Gac grows on dioecious vines and is usually collected from fence climbers or from wild plants. The vines can be commonly seen growing on lattices at the entrances to rural homes or in gardens. It only fruits once a year, and is found seasonally in local markets. The fruit itself becomes a dark orange color upon ripening, and is typically round or oblong, maturing to a size of about 13 cm in length and 10 cm in diameter. Its exterior skin is covered in small spines while its dark red interior consists of clusters of fleshy pulp and seeds.
Traditionally, gac has been used as both food and medicine in the regions in which it grows. Other than the use of its fruit and leaves for special Vietnamese culinary dishes, gac is also used for its medicinal and nutritional properties. In Vietnam, the seed membranes are used to aid in the relief of dry eyes, as well as to promote healthy vision. Similarly, in traditional Chinese medicine the seeds of gac, known in Mandarin Chinese as mùbiēzǐ (Chinese: 木鳖子), are employed for a variety of internal and external purposes. Recently, attention is being to be attracted to it in the West, because chemical analysis of the fruit suggests it has high concentrations of several important phytonutrients.
The fruit contains by far the highest content of beta-carotene of any known fruit or vegetable. Research has confirmed that the beta-carotene (vitamin A) in the fruit is highly bioavailable. In a double-blind study with 185 children, some were given a dish containing 3.5 mg beta-carotene from spiny bitter gourd, while others were given an identical-looking dish containing 5 mg beta-carotene powder. After 30 days, the former group eating natural beta-carotene had significantly greater plasma (blood) levels of beta-carotene than the latter with synthetic beta-carotene. This oil also included high levels of vitamin E. The fatty acids in the aril are important for the absorption of fat-soluble nutrients, including carotenoids, in a diet typically low in fat. Thus, gac provides an acceptable source of high levels of valuable antioxidants that have good bioavailability. Also, vitamin A is good for skin and vision.
Gac has been shown to be especially high in lycopene content. Relative to mass, it contains up to 70 times the amount of lycopene found in tomatoes. It has also been found to contain up to 10 times the amount of beta-carotene of carrots or sweet potatoes. Additionally, the carotenoids present in gac are bound to long-chain fatty acids, resulting in what is claimed to be a more bioavailable form. There has also been recent research that suggests that gac contains a protein that may inhibit the proliferation of cancer cells.
Information adapted from Wikipedia