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Introduction
Taurine is a conditionally essential amino acid that is the most abundant free amino acid in many tissues. Taurine is different than most other amino acids because it is not incorporated into proteins, but it does play many roles in the body, including bile acid conjugation, detoxification, membrane stabilization, osmoregulation, and modulation of excitatory neurotransmission and intracellular calcium levels. The mean taurine intake in humans is estimated to be around 60 mg daily, but supplementation can far exceed this amount and readily increases taurine levels in many tissues. This article will briefly cover some of the many possible utilities of taurine supplementation.
Exercise
Exercise causes a significant reduction of taurine levels in skeletal muscle, as does aging [1-3]. In these conditions, supplemental taurine improves exercise performance and electrical and contractile properties of skeletal muscle in rodents, respectively [1, 3]. In healthy rats, taurine increased running time to exhaustion from 73.8 minutes to 98.8 minutes. Additionally, the weights of the gastrocnemius and soleus muscles were increased, but the difference was not statistically significant [1]. A beverage containing taurine, caffeine, and simple carbs increases endurance performance in humans, and a significant portion of these effects is due to the taurine alone [1, 4]. However, it has not been established if it was due to a synergistic relationship, as taurine-only studies have not yet been conducted in humans. Taurine also counteracts exercise-induced weakness in a mouse model of dystrophy, and is superior to creatine in this regard [5], and it protects muscle tissue from ischemia-reperfusion injury [6]. Among the postulated mechanisms of action are membrane stabilization and modulation of ion channels or calcium ion storage capacity [1, 3-4], although there could be a wide variety of reasons for this effect (such as osmoregulation).
Cardiac health
The concentrations of taurine in the heart are very high, and it constitutes over 50% of the total free amino acid pool [7]. Among the multiple roles it plays are protecting from excessive or inadequate intracellular calcium, acting as a membrane stabilizer, and protecting from injury and oxidative stress [7-8]. Although the taurine content in the heart is tightly regulated and not as subject to deprivation as many other tissues, chronic taurine depletion can result in severe cardiomyopathy [8]. Certain conditions, such as congestive heart failure, can cause a disruption of taurine homeostasis. In a trial with 14 patients with congestive heart failure, 6 grams per day of taurine lowered the heart-failure score from 5.8 to 3.7 where no benefit was seen in the placebo group, and in an animal model of congestive heart failure, 100 mg/kg of taurine reduced mortality from 52% to 11% [7]. Both in vivo and in vitro, taurine protects the heart from ischemic damage by acting as an antioxidant and membrane stabilizer [8, 9]. Taurine also reduces arrhythmias induced by chemicals or ischemia [7, 9]. Finally, taurine administration improved heart function in rats submitted to a high stress load (forced swimming) [10].
Hypertension
Commonly used animal models for hypertension include the spontaneously hypertensive rat, the DOCA-salt rat, the salt sensitive Dahl-S rat, and the renovascular hypertensive rat, and taurine alleviates hypertension in all of these models. Taurine also prevents hypertension in rats fed a high fructose diet and rats treated with alcohol. In humans with hypertension, 6 grams of taurine daily resulted in significant reductions in blood pressure, and this was independently verified in two other studies, one of them with only 3 g of taurine. Taurine does not cause a reduction in blood pressure in normotensive humans, indicating that the mechanism of action is specific to the pathology. Both effects in the CNS and modulation of hormone levels are involved [11].
Atherosclerosis
Taurine has a beneficial effect on a number of markers of atherosclerosis, and reduces the development of atherosclerosis in animals. Animal models in which taurine has improved cholesterol levels (by reducing LDL or total cholesterol or increasing HDL cholesterol) include spontaneously hyperlipidaemic mice, rats and rabbits fed high cholesterol diets, genetic type 2 diabetic GK rats, and normal rats [12-15]. In a single-blind placebo-controlled study with healthy individuals fed a diet designed to raise cholesterol levels, 6 g daily of taurine reduced total and LDL cholesterol levels [7]. The reduction of cholesterol levels is due to increased bile acid conjugation and antioxidant effects [7, 12-14]. Additionally, taurine prevents stress on the endoplasmic reticulum induced by homocysteine [16] and improves endothelial function in young cigarette smokers, having a greater effect than vitamin C [17-18].
