From the January 2006 edition of Dr. Fuhrman's Healthy Times:

Juices and extracts of exotic fruits and vegetables such as mangosteen, gogi berries, Chinese lycium, acia, Siberian pineapple, cili, noni, guarana, and black currant are touted as wondrous super foods with a myriad of health claims. Certainly, eating exotic fruits from all over the globe can add valuable phytochemical compounds with the potential for beneficial effects. I see no reason why these fruits and their juices should not be used as part of a varied diet with a wide assortment of phytonutrients. Broadening our variety of health-supporting nutrients from exotic foods has value in building a strong immune defense against cancer.

The confusion arises when marketers claim that the juices can cure cancer or kill cancer cells on the basis of studies that show that some component in the juice or other part of the plant has been shown to kill cancer cells. Just because a concentrated chemical derived from a food can kill cancer cells in a test tube does not make that food a cure for cancer.

For example, some of the non-tasty parts of these plants have been shown to cause the death of human cancer cells in test tubes. But there have been no experiments with actual cancer patients (or patients with other diseases). It's likely that many exotic fruits are highly nutritious, but there is also the possibility that some of the touted benefits seen in scientific studies arise from the medicinal effects, which are toxic. These medicinal effects are often confused by marketers with the nutritive effects of the fruit.

If something is beneficial against cancer, then one or more of the following conditions must be met:

1. The substance must be a compound that is more toxic to cancer cells than normal cells (similar to chemotherapy, which can shrink cancerous tumors and kill rapidly growing cancer cells because these cells are more sensitive to the effects of poison than normal cells);

2. It must supply nutritive substances that increase the person's defenses against the disease, thereby curtailing the spread of cancer via immunostimulating or immunosupportive means and increasing cancer survival (typically by slowing the spread of cancer);

3. It must have hormonal blocking effects that lessen the effect of the body's endogenously produced hormones that promote the spread of cancer.

When we look at the studies presented on these natural foods, we have to attempt to differentiate between a medicinal (toxic) effect due to a noxious compound in the plant and a nutritive effect due to the phytonutrient content. Promoters of these fruits and juices tend to lump all of the studies together, trying to make their product look like a magical food that can promote wellness and also kill cancer cells. But just because the toxic part of the plant has the ability to kill cancer cells in a test tube does not mean the substance will be an effective agent against cancer.

For example, one reason why I hesitate to recommend noni juice for healthy people is because of its ability to seemingly increase one's energy and to reduce pain. For a product to have such powerful medicinal effects, it must contain natural compounds with toxic properties. As a result, it isn't wise to consume these compounds continually over a lifetime. It is too much like a drug.

Noni juice may be good as an aid for elderly people who lack energy or who regularly experience discomfort. But it is inaccurate to claim that these benefits come from the nutritive components. The drug effects come from the toxic components. Natural substances with strong medicinal effects should be avoided unless you have a medical condition that warrants this type of therapy.