Dietary taurine supplementation: Hypolipidemic and antiatherogenic effects

Abstract

Taurine supplementation may prove to be a safe and convenient method to reverse high blood cholesterol and the associated rise in atherosclerosis. Although human studies are limited, experiments using animal models provide extensive proof of the hypolipidemic and antiatherogenic effects of taurine. Examples of these animal models involve feeding with high-fat diets, genetically determined or heritable disease conditions, and artificially induced or genetic diabetes. Most importantly, the addition of taurine to the diet clearly has effects against pathological increases in serum and liver cholesterol and triglycerides. Another consistent and noteworthy effect of taurine is the simulation of cholesterol 7α-hydroxylase activity, the enzyme that is responsible for the catabolism of cholesterol into bile acids. Taurine also exhibited considerable effects on atherosclerotic lipid accumulation, perhaps through an antioxidative mechanism and through the elevation of HDL cholesterol levels. Data from animal model systems support the specific cardiovascular benefits of taurine, and hopefully, this research will be continued in human studies in the future.

Introduction

Taurine (2-aminoethanesulfonic acid) is a free acid amino that is found in high concentrations in most types of animal tissues (reviewed in [1]). The main source of taurine for most mammals is the diet, although some species are capable of surviving severe taurine diet restrictions without much harm. Various physiological functions have been attributed to taurine, among them, osmoregulation, calcium modulation and antioxidation. However, much still has to be understood about the role that taurine plays in the maintenance of life and normal function.

Dietary intake of taurine is well tolerated in humans and presents as an interesting nutritional regimen for the wide number of positive effects that it seems to have. The benefits of taurine supplementation have been reviewed with respect to diabetes, cardiomyopathy, and hypertension [2], [3], [4]. Taurine also has significant effects in alleviating dyslipidemic and atherosclerotic states, thus rounding off the benefits of what truly is a protective and restorative molecule for the whole cardiovascular system. This review aims to describe the hypolipidemic and antiatherosclerotic effects of taurine, mostly in pathological or abnormal conditions that involve hypercholesterolemia, and to suggest some possible mechanisms of action for these effects.

Early studies in humans who exhibited normal lipid metabolism appeared to show that taurine supplementation had no effect on serum lipids [5]. It was suggested that inasmuch as humans conjugate cholesterol mostly with glycine instead of taurine, increased intake of taurine would not affect serum lipid levels to a significant extent. Few studies have appeared since then examining the effects of taurine in humans, specifically on serum lipid metabolism. Most studies have been correlative [6], evaluating regional diets and eating behavior relative to general health and the incidence of disease, and not truly experimental in nature. There have been some reports on the effects of taurine supplementation on bile metabolism in infants; there were no apparent benefits with taurine (reviewed in [7]). Sadly, the paucity of human studies contribute heavily to the lack of knowledge and familiarity with taurine among the general population.

In 1983, however, a study in Japan demonstrated significant correlations between daily urinary taurine excretion and serum high-density lipoprotein (HDL) [8]. The data suggested that taurine may be involved in serum lipid metabolism, albeit in an indirect manner. A more recent study, also in Japan, studied 22 healthy male volunteers after they were given a high-fat diet designed to increase serum cholesterol levels within the 3-week period of the experiment [9]. Taurine attenuated the increase in serum total cholesterol, low-density lipoprotein (LDL), and LDL-cholesterol brought about by increased cholesterol intake. However, taurine also increased very–low-density lipoprotein (VLDL) cholesterol and serum triglycerides. The significance of these effects have yet to be explained and given practical and scientific meaning. However, the data strongly suggest that taurine modulates serum lipid levels in humans.

With animal experimental models, the effects of taurine have been studied more and are easier to demonstrate. It is worthwhile to note that with data from the rat model, the hypolipidemic effects of taurine were demonstrated in control rats given normal diets and not high-fat chow. Serum total cholesterol, LDL cholesterol, triglyceride, and hepatic cholesterol, triglyceride, and free fatty acid were effectively decreased by taurine in animals fed a normal diet [10], [11], [12]. HDL cholesterol, the antiatherogenic form of serum cholesterol, was found to be decreased by taurine in one study [10] but elevated in another [13]. Some of the hypolipidemic effects of taurine may be attributed to an increase in bile acid synthesis and in the activity of the cholesterol 7α-hydroxylase enzyme, the rate-limiting enzyme in the catabolism of cholesterol into bile acids (enzyme biochemistry reviewed in [14]). The stimulatory effect of taurine on this enzyme was observed in the rat [11], as well as in the guinea pig [15], with both model system using normal diets and no other treatments.

It was also found that taurine decreased serum and hepatic cholesterol levels in hamsters fed a normal diet [16], although this same effect was not observed in another hamster study [17]. Taurine treatment increased bile acid output and bile acid pool size while decreasing cholesterol secretion from the liver [17]. These effects would necessarily lead to the lowering of blood cholesterol levels. The positive effects of taurine are not limited to the lowering of lipid levels. Taurine increased the proportion of HDL, considered to be antiatherogenic in comparison to LDL and VLDL, which are considered to be atherogenic, in mice that were given a control diet [18]. The ratio between HDL and LDL+VLDL is known as the atherogenic index, and taurine is known to improve this index as noted in other reports. Indeed, the hypolipidemic effects of taurine lead inevitably to its antiatherogenic effects. This review hopefully will shed light and understanding on the link between taurine, hyperlipidemia, and atherosclerosis.

The human and animal data suggest that even in cases of normal fat intake, taurine supplementation may still provide some cardiovascular benefit. Available data, however, lean heavily on abnormal conditions or pathological lesions that increase lipid levels and induce atherosclerosis. The following sections describe various conditions that induce hypercholesterolemia and atherosclerosis, and the beneficial effects that taurine exhibits.