Beans to Soy

May 2000; Volume 2; 36-38

By James A. Duke, PhD

It is ironic that soybeans (glycine max), which have one of the highest fat content of edible legume seeds, are now FDA-approved for alleviating or preventing high cholesterol. Based on studies showing that 25 g/d of soy protein lowered cholesterol, in the Fall of 1999 FDA ruled that foods containing 6.25 g/serving or more of soy protein are eligible to bear a health claim stating that soy protein, as part of a diet low in saturated fat and cholesterol, may reduce the risk of coronary heart disease. Since soy contains a fat to protein ratio about 10 times higher than most other beans, it would not be surprising if other beans, with much lower fat:protein ratios, would be as good—or better—for hypercholesterolemia.

The FDA ruling approved soy protein, not the whole bean, as heart healthy. Sources of soy protein identified in the proposed rule included foods composed of or derived from whole soybeans and foods that contain processed soy protein ingredients, including isolated soy protein, soy protein concentrate, soy flour, texturized soy protein, or texturized vegetable protein.

Soy protein does reduce certain risk factors for cardiovascular disease, including total cholesterol, LDL cholesterol, and triglycerides.1 But does soy have any advantage over other beans? There is little reason to think so; soy is unique neither in protein composition nor isoflavone concentration. In fact, one comment received by the FDA noted that soy protein’s hypocholesterolemic effects may be due in part to its amino acid composition, (specifically its high arginine and low methionine content), and that other vegetable proteins, such as pea proteins, have a similar amino acid profile. FDA’s response noted that it had not reviewed any data on the hypocholesterolemic effects of other vegetable proteins, and it is true that virtually all prospective studies have examined only soy protein.

It is not clear what components in soy are beneficial in terms of reducing cardiovascular risk factors, but soy isoflavones (the phytoestrogens thought to be responsible for the beneficial effect of soy on hot flashes) may be the active component. (FDA decided not to restrict the claim to products having particular levels of isoflavones). The primary isoflavones in soy are genistein and daidzein. If these isoflavones are the most active compounds, many other legumes are better sources of genistein and daidzein than soy. Kaufman et al found that Anasazi, brown, black, navy, pinto, and turtle beans (all Phaseolus vulgaris) contain about as much as or more genistein than the soybean.2 (See Table 1 for isoflavone content of various legumes.)

Table 1-Isoflavone content of legumes
Sample Genistein (ppm Zero Moisture Basis) Daidzein (ppm)
Black dot (Psoralea corylifolia) 1528.0 539.7
Kudzu root 316.9 949.8
Yellow split pea 45.8 0.4
Black turtle beans 45.1 0.4
Baby lima beans 40.1 0.4
Large lima beans 34.4 0.3
Anasazi beans 29.8 6.5
Red kidney beans 29.3 2.7
Red lentils 25.0 5.2
Soybeans 24.1 37.6
Black-eyed peas 23.3 0.3
Pinto beans 22.3 23.2
Mung beans 21.8 0.3
Adzuki beans 21.2 4.6
Faba beans 19.9 5.0
Great northern beans 17.7 7.2
Adapted from: Kaufman,PB , et al. A comparative survey of leguminous plants as sources of the isoflavones genistein and daidzein: Implications for human nutrition and health. J Altern Compl Med 1997;3:7-12.

Want fat with your isoflavones? Go with the soybean. Interestingly, FDA actually exempted foods made from whole soybeans with no added fat from the low-fat requirement. When considering fat:protein ratios, thePhaseolusbeans are better sources of estrogenic isoflavones. Calculated on a Zero Moisture Basis, the fat:protein ratio/100 g ofPhaseolusis roughly 1:25; the fat:protein ratio of soybeans is 15:35. (See Tables 2 and 3.) Once the water of the field soy is excluded, soy contains 15 times more fat and a little bit more protein thanPhaseolus, without supplying much more isoflavones. If you are trying to avoid carbohydrates, soy looks better thanPhaseolus; soy contains only about 40% carbohydrates, whilePhaseoluscontains closer to 70%.3

Table 2-Protein, fat, and genistein content
Protein g/100 g Fat g/100 g Genistein mg/100g
Bean (Phaseolus) 25 1 2-4.5
Soybean (Glycine) 35 15 2.5
Source: Duke JA, Atchley AA. Handbook of Proximate Analysis Tables of Higher Plants. Boca Raton, FL: CRC Press, Inc.; 1986.

