasis on fruits, vegetables and whole grains, says Joan Salge-Blake, a nutrition professor at Boston University and spokeswoman for the American Dietetic Assn. "A plant-based diet is not only good for bones, it's good for a lot of other things too," including lowering the risk of hypertension, stroke and heart disease, she says.
Salge-Blake says the evidence on low-acid diets is also no reason to abandon sources of calcium and vitamin D, which are still crucial for bone health. "Bones lose calcium when the diet is too high in protein," she says, "but studies suggest that if there's enough calcium in the diet to offset this, it shouldn't be a problem."