ALL it takes is a few sessions a week at the gym, lots of calcium-packed, low-fat dairy products, and an occasional steak, and your bones and biceps will stay strong and sturdy for decades to come. Right?
Not according to a small but growing band of nutritional heretics. According to their disturbing theory, the thinning of bone and muscle that comes with age is actually a modern phenomenon triggered by modern diets-diets that may appear healthy but in reality are setting the stage for an old age in which hunched backs, hip fractures and withered muscles are as inevitable as out-of-season package holidays and reading glasses.
And it鈥檚 not a lack of calcium that has got these researchers worried. Instead, they say, the problem is diets that are high in the supposedly health-promoting cereals and breads, high in protein-including calcium-packed cheeses-and low in fruit and vegetables. They claim that such a diet produces so much acid in our bodies it corrodes our very tissues.
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鈥淚t鈥檚 like water running over a rock,鈥� says Anthony Sebastian of the University of California in San Francisco, who leads the effort examining all the evidence on bones, muscle and dietary acid. 鈥淚t鈥檚 not going to look like it鈥檚 even touching the rock, but over time it will erode it away.鈥�
The idea that an acid-producing diet might rot bone and muscle originated in the 1920s. Then, doctors noticed that patients with failing kidneys-this was in the days before dialysis-had unusually fragile skeletons. A child of 10 could have the Swiss cheese bones of an osteoporotic octogenarian, while simply bounding down stairs could break the hip of an adult. But when doctors gave their patients bicarbonate to alleviate another symptom of kidney failure-bloated stomach caused by acid build-up-their bones gradually grew sturdier.
Then, in 1968, two Harvard Medical School physicians published a little-noticed letter in The Lancet suggesting that healthy people eating diets that produced too much acid in their bodies could be putting their bones at risk. Their idea stemmed from a basic understanding of how the body deals with acid, and a back-of-the-envelope calculation.
In order to stop molecules like enzymes from going haywire, the body keeps its pH exquisitely close to 7.4-just on the alkaline side of neutral. The kidneys get rid of some excess acid by dumping it in the urine, but once the pH has sunk as low as 7.38 a more drastic response is needed. In much the same way that corporate managers might raid the retirement fund to keep a company afloat, at this stage the body compensates by breaking down bones and muscles to release carbonates, phosphates and ammonia, which neutralise acid (see Graphic). Bone, it turns out, is not just a scaffold that prevents us sloshing about like jellyfish. Its role as a storage system for acid-neutralising minerals is every bit as important.
The two Harvard docs calculated that diets that are acid enough to loot just 60 milligrams of bicarbonate from the skeleton each day-barely enough to fill a few dimples on a golf ball-would have leached 15 per cent of the skeleton鈥檚 mineral mass after one decade.
That strategy of robbing Peter to pay Paul must have been used frequently by the bodies of early humans to deal with short periods of excess acid brought on by such things as starvation, diarrhoea, and plant and veggie shortages in harsh winters.
But while excess acid in the body was only a short-lived problem for the average hunter-gatherer-the molecules that were leached from the bones and muscle were quickly replenished-for many in the West it鈥檚 a chronic problem because of our highly acid-producing diet, argue Sebastian and his colleague Deborah Sellmeyer, also at the University of California in San Francisco.
鈥淲e eat a lot of cheese, bread and meat in the Western world and all three produce a metabolic acidosis,鈥� agrees Loren Cordain, an evolutionary biologist at Colorado State University in Fort Collins. 鈥淗unter-gatherers ate a lot of meat, but their carbohydrate was primarily from fruits and vegetables that countered the acid load.鈥�
Even seemingly acidic fruits and veggies such as lemons actually produce hefty doses of bases, because they contain plenty of organic salts such as citrates, which are metabolised into bicarbonates. On the other hand, grain foods like pasta and bread-now the mainstay of Western diets-produce lots of acid because they鈥檙e high in phosphorus, which the body turns into phosphoric acid.
