Does Garlic (Allium sativum) enhance exercise performance?

Garlic (Allium sativum) is a herb best known as a flavouring for food but also commonly used as a medicine to prevent or treat a wide range of diseases and conditions. It is one of the most extensively researched medicinal plants. It has a long history as a remedy for improving strength, reducing fatigue, and increasing immunity both in prevention and treatment of infectious diseases and gastrointestinal function.

Available Forms

Garlic is available in wide range of forms. Supplements are made either from whole fresh garlic, dried, or freeze-dried garlic, garlic oil, and aged garlic extracts [1]. Not all preparations contain the same amount of active ingredients, therefore it is best to use standardized garlic products.

Effects of Garlic on Athletic Performance

A decrease whole-blood viscosity has been shown to have several advantages in sports performance (such as increased oxygen and nutrient supply to working muscle tissues, and thus increasing exercise performance [2]). A single dose of garlic was reported to further increase blood fluidity in ten trained male college endurance athletes [3]. It seems to be able to increase fibrinolytic activity [3] which has been suggested as the mechanism of decreasing fibrinogen concentration [2,4]. Decreased fibrinogen concentration results in lower plasma viscosity [5]. Naoaki and coworkers [6] reported that aged garlic extract increases the turnover of aerobic glucose metabolism, reduces oxidative stress, and promotes oxygen supply. However, it does not seem to increase maximal oxygen consumption, blood oxygen saturation and consequently performance during exercise in humans under hypoxia [7].

Different processing methods result in differences in the intensity of its anti-fatigue effect [8]. Extraction of raw garlic followed by its natural ageing seems to be the most favourable [8].

Currently available data, strongly suggest that it may be a promising anti-fatigue agent.

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Interaction With Hormones

Garlic supplementation is suggested to enhance protein anabolism and suppress protein catabolism due to hormonal regulation by the stimulation of steroid hormones [11].

Testosterone

Studies evaluating garlic’s effects on testosterone levels and male reproductive functions are mixed. It seems that high doses exhibit testicular toxicity which is followed by decreased testosterone levels [9]. Lower dose, on the other hand, is associated with protection against testicular toxicity and increased testosterone levels [10]. Another animal study [11] also reported increased testosterone in rats fed either 40 or 25% protein and 0,8% garlic, whereas testosterone was unaffected in control diet group and the garlic diet group fed the 10% protein. Mechanisms of action behind this testosterone increase remain unknown.

So far there are no human trials, however, it is highly unlikely that moderate garlic consumption would affect spermatogenesis and/or testosterone levels in humans. Caution is advised as it has been reported that human spermatogenesis is more sensitive to stress than that of rats [12], which suggests lower doses might impair male reproductive system.

Cortisol

Rats fed with high-protein diet and 8% garlic extract exhibited marked reductions in plasma corticosterone concentration (nearly halved) [11]. In patients with advanced cancer, salivary cortisol was unaffected after 6 months of aged garlic supplementation [27]. On the other hand, salivary cortisol increased significantly in control group after 3 months but due to highly varied values researchers speculated that this increase could be accidental.

Growth Hormone

A study in isolated keratinocytes found that a combination of aged garlic and pycnogenol caused a potent release of growth hormone [20]. To our best knowledge there are no available studies in living models to evaluate garlic’s use for that purpose.

Health Benefits

Common Cold

It is traditionally believed that garlic is effective for a common cold. According to Lissiman et al. [13], there is only one credible study that evaluated its effectiveness for a common cold, nevertheless, the quality of the evidence is moderate. A mentioned study assessed 146 participants over a three-month period. A study reported that garlic reduces the frequency and number of colds (24 occurrences vs 65) when taken for prevention but it failed to reduce the duration of illness. Possible bias of the study is that blinding of participants was not adequate as more of the participants in garlic group noted a smell when burping. More well-designed studies are needed to validate its use for common cold prevention.

Cholesterol

There are literally tons of studies measuring the effectiveness of garlic in lowering cholesterol and triglycerides. Many of these studies contrast each other. When looking at available meta-analyses [14-17] it is evident that garlic is superior to placebo in reducing total cholesterol levels (8 to 12% overall reduction), however, its effect is modest at best. One meta-analysis [18] reported that intake of garlic preparations did not produce any significant beneficial effects on serum cholesterol. So the value of garlic for hypercholesterolemia is questionable. Furthermore, according to MedlinePlus [19], if only the high-quality studies are considered garlic does not significantly lower cholesterol or triglyceride levels. According to subgroup analysis by an effect of garlic type on LDL cholesterol, garlic powder appears to be more effective than aged garlic extract, garlic oil and raw garlic [16]. Most of the studies included in these meta-analyses were sponsored by the manufacturers of these products. Its cholesterol-lowering effects have also been confirmed recently in 970 hypertensive subjects [28].

High Blood Pressure (Hypertension)

Garlic is widely used by patients in order to lower blood pressure. A systematic review and meta-analysis [21] on the effects of garlic on blood pressure which included nine double-blind trials with 482 individuals reported that garlic preparations may lower blood pressure in hypertensive individuals. However, the evidence is not strong. Based on data from two randomized controlled trials [22] in 87 hypertensive patients it appears that it may have some blood pressure lowering effect. However, due to minor effects noted it is unclear whether garlic provides any therapeutic advantage in terms of reducing the risk of cardiovascular morbidity and mortality. It also seems to lower blood pressure in people with normal blood pressure [19]. An updated meta-analysis and review from 2016 further confirmed that it has the potential to lower blood pressure in hypertensive individuals [28].

Anti-obesity / Weight Loss

Results of some animal studies have shown that aged garlic extract supplementation and exercise provide protection against high fat diet-induced increases in body fat, body weight and insulin resistance [23,24]. A study in obese patients with metabolic syndrome reported that administration of 1.2 g/day of aged garlic extract for 24 weeks provides non-significant (slight decrease) changes in body weight [25].

Other Benefits

Garlic may be considered an effective remedy in the treatment of common yeast and fungal infections such as Candida albicans and Aspergillus flavus [29].

Side Effects and Toxicity

Garlic is considered safe. Actually, it is highly tolerated and may be considered as a complementary treatment option for hypertension, slightly elevated cholesterol, and stimulation of immunity [28]. Some common side effects include mild but undesirable effects such as body odour, bad breath (halitosis) and gastrointestinal disturbances [18], skin rash [13], a burning sensation in the mouth or stomach and nausea [26]. Patients on blood-thinning medication should avoid it because of its effect on clotting time [29]. Clinical studies evaluating doses up to 15 grams daily for weeks report no apparent side effects [29]. Certain precautions must be followed to minimize the risk of adverse side effects but overall, as a therapeutic agent, it appears to be remarkably safe.