Neuroprotection
The brain is another area where taurine is particularly important. Taurine plays an essential role in both brain development and regeneration and promotes the survival and proliferation of neurons [19]. Additionally, supplemental taurine exerts a definite pharmacological effect in the brain [20]. Taurine is a potent neuroprotectant, protecting against glutamate excitotoxicity, cerebral ischemia, oxidative stress, and the buildup of toxins (including carbon tetrachloride and ammonia) [21-25]. Among other things, this neuroprotection is due to regulation of calcium homeostasis and apoptosis, direct scavenging of toxins, and a reduction of oxidative stress [23-26]. Taurine also holds promise as an anticonvulsant and in the prevention of epilepsy [7, 27-28].
Memory
Taurine plays an important role in long-term potentiation. Taurine improves memory in mice treated with a variety of amnestic agents, such as pentobarbital, cycloheximide, sodium nitrite, and alcohol. However, it does not improve memory in healthy, untreated mice [29]. Taurine also imrpoves memory in rats exposed to ozone [30]. Taurine increases brain levels of acetylcholine in animals, and decreased levels of taurine have been found in Alzheimer's patients [7]. It is yet to be seen whether taurine improves memory in humans, but it is likely to have an effect at least in individuals with impaired memory.
Diabetes & insulin sensitivity
When administered to mice receiving a glucose infusion for six hours, taurine inhibited the development of insulin resistance, which occured primarily in skeletal muscle [31]. Taurine also inhibited hyperglycemia and insulin resistance in fructose-fed rats [32]. For this and other reasons, taurine holds promise in the treatment of diabetes. In diabetic subjects, the amount of arachidonic acid required to induce platelet aggregation is lower, and taurine reduces this effect [7]. Furthermore, taurine improves kidney function in animal models of diabetes [59-60]. In addition to antioxidant mechanisms, taurine may also exert these benefits by increasing the excretion of nitrite and the formation of kinins [32].
Cystic fibrosis
Taurine can aid in the treatment of cystic fibrosis in two ways. The first is by inhibiting nutrient malabsorption – a double-blind study found that 30 mg/kg daily helped alleviate steatorrhea in children with cystic fibrosis [7]. Taurine supplementation also helps maintain taurine levels in the lung, and along with niacin taurine reduced lung injury and fibrosis in a mouse model [33]. Taurine also protects the lungs from oxidative stress due to ozone exposure [34].
Eye health
The retina is another area where taurine is found in particularly high concentrations, although its role there is not well established. Visual dysfunction in both humans and animals has been linked with taurine deficiency that can be reversed with supplementation [35]. Along with diltiazem and vitamin E, taurine helped reduce the progressive visual field reduction caused by retinitis pigmentosa, but as monotherapy results have not been clinically significant [7, 36]. Taurine also inhibits oxidative stress in cataractous lenses [37] and protects from retinal abnormalities due to diabetes in rat models, proving more effective on various levels than vitamin E and selenium [38-39].
Heavy metals & toxins
Taurine protects many of the body's organs against toxicity and oxidative stress due to various substances. Taurine neutralizes the toxin hypochlorous acid, thus protecting from DNA damage [7]. In the liver, taurine inhibits the toxic effects of high fructose feeding, alcohol, acetaminophen, and thioacetamide in rats; taurine also improves some markers in patients with hepatitis [7; 40-43]. In the kidneys, taurine protects against cisplatin toxicity and prevents renal damage from salt feeding in salt-sensitive rats [44-45]. Taurine also protects against ulcers caused by monochloramine, a toxin associated with H. pylori infection [46]. Accumulation of heavy metals can have a variety of toxic effects, and taurine reduces the damage caused by excess levels cadmium, copper, and lead in rats [47-49]. Taurine also reduces the toxic effect of oxidized fish oil in rats [50].
Skin health
In vitro, taurine improves recovery from burn injured skin, and a topical taurine gel accelerates wound healing in mice [51-52]. As an osmolyte, taurine helps maintain hydration in the epidermis when it is exposed to a dry environment [53], but the effect of oral taurine supplementation on skin hydration and healing has not been determined.
General health & life extension
Aging is associated with a decline in taurine content in various tissues, including liver, kidney, cerebellum, eye, spleen, blood, and skeletal muscle, and dietary supplementation with taurine can increase the taurine content to levels even above those of healthy adults [3, 54-55]. Taurine supplementation also reduces the elevated protein carbonyl levels associated with aging, aids in the restoration of protein and acid-soluble thiols in aged rats, blunts the age-related decline of IGF-1, and aids in the retention of antioxidant nutrients such as vitamin E and vitamin A [54-57].
Dosage & toxicity
Toxicity is not a concern with taurine, as no signs of toxicity have been indicated in animal studies [58]. For general health and increased exercise performance, 1-4 g daily is commonly used (with at least 1 g prior to exercise), while most clinical trials for treatment of conditions have utilized dosages in the range of 3-6 g daily.