In addition, our consumption of acid-producing protein has increased by a whopping 50 per cent over the past 40 years. When it鈥檚 not accompanied by plenty of base-producing greens, eating large helpings of meat swings the internal litmus paper too far into the red, even though breaking down the protein in your own muscles has the opposite effect. Two different chemical pathways come into play, and the one that breaks down our own muscle spews out basic ammonia, while the one that digests meat spews out sulphuric acid.
Most worrying of all, hard cheeses-which are jam-packed with calcium and widely recommended as a way to protect your bones-are also a major source of acid within the body, and so, argue Sebastian and Sellmeyer, a major suspect for corroding bones and muscle. Milk, you鈥檒l be relieved to hear, is still legit.
It comes down to how the milk is made into cheese, says Thomas Remer of the Research Institute of Child Nutrition in Dortmund, Germany, who has meticulously surveyed the amount of acid released in the body by everything from kiwi fruit to cornflakes. Milk contains roughly equal proportions of acid and base-producing nutrients, but to make hard cheese, you remove the liquid that contains the base-forming constituents.
But before we rush out and change our diets yet again, how good is the evidence that our muscle and bones are getting less healthy and that diet is the culprit?
Well, for starters, osteoporosis is clearly, and mysteriously, on the march. The rate of hip fracture in many European countries has more than doubled in the past 50 years-even after you take into account the ageing populations. Nor are men exempt. They account for 30 per cent of the 2.3 million hip fractures seen worldwide each year.
None of the researchers arguing that high acid-producing diets are helping fuel this epidemic denies that traditionally accepted risk factors, such as too little calcium or protein in the diet, menopause and a couch-potato lifestyle, are important. But according to the biggest and best study to date, the Study of Osteoporotic Fractures (SOF)-which has followed nearly 10,000 elderly women since 1986-all those factors together account for fewer than half of all hip fractures. 鈥淒ietary acid load is probably up there at the top of the list of risk factors,鈥� says Uriel Barzel, an endocrinologist at the Albert Einstein College of Medicine in New York, and one of Sellmeyer and Sebastian鈥檚 biggest proponents. It鈥檚 second only to menopause, he says, 鈥渁nd probably more important than the amount of calcium in the diet.鈥�
Then there are the Eskimos. The data is sparse, but what studies there are suggest Eskimos in the Arctic experience more rapid bone loss, and at an earlier age, than people in other parts of the world, as well as more hip fractures. Some researchers put this down to diets that are low in calcium, but Sebastian says high dietary acid is also 鈥渁 definite possibility鈥�-after all, Eskimos traditionally subsist on high-protein foods like seal, caribou and fish.
Of course, these are just tantalising hints. In a world of Pygmies, Pashtuns, Manchus and Bantus, one can always find a group in which the diet and the disease you鈥檙e claiming are connected appear together. But Sebastian and his colleagues have also conducted a huge 鈥渕eta-analysis鈥� of over 87 different studies and found that the amount of acid produced by typical diets in 33 countries can account for over two-thirds of the variation in hip fractures between those countries. The richest, like Germany and Sweden with their high-protein meat, cheese and fish diets, had 40 times as many hip fractures as poorer nations like Thailand. True, the Thai eat heaps of acid-producing rice, but that鈥檚 more than offset by the fact that they eat more fruit and veggies than Westerners, only a third as much meat, and almost no cheese. This year, Sellmeyer re-analysed a subset of more than a thousand patients enrolled in the SOF study, and found that women who ate the most acid-producing diets had almost four times as many hip fractures as women on the least acid-producing diets.
