Antioxidant, anti-inflammatory, and antimicrobial properties of garlic and onions

The Authors

Emily A. Wilson, Department of Ecology and Evolutionary Biology, University of Colorado, Boulder, Colorado, USA

Barbara Demmig-Adams, Department of Ecology and Evolutionary Biology, University of Colorado, Boulder, Colorado, USA

Acknowledgements

The authors thank Gayla Buitron for helpful editorial suggestions.

Abstract

Purpose – The purpose of this paper is to provide a comprehensive overview of multiple functions and their underlying mechanisms for two common spices, garlic and onion, containing organosulphur compounds.

Design/methodology/approach – Literature review of chemistry, physiology, molecular biology, clinical studies.

Findings – Both garlic and onions exert their effects on human health via multiple different functions, including antioxidant, anti-inflammatory, and antibacterial properties. The organosulphur compounds in these spices scavenge oxidizing agents, inhibit the oxidation of fatty acids, thereby preventing the formation of pro-inflammatory messengers, and inhibit bacterial growth, via interaction with sulphur-containing enzymes.

Research limitations/implications – Currently available information on the optimal amount for consumption for each spice is insufficient.

Originality/value – This review is unique in its comprehensive nature, considering multiple different effects of the spices examined as well as multiple studies from molecular to clinical approaches.

Article Type:

Literature review

Keyword(s):

Food products; Health foods; Diet.

Journal:

Nutrition & Food Science

Volume:

Number:

Year:

2007

pp:

178-183

Copyright ©

Emerald Group Publishing Limited

ISSN:

0034-6659

Introduction

The effects of spices and their possible health benefits have been studied for centuries. Spices are used for medicinal purposes as well as for added flavor and aroma in many food dishes worldwide. Spices are therefore of considerable economic importance and account for some 2 billion dollars (and nearly 500,000 tons) annually of United States’ imports alone (Srinivasan, 2005a). The potential medical benefits of spices include possible roles in lowering the risk for atherosclerosis, cardiovascular disease, cancer, and diabetes (Ali et al., 2000; Griffiths et al., 2002; Banerjee et al., 2003; Lai and Roy, 2004; Khanum et al., 2004; Ashraf et al., 2005; Srinivasan, 2005b; Rahman and Lowe, 2006). While spices have been used for their presumed health benefits for thousands of years (that also include purported roles in preventing earaches, hair loss, and treating warts; Griffiths et al., 2002), new medical research is now uncovering the underlying physiological and molecular mechanisms of their action as well as providing some scientific evidence of their effectiveness. While spices have specific beneficial effects, e.g. in aiding digestion by increasing the production of saliva and gastric juices (Srinivasan, 2005b), this paper will focus on more general and fundamental features, i.e. the antioxidant, anti-inflammatory, and antimicrobial properties of spices.

There is promising evidence for antioxidant, anti-inflammatory, and antibacterial properties of garlic and onion, however, a caveat at this time is that the doses used in many in vitro studies exceed those typically consumed by humans on a daily basis (Ali et al., 2000). On the other hand, epidemiological studies show an inverse correlation between garlic consumption and the risk for cardiovascular disease (reviewed in Rahman and Lowe, 2006). Optimal doses for maximal benefit to human health, while avoiding toxic effects such as anemia, remain to be defined (Banerjee et al., 2003) and controlled human trials are needed to address this issue.

Two key spices with organosulphur compounds: garlic and onion

Garlic and onion are both found in the Allium family and contain organosulphur compounds with antioxidant, anti-inflammatory, and antimicrobial properties. Garlic, Allium sativum, is a widely studied spice with many purported benefits and a long medicinal history dating back to Aristotle, Hippocrates, and Aristophane (Ali et al., 2000). Garlic typically contains three times greater levels of organosulphur compounds than onion (Benkeblia, 2004). After chopping or crushing, the enzyme allinase converts alliin (a cysteine-sulphoxide) in garlic to allicin (a thiosulphate) (Benkeblia, 2004; Banerjee et al., 2003). The latter compound is thought to confer many of garlic's medicinal effects, but garlic has also been shown to be metabolized to a number of additional organosulphur compounds (Khanum et al., 2004).

The effect of cooking on garlic is controversial at this time. Shobana and Naidu (2000) found that boiling at 100 ^○C for 30min increased garlic's overall antioxidant activity, and Benkeblia (2004) reported increased beneficial effects of garlic after heating. On the other hand, Lai and Roy (2004) concluded that some garlic antioxidants are thermally unstable and Banerjee and coworkers (2004) reported that allinase is inactivated by heat, thus preventing allicin formation.

