They include:

* Zinc. This is crucial for regulating more than 100 enzymes. One study found a significant reduction in 5-alpha-reductase activity in rat prostate tissue previously injected with zinc (Andrologia, 1993; 25: 369-75). Another study found that this inhibitory effect of zinc was increased when taken with vitamin B6 (Br J Dermatol, 1988; 119: 627-32).

However, a word of caution: stick to the recommended dosages of 15-30 mg/day, as higher amounts can lead to copper deficiency which, in turn, results in - you guessed it - hair loss.

* Azelaic acid. This naturally occurring substance, found on the skin, is a potent inhibitor of 5-alpha-reductase even at low levels. Combined with zinc, the two inhibitors had an additive effect (Br J Dermatol, 1988; 119: 627-32). Azelaic acid is available over the counter as a cream or lotion.

* Saw palmetto (Serenoa repens). This antiandrogenic herb lowers dihydrotestosterone in two ways: it cuts down its production by lowering levels of 5-alpha-reductase; and stops its absorption by blocking its receptors on cell membranes.

A study of saw palmetto extract for treating mild-to-moderate male pattern baldness found improved hair growth in 60 per cent of the subjects (J Altern Complement Med, 2002; 8: 143-52). A common dosage is around 300 mg/day.

* Gamma-linolenic acid (GLA). Along with certain other unsaturated fatty acids, GLA is thought to inhibit 5-alpha-reductase activity - at least in animals. This study also found GLA to have low toxicity and no risk of systemic side-effects (J Invest Dermatol, 1997; 109: 152-7). Nevertheless, it’s worth remembering that the results of animal studies cannot be counted on as they don’t necessarily apply to humans.

Evening primrose oil is a rich source of GLA, and the recommended dosage ranges from 50-100 mg/day to as much as 700 mg/day.

* Silica. Although not, strictly speaking, a hair-growth stimulant, silica is a mineral recognised for strengthening existing hair, and maintaining its thickness and colour. Good food sources of silica include potatoes, cucumbers, brussels sprouts, and green and red peppers.

There is no recommended dose for silica. However, most of the commercial nutritional supplements containing this trace element use around 10-15 mg per dose.

* Traditional Chinese medicine. TCM practitioners believe that hair loss is linked to weak kidneys, and the usual treatment includes tonics or herbs to strengthen those organs. Other symptoms related to weak kidneys are fatigue, lower back pain and general physical weakness.

* Aromatherapy. The results of a randomised, double-blind study suggest that three drops each of the essential oils of thyme, rosemary, lavender and cedarwood, when mixed with 30 mL each of jojoba and grapeseed carrier oils, improved the symptoms of alopecia areata (patchy hair loss). When massaged into the scalp every night for seven months, this oil mixture stimulated new hair growth in 44 per cent of users (Arch Dermatol, 1998; 134: 1349-52).

* Ayurvedic medicine. This form of traditional Indian medicine often uses Brahmi (Centella asiatica, gotu kola) or bhringarag (Eclipta alba) oils for hair loss. The Ayurvedic tradition believes that hair is a byproduct of bone formation.