Progesterone is the gestational hormone that prepares the lining of the uterus for the fertilized ovum and maintains pregnancy. It is derived primarily from the corpus luteum that is formed in the ovary from the ruptured follicle. It is also produced in the placenta during pregnancy and in small amounts by the adrenal cortex. Progesterone is a "precursor" hormone. This means that it can be converted by the body into other steroid hormones.

Artificially produced progesterone or progestins are synthetic hormones that closely resemble the body's own production of progesterone, but differ in important ways. Both natural and synthetic hormones share the ability to sustain the lining of the vagina and uterus but progestins do not have the full range of biological activity of natural progesterone. Progestin has actually been shown to inhibit biosynthesis of progesterone.
Some doctors now believe that progestin is responsible for a long list of side effects. And since many women who are engaged in hormone replacement therapy are filling their prescriptions with synthetic progesterone, they are exposing themselves to unnecessary risks.

According to Dr. David G. Williams, progestin can cause abnormal menstrual flow or cessation, fluid retention, nausea, insomnia, jaundice, depression, fever, weight fluctuations, allergic reactions, and the development of male characteristics. Natural progesterone, on the other hand, has few side effects: occasionally it may cause a feeling of euphoria, and, for some women, it may alter the timing of their menstrual cycles.

Dr. Williams recommends that women begin by taking vitamin supplements to increase their own production of progesterone. Animal studies suggest that beta-carotene can stimulate the production of this vital hormone. Likewise, a daily dose of 150 IU of vitamin E can raise progesterone levels; however, dosages of 300 to 600 IU of vitamin E can actually lower levels of the hormone.

San Francisco nutritionist Linda Ojeda advocates dietary sources of estrogen and progesterone: soybean products such as tofu, miso, and soymilk. These products contain phytoestrogens, which have different levels of estrogenic activity. Women who are reluctant to take synthetic estrogens may consider phytoestrogens as an alternative therapeutic agent. Ojeda points out that Japanese women experience a very low rate of menopausal complaints, which she attributes to their high consumption of soybeans.

For women who want more, Dr. Williams recommends natural progesterone. Unfortunately, natural hormone supplements are hard to come by, since drug companies cannot patent them and therefore are not interested in selling them commercially. Cream extracted from the Mexican wild yam (Dioscorea mexicana) has long been recognized as a natural source of estrogen and progesterone. Dioscorea is not a hormone. It is the food for hormone production in the body, and because of its effect on DHEA, it affects the production of all hormones, not only estrogen and progesterone. Products made from the ovaries of cows may help a woman who still has her ovaries to raise her own progesterone levels. Other alternatives are creams containing plant-derived estrogens and progesterones.

Dr. Julian Whitaker, medical director of the Wellness Institute in Newport Beach, California, also points to the importance of progesterone in treating menopause: "Estrogen slows down the leaching of calcium from the bone, but does not facilitate deposition of calcium in the bone to strengthen it. Progesterone does that, and given by itself, will not only prevent osteoporosis, but will even reverse it."

Like Dr. Williams, Dr. Whitaker recommends natural, topical hormone creams for both progesterone and estrogen supplements. He cites the work of Dr. John Lee, who treated a group of 100 patients over six years with transdermal ("through the skin") natural progesterone only. Lee's patients experienced no significant side effects while enjoying increased bone density and strength.