The onion, Allium cepa, is another food/spice with medicinal properties as well as uses for flavor and aroma. A major active ingredient of onion is S-propenylcysteine sulphoxide (Ali et al., 2000). Onion furthermore contains cepaenes that are best known for their inhibition of pro-inflammatory messengers (Ali et al., 2000). Onions possess antioxidant and antibacterial properties, but their antioxidant activity is less than that of garlic (Shobana and Naidu, 2000). The antioxidant activity of onion is reduced after cooking, and onion is thus most effective in its raw form (Ali et al., 2000). Interestingly, different types of onions were found to vary in their properties, with highest total antioxidant activities as well as greatest in vitro tumor cell inhibition seen in shallots and the onion variety Western Yellow (Yang et al., 2004).

Regulation of metabolism by oxidation/antioxidation and the role of garlic and onion

Spices contain the classic antioxidant vitamins ascorbic acid (vitamin C) and tocopherols (vitamin E group) but also other, very potent antioxidants, such as phenols, thiols (as sulphur compounds), and carotenoids (Sharma, 2005; Yang et al., 2004). As antioxidants, all of these are compounds able to slow down, stop, or reverse oxidation processes by scavenging oxidizing agents, such as reactive oxygen species (ROS), and recycling oxidized lipids, proteins, and nucleic acids. When present in excess amounts, ROS increase the risk of atherosclerosis and chronic diseases (Sharma et al., 2005). Oxidation of lipids can cause specific, direct effects, such as destabilization of (lipid) membranes resulting, e.g. in decreased survival of red blood cells (Yang et al., 2004; Kempaiah and Srinivasan, 2004). Allicin has been shown to act as an antioxidant by scavenging ROS and preventing lipid oxidation and production of pro-inflammatory messengers (Banerjee et al., 2003), and similar results were obtained for garlic and onion extracts (Shobana and Naidu, 2000).

A key mechanism for the multiple effects of ROS is the activation of redox-regulated gene regulatory proteins (Lavrovsky et al., 2000) that turn on genes for pro-inflammatory enzymes such as cyclooxygenase (COX) and lipoxygenase (LOX). Redox-regulated genes are controlled by reduction (via antioxidants) and oxidation (via ROS) of components of the signal transduction pathways that control their expression. Expression of COX is upregulated by a surplus of ROS and downregulated by antioxidants (such as those present in garlic and onion).

How much of these pro-inflammatory enzymes (COX and LOX) is synthesized is regulated by gene regulatory factors (transcription factors). One of these is nuclear factor kappa B or NF κB, a master control gene of the immune/inflammatory response (Janssen-Heininger et al., 2000). Under normal conditions, NF κB remains inactivated by another factor, its inhibitor I κB. When NF κB is stimulated, more COX/LOX is synthesized and inflammation is triggered. This transcription factor is, in turn, strongly regulated by dietary factors; it is activated under insufficient levels of antioxidants, particularly sulphur-containing ones (Janssen-Heininger et al., 2000).

In a study by Kempaiah and Srinivasan (2004), rats were given a high-fat diet with or without garlic, and blood levels of triglycerides (lipids with three fatty acids known to increase atherosclerosis risk) and thiols such as glutathione (amino acids or peptides with sulphur groups that recycle, or re-reduce, oxidized proteins, scavenge ROS, and have a potent effect on redox-regulated signaling pathways, such as that involving NF κB) were assessed. Food intake per se was not affected by garlic in this study. The high-fat diet increased the levels of blood triglycerides, decreased the levels of thiols such as glutathione, and increased lipid oxidation. Kempaiah and Srinivasan (2004) found that all of these adverse effects of the high-fat diet were effectively reduced by regular addition of garlic to the diet, thus presumably reducing the risk of atherosclerosis. When garlic was added to the high-fat diet, total endogenous thiols increased by 16 per cent, glutathione increased by 28 per cent, and the level of an endogenous antioxidant enzyme, catalase, which is depleted under oxidative stress, also increased (Kempaiah and Srinivasan, 2004). The sulphur compounds in garlic are thus able to protect the endogenous thiol pool (by re-reducing thiols that become oxidized). Other studies (Ali et al., 2000; Ashraf et al., 2005; Srinivasan, 2005b; Rahman and Lowe, 2006) support the effect of garlic in improving cardiovascular health, e.g. via decreases in platelet aggregation, a lowering blood pressure and cholesterol levels, and inhibition of several steps in the inflammation process as described in the present review.

Further anti-inflammatory effects of garlic and onion

Chronic over-production of either COX or LOX (and also NF κB itself) causes excess inflammation and contributes to chronic pro-inflammatory diseases such as cardiovascular disease, diabetes, and others (Goodsell, 2005). The messengers produced by LOX can also either stimulate or prevent programmed cell death. Excessive cell death is involved in e.g. neurodegenerative disease, while insufficient cell death can lead to cancer (Hannun, 1997; Tatton and Olanow, 1999).

In addition to limiting how much of these inflammatory enzymes is manufactured (see above), spices can also dampen the actual activity of existing the pool of inflammatory enzymes such as COX and LOX. Both COX and LOX convert oxidized lipids, such as arachidonic acid (AA), to pro-inflammatory, hormone-like messengers. COX produces prostaglandins that signal pain and trigger inflammation and LOX produces a related group of messengers, leukotrienes (Goodsell, 2005). Spices inhibit the activity of both COX and LOX (Goodsell, 2005).

