March
2002
Eat
Your Greens! Diet and Bone Health
By Stephen Walsh
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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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