Summary
The FDA labeled soy protein as a nutrient that could help reduce the risk of heart disease. It has also been clinically proven that diets rich in soy protein can reduce cholesterol levels. About 25 to 50 milligrams a day has been proven to benefit overall heart health and cholesterol levels. Soy protein can be found naturally in food or in supplement form.
Description
In October 1999, the Food and Drug Administration (FDA) approved a labeling health claim for dietary soy protein stating that it may reduce the risk of heart disease. The health claim that can be used on labels of products containing soy protein states: ""Diets low in saturated fat and cholesterol that include 25 grams of soy protein a day may reduce the risk of heart disease.'' In order to carry the health claim, one serving of a product must contain at least 6.25 grams of soy protein and must also be low in total and saturated fat, cholesterol and sodium.
The substantive evidence underpinning the approval of the health claim came from a meta-analysis by James W. Anderson and his University of Kentucky colleagues, published in August 1995 in the New England Journal of Medicine. The article reported that regular consumption of soy protein lowered total cholesterol by 9.3%, LDL-cholesterol by 12.9%, triglycerides by 10.5% and raised HDL-cholesterol by 2.5%. The average daily soy protein intake by those included in the meta-analysis was 47 grams. Interestingly, those whose soy protein intake was 25 grams daily—the amount approved in the health claim—demonstrated only a 5% decrease in their LDL-cholesterol levels. Subsequent meta-analyses and clinical studies of the effects of soy protein intake on serum lipids reported LDL-cholesterol decreases ranging from 3 to 5.3%. One study found a 3% lowering of LDL-cholesterol at a soy protein intake of 36 grams daily. A review of 22 randomized clinical trials by the American Heart Association Nutrition Committee and published in 2005 reported that those consuming 50 grams daily of soy protein showed no higher than a 3% decrease in LDL-cholesterol and concluded that they could not support the use of soy protein to lower cholesterol. The FDA is reconsidering the soy protein health claim.
Soy protein isolates have become popular items in the nutritional supplement marketplace. Most of these supplements also contain the soy isoflavones genistin, daidzin and glycitin. (See Soy Isoflavones, Genistein, Daidzein and Glycitein.)
Actions & Pharmacology
Actions
Soy protein has putative lipid-lowering, antiatherogenic, antioxidant, anticarcinogenic and antiosteoporotic activities.
Mechanism of Action
Diets rich in soy protein have been found to reduce serum levels of total cholesterol, LDL-cholesterol, triglycerides and apolipoprotein B (apo B). The mechanism of the possible lipid-lowering activity of soy protein is unclear. There are a few possible explanations. Soy protein is much richer in L-arginine than is animal protein, which is richer in L-lysine. Some animal studies indicate that dietary increases in L-arginine are accompanied by decreases in cholesterol levels. Further, some studies have demonstrated that, under certain conditions, e.g., hypercholesterolemia, high intakes of L-arginine could enhance endothelial-dependent vasodilation and nitric oxide or NO production (see L-Arginine). This could contribute to the possible antiatherogenic activity of soy protein.
The soy isoflavones may also contribute to the possible lipid-lowering activity of soy protein as well as its possible antiatherogenic activity. Most soy protein products contain the isoflavones genistin, daidzin and glycitin, which have weak estrogenic effects and also may have antiestrogenic activity (see Soy Isoflavones). Oral estrogens have been shown to decrease total cholesterol and LDL-cholesterol. The soy isoflavones may have similar actions.
Interestingly, a few studies have shown that when the isoflavones are removed from the soy protein, the protein itself has little hypocholesterolemic activity. Soy isoflavones themselves do not have the same hypocholesterolemic activity as the combination of soy protein and soy isoflavones. There are probably synergistic effects of these substances that are not understood at this time.
There are also other substances associated with soy protein, including saponins, trypsin inhibitor and bioactive peptides, which may also contribute to the lipid-lowering activity of soy protein. The soy isoflavones are antioxidants, and their antioxidant activity may contribute to the possible anti-atherogenic effect of soy protein. Recently, peptides in soybean protein hydrolysate produced by certain proteases have been reported to have hypocholesterolemic activity. Among the mechanisms suggested—blockage of bile acid and/or cholesterol absorption, inhibition of cholesterol synthesis and stimulation of low- density-lipoprotein receptor (LDL-R) transcription—the peptides appeared to stimulate LDL-R transcription. Stimulation of LDL-R transcription in the human liver cell line appeared to reduce serum cholesterol levels. Further research on the identity of the peptides and their mechanism of action is needed and warranted.
The possible antioxidant, anticarcinogenic and antiosteoporotic activities of soy protein are probably due, in large part, to the soy isoflavones (see Soy Isoflavones). Soy protein has been found to reduce intestinal mucosa polyamine levels in rats, which may be another anticarcinogenic mechanism. Also, a bioactive peptide has recently been isolated from soybeans and has been found to have potent antimitotic activity.
Pharmacokinetics
The digestion, absorption, distribution and metabolism of soy protein occur by normal physiological processes. See Soy Isoflavones, Genistein, Daidzein and Glycitein for the pharmacokinetics of these substances.
Indications & Usage
The FDA has allowed the following health claim for soy protein: ""25 grams of soy protein a day, as part of a diet low in saturated fat and cholesterol, may reduce the risk of heart disease.'' The isoflavone constituents of soy protein may confer some additional benefits. The results of some recent reports, however, question the FDA-allowed health claim.
Overdosage
There are no reports of overdosage.