鈥淪ebastian鈥檚 theory is very, very important,鈥� says Cordain. 鈥淸Acid] is neglected in the treatment of osteoporosis. No one even says a word about it, yet we know we can make rats osteoporotic simply by giving them a high-acid diet, and then reverse it by giving them base.鈥�
Indeed, the most impressive evidence for a link between acid in the diet and osteoporosis comes from a pilot study that Sebastian ran in 1994. For two and a half weeks, he had 18 healthy, post-menopausal women eat enough potassium bicarbonate (essentially baking soda) to neutralise the acid produced by their diets. 鈥淭he effects were major,鈥� remembers Sebastian. The women lost 27 per cent less calcium in their urine compared with controls, and all the chemical indicators suggested it was because less bone was being broken down. The loss amounts to no more than 55 milligrams of calcium a day, but over two decades that adds up to almost half a kilo-literally an arm-and-a-leg鈥檚 worth of bone calcium. What鈥檚 more, the daily dose of bicarbonate also reduced muscle breakdown.
Dialysis patients already benefit from results like these-they are often given acid-neutralising bases in their dialysis lines to help prevent their biceps, thighs and bones from wasting away.
Even so, nutritionists and osteoporosis experts-including Sebastian and Sellmeyer-are calling for more data before they are prepared to make any public pronouncements about what your average Joe or Joanna should be eating. And some researchers are openly critical of Sebastian and Sellmeyer鈥檚 ideas.
Robert Heaney, an endocrinologist based at Creighton University in Omaha, Nebraska, argues that most evidence suggests that you鈥檇 still do better worrying whether you鈥檙e getting sufficient calcium in your diet-something which very few people in industrialised countries have. That would make dietary acid irrelevant, he says, because all the calcium that鈥檚 leached from the bone to neutralise it would be replaced.
Remer, however, turns Heaney鈥檚 criticism on its head. He says it makes no sense for Western nations to be inflating their calcium intakes while eating a high-acid diet-by, for example, eating even more hard cheese. After all, people who live in countries like South Africa and Singapore consume less than a third of the recommended amount of calcium each day, but eat low-acid diets. They have far lower hip fracture rates than those in the West. High-calcium, high-acid diets are 鈥渁 little bit stupid鈥�, says Remer, 鈥渓ike you are driving a car at the highest velocity and on the other hand you are using the brake.鈥�
But Heaney also points out that a second osteoporosis study, the Framingham Osteoporosis Study that followed 615 men and women for four years, appears to contradict Sellmeyer鈥檚 analysis of the SOF study. The Framingham study found that women with the most protein in their diets had the least bone loss and vice versa-a finding that on the surface, at least, is at loggerheads with the idea that acid is a culprit in osteoporosis. But Katherine Tucker, an epidemiologist at Tufts University in Boston and one of the authors of the Framingham analysis, believes that there is an alternative explanation. She points out that the amount of protein in the diet was generally lower in women in the Framingham study compared with the SOF. Rather than contradicting one another, says Tucker, the two studies corroborate what was already suspected-that 鈥渂oth very low protein and very high protein intake is bad for bone.鈥�
In the end, says Sebastian, the best way to solve the nagging doubts is to run a large study in which elderly people receive daily doses of bicarbonate or placebo for five years, after which the groups鈥� bone loss and hip fracture incidence would be compared. Sebastian is in the early stages of planning just such a trial.
Meanwhile, Sebastian admits to tweaking his own diet just a little-not by eliminating all acid-producing foods, but by balancing items like poultry, fish and cheese with base-producing ones like broccoli, leafy salads and fresh fruit. He has also sharply reduced the amount of grain in his diet. 鈥淕rain not only has its own acid load,鈥� he explains, 鈥渂ut it displaces other foods that have a base load from your diet.鈥�
It may seem odd for such a hardcore, show-me-the-data kind of scientist like Sebastian to be putting all of his dietary eggs in the acid-base basket, but really it amounts to nothing more radical than eating a balanced diet, and it fits into his larger vision. 鈥淭he palaeolithic diet is my guiding principle,鈥� he says. 鈥淚 try to think about what our pre-agricultural ancestors must have been eating. I try to get back to our roots-our roots, tubers and fruit.鈥�
