How beneficial is the citrus?

What studies say
Scientific studies indicate compounds in citrus, including lemon, have real potential as anti-cancer agents. Studies have indicated that citrus limonoids do have potential as anti-cancer agents.

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However, it is not yet clear exactly HOW effective citrus will ultimately prove to be in preventing or fighting against cancer in humans. It is PREMATURE and INACCURATE to claim that lemon is a "proven remedy against cancers of all types." Nor, at this point, can it be said that lemon is a viable alternative to traditional treatments such as chemotherapy. But lemon, like other kinds of citrus is likely to be a healthy addition to your diet and may even reduce the risk of cancer. However, these findings do not give validity to the exaggerated and unsupported claims made wrt lemons so far. To be useful, health advice needs to be valid, accurate and be supported by credible medical sources. Sending on spurious health information is unlikely to be beneficial.

Research results we have so far
A number of studies have indicated that compounds found in citrus (including lemon) may be effective as anti-cancer agents, at least for certain types of cancer. A December 2004 Science Daily article reports:

Research by Texas Agriculture Experiment Station scientists has shown that citrus compounds called limonoids targeted and stopped neuroblastoma cells in the lab. They now hope to learn the reasons for the stop-action behavior and eventually try the citrus concoction in humans. [......]

Harris explained that flavonoids and limonoids – nutrient-packed pigments that give color and taste to fruit – may work against cancer in any of three ways: prevent it from forming, slow the growth of existing cancer, or kill cancer cells.

"The limonoids, which differ structurally from flavonoids, seem to do all three," he said of tests in his lab by one of Patil's graduate students, Shibu Poulose, who also worked in Harris' College Station lab. Their work emphasized the compounds' ability to kill the existing neuroblastoma cells with the rationale that if the method and time limonoids take to obliterate the cancer could be found, perhaps scientists could exploit it to help cure the disease.

A May 2000 report about the potential of citrus limonoids as anticancer agents explains:

The experimental results described above indicate that citrus limonoids may provide substantial anticancer actions. The compounds have been shown to be free of toxic effects in animal models so potential exists for use of limonoids against human cancer in either the natural fruit , in citrus fortified with limonoids, or in purified forms of specific limonoids . Although the initial studies are very promising , they have been conducted primarily with in vitro cell culture and animal models. Thus , research is needed to determine whether the limonoids may be useful in preventing or treating cancer in humans .

And a report on the medicinal use of citrus published on the University of Florida EDIS website notes:

Citrus flavonoids have potential antioxidant (prevents aging), anti-cancer, antiviral, anti-inflammatory activities, effects on capillarity, and cholesterol-lowering ability. The principal carotenoids in pink grapefruit are lycopene and beta-carotene. Lycopene-containing fruits and vegetables have been shown to contribute to a significant reduction in prostate and mammary cancer risk. Recent studies have further shown that limonoids inhibit the development of cancer in laboratory animals and in human breast cancer cells as well as reducing cholesterol. Researchers have also suggested that, if ingested, limonoids may not be absorbed in the large intestine, and therefore could be distributed throughout the body, with beneficial effects.

 Source: http://www.hoax-slayer.com/lemon-cancer-cells.shtml