Garlic is one of the most heavily studied of all plants and the literature shows that it possesses some very important health benefits. Researchers looking at the effects of garlic have focussed mainly on trying to understand the metabolism and chemistry of the organosulphur compounds produced by crushing garlic. The organosulphur compounds present in the whole bulb are primarily odourless and biologically inactive, but on crushing are enzymatically converted to a number of volatile compounds that give garlic its characteristic odour (here). In addition, there are a group of saponins present in garlic that may have important health benefits and metabolic effects in humans, but these are not so well understood. Garlic saponins have been shown to possess possible antifungal, antibacterial, anti-inflammatory effects, as well as the ability to lower cholesterol levels.
Researchers have investigated the saponin content of aged garlic extract (AGE) and found that it contains a large variety of compounds (table 1)1. Saponins are non-sulphurous compounds that produce a stable foam when shaken in water, are haemolytic and have a bitter taste. There are two groups of saponins found in plants named the triterpenoid saponins (e.g. ginsenocides in ginseng and glycyrrhizin in liquorice) and the steroid saponins. Steroid saponins are divided into furostanol saponins and spirostanol saponins. Furostanol saponins are easily converted into spirostanol saponins by the enzyme β‑glucosidase. Spirostanol saponins are usually found after drying of plant material, having being enzymatically converted from the furostanol saponins present in the whole plant over time2. As with the organosulphur compounds in garlic, the methods of preparation result in variation in the steroidal saponin content of garlic.
Table 1. Some of the known saponins in garlic.
Experiments using laboratory animals have shown that feeding spirostanol saponins, enzymatically derived from the furostanol saponins in the whole garlic, decreases levels of total cholesterol and LDL cholesterol. The effects of saponins on cholesterol may be due to their ability to decrease the absorption of cholesterol from the intestinal tract, although the exact mechanisms is not understood. However, simply decreasing the absorption of cholesterol from the intestine would be unlikely to reduce plasma cholesterol levels in humans, as evidenced by reduced cholesterol diets having little effect. A steroidal saponin in garlic called eruboside-B, has been shown to possess possible antifungal activity against Candida albicans and cytotoxic effects in vitro. The garlic saponin β-chlorogenin may also have the ability to decrease the aggregation of platelets in vivo, which may explain some of the cardioprotective effects of garlic consumption.
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