WASHINGTON (Reuters) - Capsaicin, which makes peppers hot, can cause prostate cancer cells to kill themselves, U.S. and Japanese researchers said on Wednesday.

Capsaicin led 80 percent of human prostate cancer cells growing in mice to commit suicide in a process known as apoptosis, the researchers said.

Prostate cancer tumors in mice fed capsaicin were about one-fifth the size of tumors in untreated mice, they reported in the journal Cancer Research.

"Capsaicin had a profound anti-proliferative effect on human prostate cancer cells in culture," said Dr. Soren Lehmann of the Cedars-Sinai Medical Center and the University of California Los Angeles School of Medicine.

"It also dramatically slowed the development of prostate tumors formed by those human cell lines grown in mouse models."

While it is far easier to cure cancer in mice infected with human tumors than it is in human beings, the findings suggest a possible future treatment. They also may offer a good excuse for men who like spicy food to eat more of it.

Lehmann estimated that the mice ate the human equivalent of 400 milligrams of capsaicin three times a week. That is about the amount found in three to eight fresh habanero peppers, depending on how hot the peppers are.

The capsaicin inhibited the activity of NF-kappa beta, a molecular mechanism that helps lead to apoptosis in many cell types.

Prostate cancer is the most common malignant cancer in U.S. men. It is diagnosed in 232,000 men every year and kills up to 30,000 of them. Worldwide, 221,000 men die every year from prostate cancer.