Although not strictly speaking an amino acid, GABA is often classified with the amino acids. It functions as the most widely distributed inhibitory neurotransmitter in the brain. The manufacture of GABA is regulated by vitamin B6. Benzodiazepine tranquillizer drugs (such as Valium), produce their anti-anxiety and muscle-relaxant effects by activating the neurons (nerve cells) and receptor sites which respond to GABA.

GABA is almost always deficient in clinical and experimental seizure disorders. It is involved in the regulation of mechanisms related to blood pressure, therefore stimulants of GABA receptors are considered useful in combating hypertension.

Availability: GABA is not readily available as a dietary supplement, but supplementation with the amino acid taurine increases the breakdown of glutamate to GABA.

Gamma linolenic acid (GLA)

(Also see Fats)

A polyunsaturated fatty acid which is an intermediate in the conversion of linoleic acid to prostaglandins. Some individuals are thought to perform this conversion inefficiently, and may therefore benefit from supplementation with GLA, which is rarely found in food. Nutrients needed for the conversion process include magnesium, zinc, biotin and vitamin B6. Deficiencies of any of these nutrients may result in a GLA deficiency in the body.

Symptoms of GLA deficiency may include dry skin, extreme thirst (where no apparent cause can be found on medical investigation), hyperactivity in children, premenstrual syndrome and eczema.

The richest food sources of GLA include human breast milk, borage seed oil, evening primrose oil and blackcurrant seed oil. The seed oils are readily available in health food shops for use as dietary supplements.

Gamma oryzanol

A phytosterol (cholesterol-like substance) found in rice bran oil and consisting of ferulic acid esters of triterpene alcohols. Gamma-oryzanol is partially metabolized to ferulic acid in the intestine but is primarily (though poorly) absorbed intact. It is sold as an intestinal health supplement. Japanese research studies claim that it has been successfully used against diseases as diverse as peptic and duodenal ulcers, psychosomatic complaints, hypercholesterolaemia, and menopausal symptoms.

Availability: from nutritional therapists.

Garcinia cambogia

This is another name for tamarind, a fruit used in Indian cookery and food preservation. It contains the substance (-)hydroxycitrate, which is related to citric acid and is sold in supplement form as an aid to weight loss. (-)Hydroxycitrate is thought to be able to partially inhibit an enzyme required for the conversion of blood glucose into fat, which results in higher glycogen (stored carbohydrate) levels instead. High levels of glycogen in the blood can help to suppress the appetite and glycogen is readily used as an energy source.

There is a shortage of human studies carried out on (-)hydroxycitrate as a weight loss aid. Individuals who have tried it obtain a varied response. It may be of most use in curbing an excess sugar consumption since sugar consumption results in early satiety if (-)hydroxycitrate supplements are taken previously.

Availability: from health food stores.