Dosage
There are several soy protein supplements available. Typically the soy protein supplements contain soy isoflavones. Dosage is variable.
A total intake of 25 to 50 grams of soy protein and 50 milligrams of soy isoflavones daily may have cardiovascular and other health benefits. This can come from nutritional intake, as well as supplemental intake.
Literature
Anderson JW, Johnstone BM, Cook-Newell ME. Meta-analysis of the effects of soy protein intake on serum lipids. N Engl J Med. 1995;333:276-282.Aoyama T, Fukui K, Takamatsu K, et al. Soy protein isolate and its hydrolysate reduce body fat of dietary obese rats and genetically obese mice (yellow KK). Nutrition. 2000;16:349-354.Carroll KK. Review of clinical studies on cholesterol-lowering response to soy protein. J Am Diet Assoc. 1991;91:820-827.Carroll KK, Kurowska EM. Soy consumption and cholesterol reduction: review of animal and human studies. J Nutr. 1995;125(3Suppl):594S-597S.Cho SJ, Juillerat MA, Lee CH. Cholesterol lowering mechanism of soybean protein hydrolysate. J Agric Food Chem. 2007;55(26):10599-10604.Crouse JR III, Morgan T, Terry JG, et al. A randomized trial comparing the effect of casein with that of soy protein containing varying amounts of isoflavones on plasma concentrations of lipids and lipoproteins. Arch Intern Med. 1999;159:2070-2076.Dewell A, Hollenbeck PL, Hollenbeck CB. Clinical review: a critical evaluation of the role of soy protein and isoflavone supplementation in the control of plasma cholesterol concentrations. J Clin Endocrinol Metab. 2006;91(3):772-780.Erdman JW Jr. Control of serum lipids with soy protein (Editorial). N Engl J Med. 1995;333:313-315.Gaddi A, Decovich GC, Noseda G, et al. Hypercholesterolemia treated by soybean protein diet. Arch Dis Childhood. 1987;62:274-278.Galvez AF, deLumen BO. A soybean cDNA encoding a chromatin-binding peptide inhibits mitosis of mammalian cells. Nat Biotechol. 1999;17:495-500.Greaves KA, Wilson MD, Rudel LL, et al. Consumption of soy protein reduces cholesterol absorption compared to casein protein alone or supplemented with an isoflavone extract or conjugated equine estrogen in ovariectomized cynomolgus monkeys. J Nutr. 2000;130:820.Sacks FM, Lichtenstein A, Van Horn L, et al. Soy protein, isoflavones, and cardiovascular health: a summary of a statement for professionals from the American Heart Association Nutrition Committee. Arterioscler Thromb Vasc Biol. 2006;26(8):1689-1692.Sacks FM, Lichtenstein A, Van Horn L, et al. Soy protein, isoflavones, and cardiovascular health: an American Heart Association Science Advisory for professionals from the Nutrition Committee. Circulation. 2006;113(7):1034-1044.Teixeira SR, Potter SM, Weigel R, et al. Effects of feeding 4 levels of soy protein for 3 and 6 wk on blood lipids and apolipoproteins in moderately hypercholesterolemic men. Am J Clin Nutr. 2000;71:1077-1084.Wang W, Higuch CM. Dietary soy protein is associated with reduced intestinal mucosal polyamine concentration in male Wistar rats. J Nutr. 2000;130:1815-1820.Research & Summary
The FDA-approved health claim—see Indications above—that soy protein, in adequate amounts, may help protect against heart disease is based upon numerous in vitro, animal, epidemiological and human studies. Evidence has accumulated over many decades showing that soy protein, but not animal protein, has significant cholesterol-lowering properties in animal studies. However, more recent results question some of the older evidence.
In a 1995 meta-analysis of clinical studies, most of them well-controlled, investigators concluded that soy protein significantly lowered serum concentrations of total cholesterol, LDL-cholesterol and triglycerides without significantly altering HDL-cholesterol concentrations.
Since the meta-analysis cited above was conducted, some, but not all, clinical research has continued to confirm the lipid-lowering ability of soy protein. One study demonstrated that administration for six weeks of as little as 20 grams of soy protein per day, in place of animal protein, achieved significant reductions of non-HDL-cholesterol and apolipoprotein (apo) B in moderately hypercholesterolemic men. However, the results of some more recent clinical studies and meta-analyses show much lower cholesterol-lowering activity of soy protein, and the FDA is reconsidering the allowed health claim for soy protein. However, even if the LDL-cholesterol-lowering activity of soy protein is much lower than what was originally thought, it may still be enough to confer some cardiovascular benefits. We know from prior studies that even a 1% reduction in LDL-cholesterol can result in a 2 to 4% percent reduced risk for heart disease and that could have enormous public health implications. Also, there may be other than LDL-cholesterol-lowering activities in soy protein that are also beneficial for cardiovascular health. Clearly, more high-quality clinical trials are needed and warranted to settle these important issues.
Contraindications, Precautions & Adverse Reactions
Contraindications
Soy protein supplements are contraindicated in those who are hypersensitive to any component of a soy protein-containing product.
Precautions
Pregnant women and nursing mothers should discuss the use of soy protein supplements with their physician before starting them.
Women with estrogen receptor-positive tumors should exercise caution in the use of soy protein supplements and should only use them if they are recommended and monitored by their physicians.
Interactions
Nutritional Supplements and Foods
Soy contains phytic acid, which may bind with certain minerals, such as calcium, magnesium, manganese, zinc, copper and iron, reducing their availability.