Table 3 Protein:fat ratios of beans
Bean Where Consumed Rounded P:Fat Ratio (g/100 g Zero Moisture Basis)
Cow pea (Vigna unguiculata) IndoAfrican originally 25:1
English pea (Pisum sativum) Middle Eastern origin 25:1
Faba bean (Vicia faba) Mainly Mediterranean 30:2
Kidney bean (Phaseolus vulgaris) Latin American originally 25:1
Lentil (Lens culinarus) Middle Eastern origin 25:1
Lima bean (Phaseolus lunatus) Latin American originally 25:1
Mung bean (Phaseolus aureus) Asian originally 25:1.5
Peanut (Arachis hypogaea) American originally 25:40
Soybean (Glycine max) Once strictly Asian 35:15
Source: Duke JA, Atchley AA. Handbook of Proximate Analysis Tables of Higher Plants. Boca Raton, FL: CRC Press, Inc.; 1986.

To get the 6.25 g soy protein the FDA requires for the health claim, you’d need only about 17.5 g dry soybean, giving you almost 3 g fat and less than 1 mg genistein. To get 6.25 gPhaseolusprotein, you’d need 25 g dry bean, giving you only 0.25 g fat and 0.25-1 mg isoflavone. There is a difference among soy products. Fresh or dried soybeans, as well as the so-called "per nut" or "soy nut" (soybean toasted like a peanut), and products made from whole beans are high in fat. However, many processed soy products have had the oil extracted so that it can be sold separately.

Dried peanuts are 33.6-50.3% fat,3 including about 31.4% polyunsaturated fatty acids (PUFA) and about 13.6% saturated fat.4 Most beans are 1% fat, while the amount in soy is 18-20% (15% saturated, 23% monosaturated, 58% PUFA—of which 51% is linoleic and 7% is alpha-linolenic).5 It bears noting that not all fats are equal, and some appear to be beneficial. Alpha-linolenic acid (7%) is an omega-3 fatty acid that appears to have favorable effects on platelet reactivity. Linoleic acid has effects on lipid peroxidation, platelet aggregation, and eicosanoid synthesis, but it is not clear whether these effects are beneficial or deleterious. Saturated fat should clearly be avoided.

Except for the soy and the peanut in Table 3, legumes tend to cluster in the neighborhood of 25 g of protein per gram of fat. Want to avoid fat in your beans? Avoid peanut and soy. That’s why we have soy oil and not black bean oil.

Dr. Duke is a Senior Science Advisor for Nature’s Herbs in American Fork, UT, and in Laurel, MD.


1. Anderson JW, et al. Meta-analysis of the effects of soy protein intake on serum lipids. N Engl J Med 1995;333:276-282.

2. Kaufman PB, et al. A comparative survey of leguminous plants as sources of the isoflavones genistein and daidzein: Implications for human nutrition and health. J Altern Compl Med 1997;3:7-12.

3. Duke JA, Atchley AA. Handbook of Proximate Analysis Tables of Higher Plants. Boca Raton, FL: CRC Press, Inc.; 1986.

4. Pennington JAT. Food Values of Portions Commonly Used. 15th ed. New York: Harper and Row; 1989:158.

5. Anderson JW, et al. Cardiovascular and renal benefits of dry bean and soybean intake. Am J Clin Nutr 1999;70(suppl):464S-474S.

Funding of Reviewed Studies

Reference 1: Protein Technologies International, Inc. Reference 2: University of Michigan Undergraduate Research Opportunity Program (UROP). Reference 5: Not stated.