By David Kiefer, MD, Editor

Clinical Assistant Professor, Department of Family Medicine, University of Wisconsin; Clinical Assistant Professor of Medicine, Arizona Center for Integrative Medicine, University of Arizona, Tucson

Dr. Kiefer reports no financial relationships relevant to this field of study.

SYNOPSIS: Seven spices were found to contain numerous antioxidant phytochemicals and have an overall, but variable, prebiotic effect on 88 known species of microbiome bacteria.

SOURCE: Lu QY, et al. Prebiotic potential and chemical composition of seven culinary spice extracts. J Food Sci 2017;82:1807-1813.

The recent scientific literature has highlighted the physiological effect of spices focusing on their antioxidant and anti-inflammatory effects. This review, alternatively, ties spices into the burgeoning field of the microbiome, using a prebiotic mechanism as the connection. The researchers tested seven spices (black pepper, cayenne pepper, cinnamon, ginger, Mediterranean oregano, rosemary, and turmeric) and documented their chemical composition, antioxidant effect, and possible prebiotic effects. The authors defined prebiotics as “…substances that induce the growth or activity of microorganisms that contribute to the well-being of their host.”

The researchers extracted each of the plants in hot water (presumably, at least partly replicating a culinary use) and tested those extracts on 88 species of bacteria found in the large intestine (representing the microbiome) to determine the Minimum Inhibitory Concentration (MIC). The MIC was the lowest concentration found to result in no growth or a “marked change” in the appearance of growth as compare to control agar growth plates. The “marked change” could indicate growth stimulation, which they would document. They found that all plants, except turmeric, promoted the growth of Bifidobacteria and Lactobacillus species (“good” bacteria), whereas all seven plant extracts inhibited the growth of Ruminococcus species (a “bad” bacteria). There were variable effects on two other “toxigenic” genera, Clostridia and Fusobacteria. (See Table 1.) In the paper, large tables with MIC details are presented, the accuracy of which is to be perhaps doubted; the abstract states that cinnamon, oregano, and rosemary inhibit Fusobacteria, whereas the text states that cinnamon, ginger, and oregano have inhibitory effects (their Table 3 supports the abstract). Surely, it is difficult to keep track of the effect of seven plants of varying concentrations on 88 bacteria.

There are a multitude of antioxidant tests available, but in this study, the researchers used the Trolox equivalent antioxidant capacity (TEAC), for reasons that were not documented. Of the seven plants, rosemary and oregano were found to have the highest antioxidant effect by this assay, and ginger the lowest, ranging from 140.4 to 13.1 millimolar trolox equivalents. The researchers also found that the antioxidant effects correlated directly with the total chemical composition. On this note, the researchers quantified the total concentration of phenolic and other compounds in each of the plant samples. Phenolics are a well-known family of compounds thought to have physiological, and resulting clinical, effects. The specific compounds were identified using high-performance liquid chromatography methods; 14 major phenolics were found in cayenne pepper, ginger, oregano, rosemary, and turmeric, while piperine, an alkaloid, was found in black pepper, and cinnamic acid and cinnamaldehyde were found, not surprisingly, in cinnamon.

All told, these results are interesting by corroborating recent advancements in the knowledge that diet affects the microbiome, but taking it one more step. Prior to this, most dietary prebiotic effects were thought to be due to specific fiber and carbohydrate-containing foods; the fact that spices, plants used in small amounts to flavor or preserve food, also could have that effect is novel and potentially important.

What is unclear from this study is dose. Do the concentrations of the extracts used correspond to clinically significant changes in the diet with spice-concentration ingestion? Also, we know that some, mostly hydrophobic, compounds are not particularly well dissolved in aqueous solutions, and so may not be appropriately analyzed in this study. Furthermore, we don’t always cook with a water base; thinking about oil-based foods and cooking methods, the next analysis might examine how those spices affect the microbiome when ingested in this way. We also might take issue with the antioxidant testing; it provides little new information and may not be accurate. There is controversy about the use of any antioxidant testing, so much so that a major scientific journal recently decided to not publish any antioxidant analyses.¹ The antioxidant conclusions drawn by the authors probably should be taken with a grain of salt (pun intended). Finally, we should keep the researchers to task with their botanical identification; identification to species allows us to know exactly which plants were used in this study, but this information wasn’t provided. There are many different culinary oregano species used, and at least two cinnamon species available in the marketplace; the next iteration of this work should provide the Latin scientific names of these to be complete.

The microbiome effects of diet, lifestyle, pharmaceuticals, and countless other variables, are complex, making it difficult to know exactly what to tell patients in clinic. This paper seems to indicate an overall positive physiological effect of spices, at least in the laboratory. The next steps will be to determine the exact microbiome effects on individuals when those plants are ingested as spices in the short and long term. We may learn in the future that certain patients, with a certain microbiome environment, would benefit from the use of specific spices in specific doses. Until that scientific guidance arrives, there seems to be little harm in weaving in these plants, as spices, to our diet.

REFERENCE

1. Harnly J. Antioxidant methods. J Food Comp Analysis 2017;64:145-146.