In medieval Europe, rosemary was much beloved, signifying friendship and remembrance. Modern science shows that this reputation has some substance—this flavorful culinary herb contains phytochemicals that have potential for treating and preventing Alzheimer’s disease and related dementias.
The mechanics of memory
Research shows that rosemary affects a neurotransmitter in the brain called acetylcholine that transmits nerve impulse signals from one neuron to another. Acetylcholine is crucial in the areas of the brain where memories are created and stored. If nerves and neurons are a sort of living wiring system, acetylcholine and other neurotransmitters are “switches” that turn on to make a connection between two nerve “wires,” then turn off after the connection is made and the information has been correctly transmitted.
After acetylcholine transmits a nerve signal, it’s broken down rapidly by an enzyme called acetylcholinesterase. This cycle of breaking down and rebuilding acetylcholine prevents the neuron from “shorting out” after receiving a signal. If the acetylcholine switch stays on, the brain can’t tell where one signal ends and the next begins.
In the case of Alzheimer’s, it’s thought that acetylcholine levels are either too low or that acetylcholine is broken down too fast, so some nerve impulses carrying memory or learning information can’t be fully transmitted. In either case, slowing or inhibiting the action of acetylcholinesterase may help improve memory, and researchers are on the lookout for compounds that do this safely. Drugs with this action are among the few that have proven successful in Alzheimer’s treatment. Rosemary was one of the first plants identified as having the same inhibiting power.
Rosemary is not the only herb with this talent. In fact, most of rosemary’s cousins also inhibit acetylcholinesterase. Rosemary (Rosmarinus officinalis) belongs to the mint family and is related to peppermint, spearmint, and pennyroyal, although these mints are from the genus Mentha. A 1998 Japanese study tested essential oils from thirty-one Mentha species and varieties and found that most inhibited acetylcholinesterase, with the most effective being water mint (M. aquatica). Although various phytochemicals in the mints’ oils were tested individually, the whole essential oils were more effective, indicating that components in the oils act synergistically.
Maybe this follows the common sense of our senses: Doesn’t smelling or tasting these herbs pick you up? The appeal of mints and rosemary may be deeper than emotional or food-oriented attachments—it may extend all the way down to the level of brain chemistry.
There’s still research to do, though, because the chemicals in the mint family that inhibit acetylcholinesterase haven’t yet been shown to cross the blood-brain barrier, a protective “gate” or “filter” system that screens out many chemicals. In other words, they show potential in a petri dish, but we haven’t yet proven they work in the body.
Other memory minders
Rosemary and other mints also may help prevent dementia or poor memory because they are potent antioxidants. Studies have shown that higher blood levels of antioxidants from vitamins or plants are associated with better cognition in older people.
The most powerful antioxidant in rosemary is carnosic acid. A 1998 Spanish study of carnosic acid in rosemary varieties found that the antioxidant was concentrated in the young growing leaves, with less in older leaves and hardly any in old woody stems. Levels also peaked during the summer. These patterns show that the plant maximizes its antioxidant protection in young, vulnerable, fast-growing leaves. The highest levels were seen in summer because the plants were using antioxidants to protect themselves from sun damage. In autumn, an abrupt drop in carnosic acid begins with leaf fall, and low levels are maintained throughout winter.
Maybe there’s a lesson here—aging plants have less antioxidant protection, and evidently so do aging people. Unlike plants, we can’t regenerate ourselves from cuttings or protect ourselves with dormancy, but what the plants use to protect themselves, we can use too, to retard aging. Hopefully, mint gardens will have a place in nursing homes of the future.
Chen, Q. H. Shi, and C. Ho. “Effects of rosemary extract and major constituents on lipid oxidation and soybean lipoxygenase activity.” Journal of the American Oil Chemists’ Society 1992, 69:999–1002.
Hidalgo, P. J. et al. “Determination of the carnosic acid content in wild and cultivated Rosmarinus officinalis.” Journal of Agricultural and Food Chemistry 1998, 46:2624–2627.
C. Leigh Broadhurst holds a doctorate in geochemistry and is a nutrition consultant in Clovery, Maryland.