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There are countless dietary supplements and herbs out there with countless clinical claims, but often little in the way of plausibility or supporting clinical evidence. The latest “mole” to pops its head up in this endless game to whack them is black cumin (also referred to as black seed, black caraway, black sesame, and other common names). These are the seeds of the Nigella sativa plant, a traditional medicinal herb common in the Middle East and parts of Asia. According to one review:

It is very popular in various traditional systems of medicine like Unani and Tibb, Ayurveda and Siddha…In Islamic literature, it is considered as one of the greatest forms of healing medicine.

The same reference indicates:

…a wide spectrum of its pharmacological actions have been explored which may include antidiabetic, anticancer, immunomodulator, analgesic, antimicrobial, anti-inflammatory, spasmolytic, bronchodilator, hepato-protective, renal protective, gastro-protective, antioxidant properties, etc.

The claims made for black cumin follow many of the themes and tropes common to popular herbal products. Most significantly, black cumin is alleged to treat or cure an implausibly wide range of unrelated conditions. It’s almost as if it works for everything, which is a very good reason to suspect that it more likely works for nothing. Many of the conditions have completely different mechanisms – why would one product treat asthma and diabetes, cancer and liver disease?

Further, many of the conditions listed are contradictory. It does not make sense that one substance would simultaneously be antimicrobial and anti-inflammatory. The anti-inflammatory treatments that work have as a potential side effect reducing immune activity against infections. It is theoretically possible to have direct anti-microbial effects and be anti-inflammatory, but this gets back to ascribing numerous pharmacological properties to one product. Further, proponents claim that black cumin is effective against bacteria, viruses, fungi, and parasites – which would make it the only substance in the world with such broad spectrum activity. We can’t even engineer a drug to be effective against all bacteria, let alone also viruses and fungus, which are very different forms of life with different vulnerabilities.

The other blaring contradiction is that black cumin is alleged to be both antioxidant, and anti-cancer. This gets back to the inherent problem with antioxidant hype – the immune system actually uses oxygen free radicals to fight invaders and cancer cells. Further, cancer cells have high metabolism, and so are under increased oxidative stress. It turns out, if anything, antioxidants may be protective of cancer cells, and therefore counterproductive in cancer.

Another problem that plagues herbal products is pharmacokinetics, such as bioavailability – how much of the active ingredient actually gets absorbed and distributed to the target tissue? This is the fatal problem for turmeric, for example. I could not find any studies looking at pharmacokinetics in humans, only studies calling for such research to be done (a good indication that so far it hasn’t been). I did find a study looking at the pharmacokinetics of thymoquinone (thought to be the primary active ingredient in black cumin) in chickens. They found an 85% bioavailability with oral use, which is good, but they also found that the half-life was 1 hour, which is bad.

In order for pharmaceuticals to be medically useful, they need to have a host of properties all at the same time. They need reasonable bioavailability for the route of administration, they need to be chemically stable, both on the shelf and after introduction into the body, they need to get to the target tissue in the body, they need a useful half-life (the time it takes for the body to eliminate half of the blood level), and, of course, useful therapeutic effect at a dose with tolerable side effects and toxicity. It’s also good to know about drug-drug interactions. A really short half-life is one of the deal-killers for useful pharmaceuticals.

This is exactly why active ingredients are identified and studied. Almost always the chemical structure needs to be tweaked in order to achieve all of the desirable characteristics at the same time, with no deal-killers.

Another feature of the claims made for black cumin that is very common is that they are closely tied to a cultural belief. (Sometimes a cultural connection is false, manufactured for marketing purposes, but in this case it appears to be true.) My concern here is that a strong cultural belief is a powerful thumb on the scale of scientific research. This is not a hypothetical concern. It has been established, for example, that 100% of acupuncture studies coming out of China are favorable, a statistically unlikely outcome that is a blaring signal of researcher and publication bias.

Similarly, while I have not done a statistical analysis, in my review of published studies of black cumin or thymoquinone, nearly 100% of studies are both published by Middle Eastern authors and favorable. This would certainly explain why black cumin seems to work for everything. This is also a good rule of thumb – if your process results in 100% positivity, don’t assume you have discovered a miracle, assume your process is flawed.

The aforementioned review says of black cumin, “Due to its miraculous power of healing, N. sativa has got the place among the top ranked evidence based herbal medicines”. That hardly sounds like a sober scientific review, rather more of a point of cultural pride.

Despite all of these concerns, what does the research actually say about the clinical effects of black cumin (or thymoquinone)? Most of the research is pre-clinical, and the claims made are based on unreasonable extrapolation from the petri dish to the living organism. As we have pointed out here multiple times, this is not appropriate. Basic science findings rarely translate directly to clinical outcomes, partly because of all the pharmacokinetic issues I discussed above, but also because whole organisms are complex dynamic homeostatic entities.

The clinical research is mostly preliminary, and again focusing on dozens of unrelated claims. Every single study I found was favorable, but was also not convincing due to methodological limitations. This is also a common feature of fringe treatments that have not garnered mainstream acceptance (for a reason) – weak research methodology with positive findings but generally small effect sizes. One review, for example, notes that there is evidence for some metabolic and anti-inflammatory effects, but “the amount of benefit it confers is too small for it to be considered a good intervention.”

This is a familiar pattern, because even a little bit of researcher bias can manufacture apparent small positive effects. When we combine the low plausibility (especially given the number of claims), the apparent isolation to one culture, the weak methodology with small effect sizes, we see an overall picture consistent with herbal medicine hype, not scientific reality. It is certainly possible that thymoquinone or some other active ingredient may ultimately have useful medical purposes, when properly studied and developed. That remains to be seen. But honestly, by existing in the fringe of alternative/herbal/cultural medicine, it is less likely to actually be developed as effective medicine.

For now, there is no evidence to support black cumin as an herbal product for any indication, and there are good reasons to be skeptical.

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Posted by Steven Novella

Founder and currently Executive Editor of Science-Based Medicine Steven Novella, MD is an academic clinical neurologist at the Yale University School of Medicine. He is also the host and producer of the popular weekly science podcast, The Skeptics’ Guide to the Universe, and the author of the NeuroLogicaBlog, a daily blog that covers news and issues in neuroscience, but also general science, scientific skepticism, philosophy of science, critical thinking, and the intersection of science with the media and society. Dr. Novella also has produced two courses with The Great Courses, and published a book on critical thinking - also called The Skeptics Guide to the Universe.