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March 2002
Eat Your Greens! Diet and Bone Health

By Stephen Walsh

 


In all developed countries with ageing populations, thinning of bones due to loss of calcium is a major public health issue. As bones thin, risk of fracture increases. Osteoporosis, the thinning of the bones to the extent that they become porous and brittle, and vertebral fractures give rise to the familiar loss of height with age and to the painfully familiar bent-over posture of many elderly people. In addition, bones can become extremely brittle with age, sometimes breaking and causing serious injuries—hip fracture is a particularly devastating example. Thus, measures to promote bone health are important throughout life, to promote bone growth in the young and reduce bone loss later in life.

Dietary recommendations have focused almost exclusively on increasing calcium intake. Increasing calcium intake is not wrong in itself but, in relation to bone health, its undue pre-eminence over reducing sodium intake, increasing vitamin K and potassium intakes, moderating protein intake, increasing physical activity and adequate sun exposure is a serious error in public policy.

Calcium is naturally lost from the body in urine, gut secretions and sweat. The key to avoiding bone loss is to ensure that calcium absorbed from food balances out with calcium loss. Otherwise, the body will take calcium from bone to maintain the level needed in the blood.

In the typical North American and European diets, calcium loss is equally driven by high sodium (salt) and protein intakes and low potassium intakes. Variations in these three components can cause daily calcium requirements to range from 240 milligrams to over 2,000 milligrams .

Generally, 600 to 1,000 milligrams of calcium per day from calcium-rich foods, plus smaller amounts from other foods and supplements, will provide an adequate intake. The following are recommendations for healthy sources of calcium with explanations of the benefits and adverse effects of certain foods on calcium levels and bone health.

Ideal Sources for Calcium
For an individual trying to improve calcium balance, fruits and vegetables are the best foods, as they are rich in potassium, which reduces calcium loss. For example, 100 grams each of red peppers, bananas, oranges, and leafy greens will boost retained calcium by about 40 milligrams per day—enough to reverse a one percent loss of bone calcium per year. A daily pint of cow’s milk would have a similar effect for the average person, but only about half the benefit for individuals with low calcium absorption—those who are at particular risk of osteoporosis.

The ideal foods for bone health are those that are not only high in calcium but also reduce calcium loss. Low oxalate (oxalates are natural substances that bind strongly to calcium) and calcium-rich dark green leafy vegetables such as kale, collard and turnip greens, and spring greens are the best example. Some vegetables such as spinach, purslane and rhubarb are also high in oxalate, which hinders absorption of their calcium. Dark green leafy vegetables provide about 150 milligrams of calcium per 100 grams in raw weight. While many modern cultivated foods are sadly much less rich in calcium than the wild plants with which we evolved, green leafy vegetables are an exception and therefore of particular importance for modern humans.

Human use of dairy products is a recent and unnecessary development: a diet low in salt and rich in vegetables, fruits and root crops provides the best path to healthy bones. Dairy foods increase calcium loss as well as provide calcium. For example, cheddar cheese causes a net calcium loss in high-risk individuals, and cottage cheese causes calcium loss for most people in general. Foods such as meat, fish and eggs have a strong adverse effect because they are low in calcium and also cause high loss; foods that are low in calcium but also reduce losses, such as peppers, bananas and oranges, provide everyone with at least a modest boost.

Almonds, carob and molasses each provide about 250 milligrams of calcium per 100 grams. While these foods are too concentrated in either calories or pungency to consume in large amounts, they can make a useful contribution. They will also provide alkali to boost blood pH.

Some tofu is highly salted and is high in calcium only if calcium has been used in making it. Tofu can therefore vary from substantially increasing calcium balance to substantially decreasing it. The calcium content of tahini is also highly variable. The amounts of calcium and sodium in these foods should therefore be checked on the labels and not taken for granted: for a reliably beneficial effect on bone health there should be at least as much calcium as sodium.

Calcium-fortified foods or calcium supplements provide a further convenient source of calcium. If phosphate intakes are low (unusual for vegans), calcium phosphate may be preferable to calcium carbonate or calcium citrate. Calcium carbonate should always be consumed with meals. If stomach acid is low, something other than carbonate should be used.

Overall, it is hard to imagine a food more supportive of bone health than leafy greens. In addition to being abundant sources of calcium, they also protect and strengthen bone by raising blood pH and providing vitamin K and vitamin C. Vitamin K is particularly beneficial for postmenopausal women not using hormone replacement therapy. One hundred grams of leafy greens or broccoli per day may halve risk of bone fracture. Leafy greens are a good source of plant carotenes, which meet the body’s needs for vitamin A safely and naturally. Green leafy vegetables are also high in folate, which is highly beneficial to general health. Using the green stuff instead of the white stuff also avoids the adverse effects of dairy fat on cardiovascular health.

