Are the health benefits of turmeric too good to be true?

"There are plenty of studies currently being done but already good evidence that turmeric can help control knee pain from arthritis as well as decrease the likelihood of a heart attack after bypass surgery," said Dr. Sanjay Gupta, CNN's chief medical correspondent.

Turmeric is one of many plants used in ayurveda, a traditional South Asian system of medicine, according to the National Institutes of Health. It is used to treat issues such as breathing problems, rheumatism, fatigue and pain.

"There's a distinction that's very important to make between turmeric, which is the plant and the spice, and what people often study, which is the curcumin, which is the proposed active constituent in turmeric," Hopp said. "And even curcumin, as it's usually sold or researched, is not a single compound. It's usually a collection of three or four compounds that are called curcuminoids, collectively."

The exact amount varies, but the turmeric root contains up to 5% of these curcuminoids typically, the National Institutes of Health says.

Extracting the curcumin and translating its power into a successful treatment is still a major challenge for researchers, experts say. There is epidemiologic evidence that people who eat a diet rich in turmeric can potentially attribute their substantial health benefits to the spice, Hopp said, citing a lower incidence of colon cancer in the Indian subcontinent.

"But it's very difficult to sort of project what you see in terms of an activity in a cell to what's going to actually happen in the human," he added. "There's a sort of a disconnect between what appears to be a lot of very promising activity in vitro, which is just in the cells. And contrast that with where it's been studied in clinical trials as humans, where there's been virtually no evidence of benefits."

One reason for that disconnect is that apart from turmeric, curcumin has biological properties that make it poorly bioavailable: It is rapidly metabolized and excreted, and very little of it gets absorbed into the body. The chemical doesn't make it to the places where it could be of help.

The context in which turmeric is traditionally used is important as well, Hopp said. Black pepper is often found alongside turmeric. Piperine, the substance that gives pepper its bite, increases curcumin's bioavailability.

"It keeps the door open," Hopp said. "As things go into and out of cells, piperine is sort of like a doorstop that allows things to go in and out of the cells much more readily."

Curcumin also does not dissolve easily, and much of it does not enter the bloodstream, said researchers in a study investigating the effects of turmeric in treating glaucoma, recently published in the journal Scientific Reports. One would need to take as many as 24 500-milligram tablets of curcumin a day to get an effective dose, increasing the risk of gastrointestinal side effects like vomiting and diarrhea.

"If you think about it, in a curry, there's only 700 milligrams of turmeric," said Dr. Francesca Cordeiro, professor of ophthalmology at Imperial College London and one of the authors of the study. "You'd need to eat 200 curries a day to get that therapeutic level."

Instead of choosing capsules or cuisine as their method of treatment delivery, Cordeiro's team used eye drops infused with a stabilizer that increased the curcumin's solubility on their rat subjects twice a day.

"We used nanotechnology," Cordeiro said. "The advantage of it being so small is it can cross into the eye as an eye drop into the back of the eye. Once it enters, it can affect the nerve cells there, and that direct effect can lead to them not dying. It's what we call neuroprotection."

Three weeks later, the untreated control group had a 23% reduction in retinal cells compared with the eye drop group. This loss was prevented by treatment with curcumin, Cordeiro explained.

The researchers' next steps include clinical trials and exploring the possibility of using the retina as a "window to the brain" by developing the drops into a diagnostic resource for Alzheimer's disease.

"Curcumin is fluorescent," Cordeiro said. "If you put the correct wavelength to it, it fluoresces, and it binds to the parts that are implicated in Alzheimer's, the beta amyloid plaque," one of the substances in the brain that is a hallmark of the condition.

Even if the risks of taking turmeric as a supplement appear to be limited, Hopp recommends discussing such treatments with a doctor, "especially if they're taking other medications, so that the doctor has a full picture of what the patient is consuming and can manage that care properly," he said.