Index

• Introduction
• Effects of Soyfoods on Circulating Lipid Levels
• Beyond Effects on Lipid Levels
• Practical Implications
• References

Effects of Soyfoods on Circulating Lipid Levels
Soyfoods have long been appreciated as excellent sources of protein. Over the past 15 years, the effect of this protein on blood cholesterol levels has attracted widespread research attention. The first rodent studies^{4, 5} showing that soy protein lowered cholesterol levels were published more than 60 years ago and the first clinical trial demonstrating this effect was published in 1967.⁶ However, it was not until 1995 that the hypocholesterolemic effects of soy protein began to receive widespread recognition.

In that year, a meta-analysis of clinical data, which included 38 different comparisons, found that soy protein reduced low-density-lipoprotein cholesterol (LDLC) by approximately 13 percent.⁷ This reduction was independent of the fatty acid content of soyfoods and was greater than any other single non-pharmacological approach. At the time (before statins became widely available), this effect was essentially equal to that of the available cholesterol-lowering drugs.

The results of the 1995 meta-analysis prompted much investigation into the cholesterol-lowering effects of soy protein. Some of this research was directed at identifying the specific components and mechanisms responsible for cholesterol reduction, while other research explored the responses to soy protein by different subpopulations, such as hypercholesterolemic subjects, or premenopausal and postmenopausal women. Some data suggest that cholesterol reduction is a result of upregulation of hepatic LDLC receptors by the peptides formed from the digestion of soy protein.^{8, 9} Researchers also continue to explore whether isoflavones in soybeans impact the cholesterol-lowering effects of soy protein.¹⁰



In 1999, on the basis of their own analysis of the data, the U.S. Food and Drug Administration (FDA) awarded a health claim for soy protein and CHD based on the cholesterol-lowering effects of soy protein.¹¹ In total, more than 100 clinical trials investigating the hypocholesterolemic effects of soy protein have been published. Surprisingly, and despite the health claim, questions have recently been raised about the efficacy of soy protein. This is partly due to the fact that estimates of the degree to which soy protein reduces LDLC are lower than initially thought.

Recent research indicates that soy protein lowers LDL cholesterol by 3 to 5 percent. On a population level, each 1 percent decrease in LDL reduces CHD risk and/or mortality by 2 to 5 percent. Therefore, even a 3 percent reduction in LDL could lower risk for heart disease by as much as 10 percent.

Meta-analyses published between 2004 and 2007, including those by the American Heart Association¹² and the federal Agency for Healthcare Research and Quality,¹³ have concluded that soy protein lowers LDLC by 3 to 5 percent,¹⁴ which is similar to the effects of soluble fiber.¹⁵ These estimates agree with a recently presented analysis which covered the relevant literature published since 1978.¹⁶ The scientific guidelines recommended by the FDA for the evaluation of health claims were used to analyze the data. This meta-analysis indicated the net reduction in LDLC was approximately 7.1 mg/dl or about 5 percent.

Substituting soyfoods for protein-rich foods higher in saturated fat can lead to a decrease in blood cholesterol.

The reasons for the lowered estimates are not entirely known, although compared to earlier studies, the studies published after the 1995 meta-analysis involved subjects with lower baseline cholesterol levels and used somewhat lower amounts of soy protein. Also, many of the earliest studies were conducted by an Italian research group that may have used a soy protein processed differently than the products used in most other studies. However, the extent to which each of these factors influences the cholesterol-lowering effects of soy is unclear.

It is important to note that, from a public health perspective, a 3 to 5 percent reduction in LDLC is quite meaningful. Over time, each 1 percent decrease in LDLC reduces CHD risk and/or mortality by as much as 2 to 5 percent.^{17, 18} Therefore, a 3 percent reduction in LDLC levels could lower risk for heart disease by as much as 10 percent.

The amount of soy protein needed to lower cholesterol is uncertain. Twenty-five grams per day is generally considered to be the threshold intake required for cholesterol reduction since most trials have used at least this much. However, there is evidence suggesting that lower amounts are also efficacious.¹⁹

The advantages of incorporating soyfoods into heart-healthy diets extend beyond the direct effect of soy protein on cholesterol levels. Soyfoods, which are typically low in saturated fat, can replace other protein sources with higher saturated fat content. In fact, comprehensive dietary approaches to lowering cholesterol that rely heavily on soyfoods to provide protein and lower cholesterol have resulted in reductions in LDLC ranging from 20 to 30 percent.²⁰

Approximately 84 percent of the lipids in soybeans are unsaturated. Therefore, substituting soyfoods for protein-rich foods higher in saturated fat will lead to a decrease in blood cholesterol levels as a result of both the reduction in saturated fat intake and the increase in polyunsaturated fat.²¹

The predominant fatty acid in soybeans is the essential omega-6 fatty acid linoleic acid, which reduces blood cholesterol levels.²² In the past, some health professionals have recommended it should be limited in diets since it may increase heart disease risk by increasing inflammation. Recently however, the American Heart Association rejected concerns about the pro-inflammatory properties of omega-6 fats and concluded that these fats play a critically important role in heart-healthy diets.²³

In addition to providing omega-6 fats, soybeans are a good source of the essential omega-3 fatty acid, alpha-linolenic acid (ALA).²¹ Although ALA does not possess all of the same properties as the long-chain omega-3 fatty acids found in cold-water fish, evidence suggests that ALA has direct coronary benefits.^{24, 25}

Finally, in response to the ingestion of soy protein, meta-analyses have found very modest increases (1 to 3 percent) in high-density-lipoprotein cholesterol (HDLC) and modest decreases (5 to 10 percent) in triglyceride levels. Each 1 percent or 1 mg increase in HDLC lowers CHD risk by 2 to 3 percent.^{26, 27} Although there is debate about whether an elevated triglyceride level is an independent predictor of CHD risk,²⁸ recent evidence suggests that the role of fasting triglyceride levels in the etiology of CHD has been underestimated.^{29, 30}

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