Have you heard the old saying about some sperm being “strong swimmers”?

Well, researchers from the University of California, Berkeley, have been trying to find a way to prevent that strong final “kick” in order to produce a new form of birth control.

A study published in the May 30 issue of the Proceedings of the National Academy of Sciences concluded that a pair of plant-derived chemicals could potentially be used to block this biological mechanism.

The calcium channel of sperm, called CatSper, is activated by the hormone progesterone when sperm get close to an egg.

However, researchers Nadja Mannowetz, PhD; Melissa R. Miller, PhD; and Polina V. Lishko, PhD, of the Cal Berkeley Department of Molecular and Cell Biology, found that the plant-based triterpenoids pristimerin and lupeol — sometimes found in contraceptive folk medicines — can prevent fertilization.

“Because these two plant compounds block fertilization at very, very low concentrations — about 10 times lower than levels of levonorgestrel in [the emergency contraceptive] Plan B — they could be a new generation of emergency contraceptive we nicknamed ‘molecular condoms,’” said Lishko, the lead study researcher, and an assistant professor of molecular and cell biology, in a press statement.

“This could be something really exciting,” added Mannowetz. “If you think of the classic birth control pill, you’re loading your body up with extra hormones. I’m not sure that’s a good thing to be doing over decades and decades, so this would be a big advantage.”

Read more: Understanding the sperm ‘power kick’ »

Inhibiting sperm naturally

The researchers initially found that the steroids testosterone and hydrocortisone impede the release of progesterone and activate CatSper.

That could help explain why elevated levels of these steroids (sometimes caused by stress) decrease female fertility.

It was this discovery that led the researchers to investigate other compounds that might be used to inhibit the motility of sperm.

The book “Anti-Fertility Plants of the Pacific,” provided the inspiration to look at plants like Tripterygium wilfordii, the “thunder god vine” used in Chinese folk medicine that contains pristimerin, as well as lupeol, which is found in mango, dandelion root, and grapes.

“A study of Chinese men who took a thunder god vine extract for arthritis interestingly found that they also developed reversible infertility,” noted Mannowetz.

The study found that both pristimerin and lupeol prevented progesterone from binding to a protein called ADHD2, which opens the CatSper calcium channel.

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Eliminating side effects

Genetic research conducted in the early 2000s first showed that blocking the action of CatSper on sperm could cause infertility in male mice.

The biggest advantage of a contraceptive based on pristimerin or lupeol is that it would be narrowly focused on CatSper — which acts only on sperm cells — and is not steroid- or hormone-based.

That eliminates side effects like blood clots associated with “the pill,” says Mannowetz.

“Targeting CatSpur is a most exacting way to prevent conception,” she says. “Plan B is a 100 percent synthetic compound, and the body has a great deal of difficulty breaking it down, whereas a plant-based contraceptive can be broken down very easily.”

Both of the plant-based triterpenoids were effective at low doses.

Researchers are looking at whether pristimerin and lupeol extracts can be synthesized affordably for use as contraceptives.

Mannowetz says that the downside of the folk medicines she studied is that, “If you’re taking a non-purified plant extract, you’re not only getting the compound you want but a lot of other toxic compounds, too.”

Read more: Will men ever embrace male birth control? »

Lots of questions for future studies

The Cal Berkeley researchers studied the effects of pristimerin and lupeol on individual sperm cells.

Follow-up primate studies will be needed to see if they prevent in vitro fertilization, confirm there are no dangerous side effects, and determine the best delivery method for the contraceptive — and which sex should receive it.

Although pristimerin and lupeol act upon sperm, it’s unclear whether an oral contraceptive for men would work as opposed to a pill, transdermal patch, or vaginal ring for women.

Researchers also need to determine whether the plant extracts could work as a long-term contraceptive or only as an emergency preventative taken immediately before or after intercourse.

“This method of action is at a later stage, which means it could work much faster than other new male contraceptive approaches,” Aaron Hamlin, MPH, executive director of the Male Contraception Initiative, told Healthline. “For instance, trying to lower sperm count — whether by a hormonal or non-hormonal approach — takes a number of months and comes with other issues. You don't have that obstacle here, and that's a good thing.”