Can Soy Prevent Osteoporosis?


Postmenopausal women over 50 years of age are especially vulnerable to osteoporosis, a bone-thinning disease, as a result of the rapid and progressive bone loss associated with estrogen deficiency at menopause, followed by sustained bone loss with aging. Some women experience an early menopausal acceleration of bone loss of 1 to 3 percent a year,[ 1-3] followed by an age-related bone loss of approximately 0.7 to 1 percent a year.[ 4-6]

Osteoporosis can be defined as a systemic disease characterized by low bone mass and microarchitectural deterioration of bone tissue, with a subsequent increase in bone fragility and susceptibility to fracture. Estrogen replacement therapy (ERT) has been used effectively in stabilizing skeletal bone mineral density (BMD) and preventing vertebral fractures,[ 7, 8] but because of undesirable side effects and potential health risks, the compliance to ERT by postmenopausal women is only 10 to 20 percent.[ 9] The search for alternatives is ongoing.

Phytoestrogens and Osteoporosis
Although research to date is inconclusive, results indicate that phytoestrogens may provide an alternative to ERT as a favorable therapeutic agent in the treatment of osteoporosis. Phytoestrogens, including isoflavones, are estrogen-like molecules abundant in plants.[ 10] One of the richest sources of isoflavones is the soybean and soy foods. Isoflavones, the most studied phytoestrogens related to bone health, possess weak estrogenic activity and have an affinity for estrogen receptors, specifically estrogen receptor-? (ER-?) which is present in bone, the brain, the bladder, and the vascular epithelia.

The limited amount of data on the skeletal effects of isoflavones suggests agonistic effects. Ecological studies show a lower incidence of hip osteoporosis among Asian populations consuming diets high in soy compared with Western populations.[ 11] Studies using the ovariectomized rat model have shown comparable bone-sparing effects of 17?-estradiol and soy protein isolate,[ 12] genistein or daidzein,[ 13] or their respective succinylated products obtained after soybean fermentation.[ 14]

B. H. Arjmandi et al.[ 15] showed that feeding isolated soy protein increased messenger ribonucleic acid (mRNA) insulin growth factor I (IGF-I) biosynthesis, suggesting that IGF-I levels would increase. IGF-1 is a protein involved in the bone formation process; therefore, an increase in IGF-1 suggests increased bone formation.

In two short-term published studies of the effects of soy isoflavones on BMD in humans, soy protein isolate was used with isoflavones as the treatment. S. M. Potter et al.[ 16] studied 66 hypercholesterolemic post-menopausal women and L. Alekel et al.[ 17] studied 69 perimenopausal women. Both studies reported a positive effect with approximately 90 milligrams (mg.) of isoflavones a day on spine BMD of 1 to 2 percent relative to control women who were fed a milk protein.

Biomarkers of bone turnover are useful for determining changes in bone formation and bone resorption. An increase in markers of bone formation indicates that bone is being deposited; an increase in markers of bone resorption indicates that bone is being degraded. Bone biomarkers help in assessing metabolic bone disease and therapeutic efficacy.

K. E. Wangen et al.[ 18] found that markers of bone turnover were affected by soy isoflavones at 65 and 130 mg./day in both premenopausal and postmenopausal women. In premenopausal women, markers of bone resorption significantly increased on both isoflavone diets. In post-menopausal women, bone formation markers were significantly decreased with both isoflavone diets. The changes observed in this study were of small magnitude and were probably not clinically significant.

One clinical study in humans is under way to assess the effects of soy isoflavones on calcium metabolism, including kinetics. This information is essential in ascertaining the mode of action for the skeletal effects of isoflavones.

Soy Protein and Urinary Calcium
The profile of sulfur-containing amino acids determines the effect of protein on urinary calcium excretion.[ 19] Relative to animal protein, soybeans have a lower content of the sulfur-containing amino acids cystine and methionine, However, relative to other legumes, soybeans have a higher content of sulfur-containing amino acids.[ 20] Table 1 shows the amounts of sulfur-containing amino acids in some proteins.

The metabolism of sulfur-containing amino acids generates sulfate, which increases urine acidity and therefore urinary calcium excretion.

S. A. Schuette et al.[ 21] showed a high correlation between urinary sulfate and total renal acid excretion and urinary sulfate and urinary calcium excretion in elderly men and postmenopausal women using purified proteins (proteins isolated from whole foods).

Similarly, M. B. Zemel et al.[ 22] found that an addition of purified sulfur amino acids to a protein diet significantly increased urinary calcium excretion in adult males. N. A. Breslau et al.[ 23] further demonstrated significant urinary calcium, urinary sulfate, and net acid excretions in an animal protein-rich diet relative to ovo-vegetarian and vegetarian diets. The ovo-vegetarian and vegetarian diets contained soymilk, soy cheese, and textured vegetable protein.

Soy Foods and Calcium Bioavailability
Soybeans have high calcium content relative to other plants and high fractional absorption of calcium, despite their high content of oxalate and phytate. Both oxalate and phytate inhibit calcium absorption, but oxalate is a more potent inhibitor because of the degree of insolubility of the oxalate-calcium complex. Moreover, phytate is the storage form of phosphorus in plants, which lowers the bioavailability of calcium by forming phosphorus-calcium complexes. Phytate appears to substantially reduce calcium absorption only in phytate-concentrated wheat bran cereal.[ 24]

Table 2 lists examples of natural sources of calcium and the amounts necessary to equal the calcium contained in one cup of milk. The number of servings required to replace one cup of milk for absorbable calcium are approximately three servings of cooked soybeans, eight servings of pinto beans, 10 servings of red beans, and four servings of white beans.

Some calcium-fortified foods have fractional calcium absorption values (the amount of calcium that can be absorbed from these foods) that are similar to the calcium absorption value of milk.[25[ For example, Table 2 shows that the fractional absorption of calcium-set tofu is 31 percent relative to 32.1 percent for milk and the milk equivalency of calcium-set bean curd (tofu) is 1.2 servings.

Soy-based meat products, calcium-fortified soy cheese, soy yogurts and ice creams, and soy nut butter as well as soymilk are most likely more conveniently substituted for milk and dairy products than cooked soybeans. With the exception of soymilk, the fractional absorption of calcium in these products relative to cow's milk is currently unknown.

Soy foods provide calcium essential to building and maintaining bone and may also protect against osteoporosis. However, the current understanding of the role of soy in bone health is still in its infancy. We have much more to learn about which compounds in soy protein are bioactive and at what dose, whether soy proteins have a bone-protective effect, and the mode of action on the bone tissue. Long-term randomized, controlled trials are necessary to determine the role of soy protein and soy isoflavones on BMD and fracture incidence.

Table 1 Sulfur-Containing Amino Acids in Various Protein Sources
Source Amount (mg./g.74)
Soybeans 30
Black, great northern, kidney, navy 26
Chickpea 27
Cowpea 25
Lentil 22
Lima, baby, and large 24
Fruit 38
Cereals 28
Animal foods 39
Nuts and seeds 46
Table 2 Comparison of Other Natural Sources of Calcium with Milk
Food Serving Calcium Fractional Milk
Size (g.) (mg.) Absorption Equiv. (cup)
Milk 240 300 32.1 1.0
Pinto 86 44.7 26.7 8.1
Red 172 40.5 24.4 9.7
Soy 86 100 31-42 2-3
White 110 113 21.8 3.9
Broccoli 71 35 61.3 4.5
Kale 85 61 49.3 3.2
Spinach 85 115 5.1 16.3
Tofu with
Calcium 126 258 31.0 1.2


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