Here’s How Many Microbes You Eat in a Single Day

Bad news, hypochondriacs: Your food is teeming with life. That's right, not only does your McDonald's Big Mac, large fries and coke contain 1,174 calories, the meal harbors over 230,000 living, breathing bacteria, as well as a couple thousand mold and yeast. And that's just the tip of the iceberg. If you eat a USDA-recommended diet, you're ingesting upwards of a billion hapless microbes every single day.

These are the findings of researchers at University of California, Davis, who, for the first time, tried to pin down the number and types of microbes present not just in individual foods, but in an average American diet. Their st​udy on the microbes we eat appeared last week in the journal Peer-J.

"Up until now, the question of whether the microbes we eat impact our health has been largely ignored, except for the case of overt pathogens on the one hand and probiotics on the other hand," lead study author Angela Zivkovic told me.

But to learn how the everyday microbes we eat impact our health, we first need to answer a more basic question: How many microbes do we actually eat on the daily?

To find out, the researchers prepared or purchased a day's worth of meals and snacks that represented three different "typical" American diets: A processed food-only diet, a USDA-recommended diet and a vegan diet. The processed food diet included items from Starbucks, McDonald's, and Stouffer's frozen foods, while the USDA diet contained a variety of fruits, vegetables, animal products and whole grains. The vegan diet included a protein shake, a tofu soup and a portabella burger. Each meal was pulverized in a blender, and the resultant food-goop used to inoculate microbial cultures.

The USDA-recommended meal plan contained the most microbes, roughly 1.3 billion. The vegan diet harbored 6 million critters, while the fast-food diet contained a mere 1.4 million. In most cases, the vast majority of these organisms were bacteria, although certain foods, including the USDA lunch (a roast turkey sandwich) contained a fair shake of yeast, and a bowl of Kashi cereal with milk and raspberries harbored an unsavory 1.5 million mold. These estimates probably low-ball the actual numbers, because culture methods miss a substantial fraction of microbes that can't be grown in the lab.

It's important to take the study's findings in context. The three diets contained only a limited number of different foods. The USDA diet, for instance, included microbe-laden yogurt and cottage cheese, which by and large accounted for its high microbial count. Had the vegan or processed food meal plan also included a fermented food, the numbers might have looked pretty different.

Still, the authors' findings are enough to give one pause. I, for one, never bothered to consider the lives of the half million microbes in my mocha frappuccino, nor the consumption of a small city's worth of bacteria in a fistful of hazelnuts. Now, I can't help but wonder what sort of an impact these little guys are having on me.

"No one knows, but we are very eager to find answers," says Zivkovic.

The microbes we eat may influence our health by integrating into our gut microbiome—the community of organisms that helps break down our food and ward off pathogens. But study co-author Jonathan Eisen cautions that we can't directly relate the number of microbes in a meal to an impact on our gut fauna. That's because the vast majority of critters we eat suffer a swift and brutal death upon contact with our stomach acid.

"Most microbes don't make it through the stomach, and of those that do, many won't grow well in the gut," Eisen tells me.

Instead, health impacts of the microbes we eat will be determined by a combination of factors, including how many, who they are, and how they interact with our established microbiota.

"This paper will hopefully convince the powers that be that the next study, actually looking at how different foods or diets with differing microbes affect people, should be funded soon," Zivkovic told me.

For now, it's probably worth making peace with the inevitable: Our food, like our own fleshy bodies, is really just a vessel for vast ecosystems we neither see nor control.