Allium sativum - a model of herbal complexity

Garlic

Allium sativum, commonly known as garlic has been used both as food and medicine by more human cultures and for longer than almost any other plant in the world.

It performs functions beyond that of nourishment, including risk reduction in cardiovascular disease and cancer, influencing liver detoxification pathways and blood sugar modulation. It is an antioxidant, an antimicrobial and it is prebiotic (prebiotics are substrates that are selectively utilised by host microbes conferring a health benefit).

Whilst the beneficial impacts of garlic on health work via many pathways, modulating our gut microbiota and their metabolites is one of them. Our gut microbes are a malleable interface which influence all areas of human health. Consumption of medicinal herbs/foods that are familiar, cheap, easily obtainable and which support health through pathways including gut microbiota/metabolite modulation seems like a very sensible idea. And garlic is a prime candidate.

Garlic is known as an antimicrobial and a prebiotic. This has always interested me, because antimicrobials can kill microbes and prebiotics help certain microbes to thrive. How can garlic be both things?

Garlic as a prebiotic

Garlic contains prebiotic fibre in the form of long and short chain inulin (the latter is known as fructo-oligosaccharide or FOS). Humans lack enzymes to hydrolyse these carbohydrates, making them indigestible to us. Some of our intestinal microbiota, such as Bifidobacterium and Lactobacillus, can digest these prebiotics – they are a fermentable substrate that “feeds” these microbes and some of the by-products produced through this process benefit human health through various mechanisms of interaction. These microbes are also regularly interacting directly with our immune system.

Garlic as an antimicrobial

Garlic’s role as an antimicrobial seems diametrically opposed to its role as a prebiotic. A striking feature of garlic is its high organosulfur compound content, four times higher than other sulphur-rich vegetables like onion or broccoli. This is mostly due to the presence of the sulphur-containing amino acid cysteine. Much of the research into garlic has focussed on these pharmacologically active sulphur compounds, a key one being allicin. The characteristically pungent allicin is not present in undamaged garlic bulbs, but is produced when the odourless cysteine sulfoxide, alliin, contained in plant storage cells, comes into contact with the enzyme alliinase through bulb damage (e.g. chewing).

Whilst garlic’s antimicrobial activity has been largely attributed to allicin’s direct effect on microbes, other compounds are also likely to be involved because extracts lacking in allicin can still have an antimicrobial impact. Non-sulphur compounds may affect microbes indirectly by stimulating host immune function.

Clearly, the constituents of garlic are complex and there appears to be the potential for them to work in a paradoxical way. Garlic’s antimicrobial qualities affect different microbial species and strains in different ways, depending on the physiological characteristics of those microbes. The manner in which garlic has its antimicrobial effect appears to be through cell membrane/cell wall structure damage, as well as through enzyme function and damage to DNA/RNA integrity. At sufficient concentrations relevant to each strain, it affects cell viability. But the relationship is far from straightforward; in some cases, exposure to highly concentrated garlic extract actually increases growth levels of bacteria or produces attenuated levels of bacterial inhibition. In other cases, lower concentration exposure over longer time periods results in initial inhibition, followed by microbial recovery.

Garlic as a prebiotic and an antimicrobial

Microbial species and strains vary in the extent to which they ferment the prebiotic fibre in garlic and the extent to which they can withstand garlic’s antimicrobial effects. The evidence currently available also suggests that those who eat garlic regularly may find that its antimicrobial effects are reduced because their resident microbiota may develop resistance to garlic’s antimicrobial effects, possibly leaving them to benefit from the prebiotic effect alone. Accordingly, garlic may not be the best herbal antimicrobial to use in people who already consume it regularly. Equally, it seems possible that it is not only beneficial microbes that benefit from garlic’s fibre content.

Different areas of the human gastrointestinal tract are very individual environmental niches. During digestion, garlic spends different periods of time in each region and garlic’s constituents have widely diverging actions; the volatility of its organosulfur compounds compared to digestion-resistant inulin, for example. Garlic’s actions on human physiology may well differ at different stages in that digestive journey. Add to that the fact that the microbiota and their behaviour in the various regions of the gut will also differ and it is clear there is an inordinately complex relationship to be unpicked if we are to fully understand garlic’s relationship with our physiology and our microbes.

Further research on the impact of garlic in humans would be welcome. For example, we do not yet understand how, after consumption of raw garlic, neither allicin nor its metabolites are found in the blood, urine or stool, but do appear as components of human breath, indicating systemic metabolism.

Garlic is complicated

Clearly, If we want a complete picture of garlic’s activities, we cannot consider garlic’s prebiotic effects without also considering its antimicrobial effects and it is oversimplified to think that garlic’s antimicrobial effects are only felt by troublesome bacteria within us and that the inulin and FOS it contains only benefits those microbes that support health. We know that garlic can have antimicrobial effects on beneficial bacteria such as Bifidobacterium and Lactobacillus as well as on unhelpful bacteria and it is likely that microbes that are problematic for us may benefit from the inulin and FOS it contains.

It is quite instructive to realise that even with the most used and best understood of medicinal herbs, we are a long way from fully understanding all the constituents and the mechanisms by which they impact our health. Garlic has a profound ability to support health and the more we understand about its impact on human physiology, the better we will understand the nuances of this complicated relationship.

Until we have that perfect information though, we can be sure that, after millennia of use, garlic is safe, well tolerated by the majority of us, causes few negative side effects (I don’t count garlic breath as a negative thing) and has many health benefits via many pathways. Raw or cooked, it is worth incorporating into your meals.