Onion, apparently via its thiosulphinate and cepaene content, inhibits the production of AA as well as its conversion to pro-inflammatory prostaglandins and leukotrienes (Ali et al., 2000). More specifically, onion cepaenes were shown to inhibit COX and LOX activity as well as blood platelet aggregation (Ali et al., 2000). The same study also showed that onion extract can decrease the onset and development of tumors as well as have antiasthmatic effects (the latter again via COX inhibition).

Allicin inhibited the production of pro-inflammatory cytokine messengers in a study of inflammatory bowel disease, apparently by inactivating the pro-inflammatory factor NF κB via its I κB inhibitor (Lang et al., 2004). By virtue of sulphur-based antioxidants found in garlic, NF κB was maintained in its inactive state, thus preventing synthesis of excess COX/LOX.

Antimicrobial agents and specific affects of garlic and onion

In addition to being antioxidants and anti-inflammatory agents, spices also have antibacterial/antimicrobial properties (Lai and Roy, 2004). The antibacterial properties of garlic can be eliminated by inhibition of the allinase enzyme and prevention of allicin formation (Jonkers et al., 1999). The antibacterial effect garlic apparently results from interaction of sulphur compounds, like allicin, with sulphur (thiol) groups of microbial enzymes (such as trypsin and other proteases), leading to an inhibition of microbial growth (Jonkers et al., 1999; Bakri and Douglas, 2005). Many bacterial strains, both gram-positive and gram-negative, can be inhibited with garlic, and some strains were inhibited much more strongly by allicin or garlic extract compared to antibiotics (Bakri and Douglas, 2005; Lai and Roy, 2004). The bacterial strain Staphylococcus aureus causes pus-producing infections, such as boils, as well as pneumonia and urinary tract infections (Todar, 2005). Cultures of this strain (as well as Salmonella enteritidis, the bacterium responsible for salmonella food poisoning, and several fungi) are effectively inhibited by garlic and onion oil or extracts (Benkeblia, 2004). Other microbes inhibited by garlic include Bacillus subtilis, a gram-positive bacterium found in soil, Escherichia coli, a common toxin-producing, food-borne bacterium, and Saccharomyces cerevisiae, a yeast species (Lai and Roy, 2004). Remarkably, mouthwash containing garlic significantly reduced total salivary bacteria, including Porphyromonas gingivalis, the bacterium causing gingivitis (Bakri and Douglas, 2005).

Onions possess antibacterial properties as well. Although less research is available on the antibacterial activity of onion, it is suggested that S-propenylcysteine sulphoxide is the compound that inhibits antibacterial metabolism by the same mechanism as garlic (Kyung and Lee, 2001). Onion extract, the activity of which remained stable for 48h, inhibited Streptococcus mutans, a bacterium that causes strep throat, tonsillitis, bacterial pneumonia, as well as other diseases (Ali et al., 2000).

Adverse effects of excessive doses of garlic and onion

Excessive consumption of concentrated formulations of garlic can lead to adverse effects on health, such as anemia, weight loss, and toxicity to the heart, liver, and kidney as well as breaks in chromosomes (Banerjee et al., 2003). Doses of 4ml/kg, for raw garlic juice, or of 100mg/kg, for garlic oil, were lethal to rats (Banerjee et al., 2003). Problems can arise from high doses of onion as well. One study showed that high doses (500mg/kg) given orally caused lung and tissue damage in rats (Ali et al., 2000). Another issue, as with garlic, are dermatological problems. Contact dermatitis, irritation of the skin due to allergens, could by induced with garlic (Sahu, 2002). The specific allergens have not been identified, but both Allium spices can cause burns after external application (Ali et al., 2000).

Conclusions

Garlic and onion each possess antioxidant, anti-inflammatory, and antibacterial properties. The effectiveness of these spices in decreasing pro-inflammatory diseases is rooted in their nature as modulators of metabolism, for example as COX and LOX inhibitors. While the available evidence is encouraging, controlled human trails are needed to establish the effectiveness of these spices in disease prevention. Many of the available studies utilized relatively high doses of the effective compounds in garlic and onion, and it remains to be seen whether a moderate level of consumption, that avoids the toxic effects of excessive doses, is effective. Until such trials are available, it seems safe to conclude that garlic and onion should be included in the human diet as whole foods and spices, while high-dose extracts should be used with caution.

Spices have been used for thousands of years and will likely continue to be used for their aroma, flavor, and medicinal purposes. The research summarized here supports the ancient wisdom of Aristotle and Hippocrates who recommended garlic for medicinal purposes and generally promoted the use of food as medicine.

References

Ali, M., Thomson, M., Afzal, M. (2000), "Garlic and onions: their effect on eicosanoid metabolism and its clinical relevance", Prostaglandins, Leukotrienes and Essential Fatty Acids, Vol. 6 No.2, pp.55-73.