Our prehistoric ancestors got enough calcium every day from plants. Their high intake of vegetables, fruits, roots and flowers also provided abundant potassium, magnesium, vitamin K and vitamin C, all in quantities far above modern norms. Salt was notably absent, as were dairy products.

Plant Protein
Adequate intake of protein is essential for healthy bones. If protein intake is inadequate (less than 0.8 grams of protein per kilogram of body weight), the body will lack the building blocks for muscle and bone; and growth hormones, which stimulate muscle and bone building, will decline to undesirable levels. Consuming less than the recommended amount of protein in order to reduce calcium loss is therefore a false economy. Although protein excess is more common than protein deficiency in Western society, individuals with a low calorie intake, such as the very old, are particularly at risk of getting insufficient protein.

The choice of protein source can make a great deal of difference. Overall, most plant protein sources (fruits, vegetables, legumes and many nuts and seeds) have a positive or neutral effect on calcium balance. A person trying to increase protein intake using meat or fish, for example, will lose about 25 milligrams of calcium from their body for every 100 grams eaten. In contrast, a 100-gram portion of beans (by dry weight) has an approximately neutral effect on calcium balance while providing the same amount of protein.

There are a wide variety of plant-based sources of protein. A cup of soymilk provides about eight grams of protein. Wheat is higher in protein than rice and potatoes, and using nuts and seeds rather than oils and fats will boost protein intake. Nuts which are high in monounsaturated fat, such as almonds, filberts and cashews, are ideal as they will also promote cardiovascular health. Almonds are the most beneficial for bone health as they have the most positive effect on calcium balance.

Some highly processed plant protein sources, such as certain soy protein isolates, actually have an adverse effect on calcium balance due to a loss of beneficial minerals and an addition of sodium during processing. Highly salted nuts also have an adverse effect.

Speaking of Salt
Simply reducing salt intake by half will substantially reduce calcium loss. In typical Western diets most salt is hidden in processed foods such as prepared meals and fast foods. No food with more than 0.5 grams of sodium per 100 grams should be a major part of the diet.

If you use salt, substitute one of the widely available low sodium alternatives, such as LoSalt, containing at least twice as much potassium as sodium by weight. Use low sodium bread or consume bread less frequently, as bread is a major source of sodium. Use herbs and spices instead of salt and salty pickles. There are often similar products in terms of taste with very different salt levels.

Get Vitamin A From Plants, Not Retinol
Retinol (pre-formed vitamin A) is found naturally only in animal products and makes bone loss more likely. Some dietary supplements and fortified foods contain retinol or related compounds (all beginning with “retin”). Low-fat and skimmed milks in the U.S. are usually fortified with retinol, undermining and possibly overturning the benefit of their calcium and potassium content. On the other hand, plant carotenes allow the body to make as much vitamin A as it needs without risk of adverse effects. Good sources of plant carotenes include carrots, pumpkin, sweet potato, red peppers, and dark green leafy vegetables. One hundred grams per day of any combination of these will easily meet vitamin A requirements.

Spend Time in the Sun
Vitamin D is important as it facilitates the absorption of calcium from the intestines. Sunlight causes a chemical reaction in the skin that generates vitamin D. A daily 15-minute exposure to the sun when it is well above the horizon is ample to boost vitamin D stores to ideal levels while avoiding damaging sunburn. However, during winter at high latitudes, vitamin D stores may substantially decay. Plenty of sun exposure during the summer will provide sufficient stores for three to four months of winter, but if such sun exposure is not possible or the winter is more prolonged, either take a mid-winter holiday somewhere sunny or take 10 micrograms of vitamin D2 (ergocalciferol) per day.

Lose the Caffeine
Limit or omit caffeine consumption. Caffeine has been shown to reduce calcium absorption, especially when consumed at mealtimes. Low caffeine and many non-caffeinated herbal teas provide a tasty and healthful alternative.

Exercise!
Finally, don’t forget physical activity. Just as exercise helps to build and maintain muscle, it also helps to build and maintain bone.
As well as benefiting bone, these recommendations will benefit overall health. Increased potassium and calcium intakes and reduced sodium intake strongly promote lower blood pressure and reduce risk of stroke and kidney disease. Increased calcium and vitamin D appears to reduce risk of colorectal cancer and may also reduce risk of breast cancer. Increased vitamin D may also reduce the risk of prostate cancer and auto-immune diseases. Increased consumption of foods rich in plant carotenes is associated with reduced risk of cancer.

Stephen Walsh is a trustee of the Vegan Society, UK (www.vegan society.com). This article is supported by a 30-page Vegan Society briefing paper which is available at www.vegansociety.com/briefings/dietandbone/dietandbone.doc.

 


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