Scientists made the startling assertion this week that RNA from our food can survive digestion, sneak into our cells, and control our genes. Tiny molecular messengers made inside other species--even other kingdoms of species--work just fine in our bodies, latching onto our genetic material and causing system-wide change. Our understanding of diet and nutrition may be in for a shake-up.
A group of researchers in China has been studying microRNAs (abbreviated miRNAs). These stunted nucleotide chains, instead of carrying genetic material themselves, regulate how genes are expressed. MicroRNAs bind to our genes and affect their activity, usually dialing it down. Unlike non-micro DNA or RNA, miRNAs can leave the cell and circulate through the body with the blood.
Looking at a group of healthy Chinese men and women, the researchers identified about 30 miRNAs circulating in their blood that weren't human, or even mammalian, in origin: They came from plants. A structural difference confirmed that the tiny molecules were made in plants, and weren't just animal mimics. Checking the blood of other mammals including mice, horses, and cows, the researchers found more plant miRNAs.
Since a couple of the plant miRNAs that cropped up most often in subjects' bodies are also present in rice, the researchers guessed they were coming from the rice the subjects ate. They used mice to test their theory. The rice miRNA was already present in laboratory mouse chow--explaining its presence in mouse circulatory systems--but there's much more of it in rice. So the scientists fed fresh rice to mice and measured the miRNAs in their systems several hours afterward. Sure enough, higher levels of rice miRNAs appeared throughout the digestive systems of the mice.
The miRNAs seemed to be able to survive both cooking and digestion. Researchers tried leaving them in acid for six hours, simulating the effect of stomach acid, but the miRNAs still didn't break down.
Once intact plant miRNAs left the digestive system and passed into body tissues, were they doing anything? Looking at human cells, the researchers found that the miRNAs from rice latched onto a certain gene that's active in the liver. The gene is responsible for removing LDL ("bad cholesterol") from the circulatory system. When the miRNAs from rice bound to the gene, it was less active. In mice that had eaten rice, the same liver gene was less active--and a few days later, the mice had higher levels of LDL cholesterol in their systems. The liver genes had been dialed down by plant miRNAs, and circulating cholesterol had risen as a result.
The authors think cells in the small intestine take up miRNAs from our digestive tracts, package them into little bubbles called microvesicles, and send them into our circulatory systems. From there, miRNAs find the tissues and cell types they fit best with.
What's incredible is that RNA molecules from an entirely different kingdom of life can affect our genes. The last time we shared a common ancestor with a rice plant, it was single-celled. Almost nothing about us is the same. But their keys still fit in our locks. Plant miRNAs may do a different job in our bodies than in the plants they come from, but we've been evolving with these visitors all along. Our bodies must expect them, and even need them, to enter with our food.
If miRNAs from plants can function in our body, then any and every other food source could be passing us miRNAs that tweak the activity of our genes. "Food-derived miRNAs may serve as a novel essential nutrient," the authors say, as important to our diet as vitamins and minerals. MicroRNAs could be added to foods as fortification. Illnesses could be tied to miRNA deficiencies in our diets. We could take Flintstones chewable miRNAs to stay healthy.
MicroRNAs seem nearly indestructible--they apparently handle being cooked and digested with no problem. But it's possible that our treatment of certain foods destroys their miRNAs. When we eat highly processed foods, are we depriving our bodies of nutrients we never knew existed? And as genetically modified crops become more ubiquitous, we'll want to consider whether we're modifying those crops' miRNAs as well, and how those changes might help or harm us. Staple crops such as rice and corn aren't just foods on our plates; they're also old acquaintances that share responsibility for regulating our genes. Whatever we do to our food, it'll be best if we can still recognize each other.
Zhang, L., Hou, D., Chen, X., Li, D., Zhu, L., Zhang, Y., Li, J., Bian, Z., Liang, X., Cai, X., Yin, Y., Wang, C., Zhang, T., Zhu, D., Zhang, D., Xu, J., Chen, Q., Ba, Y., Liu, J., Wang, Q., Chen, J., Wang, J., Wang, M., Zhang, Q., Zhang, J., Zen, K., & Zhang, C. (2011). Exogenous plant MIR168a specifically targets mammalian LDLRAP1: evidence of cross-kingdom regulation by microRNA Cell Research DOI: 10.1038/cr.2011.158