Among the people I know, digestive issues are the biggest cause of health complaints. I usually recommend a good-quality probiotic, based on the reams of solid evidence that I've read about in the last three or four years — and my personal positive experience with them.

A quick primer: Probiotics are in your body right now — we are all born with some, build some up over our baby- and childhoods. We consume them when we eat, especially when we eat fermented foods, and some may come from our local environments. Some people call them "good bacteria" and they are a normal, healthy, and important part of a healthy digestive system — they help you digest your food more effectively and also help fight bad bacteria that can make you sick. We've only known about the existence of these organisms for about a hundred years, but they have been with human beings since our earliest beginnings. Besides being key to balanced healthy bodies, they also convert some of our favorite foods to longer-lasting forms; milk to yogurt, cucumbers to pickles, cabbage to kimchi, to name just a few of my own favorite foods.

So if they occur naturally in some of our tastiest treats, why do we see them sold in supplement form or added to foods? With the introduction of modern agriculture and food preservation (especially pasteurization and other types of sterilization), we got rid of many of the legitimately problematic diseases that haunted foods of the past that made people sick — and sometimes even killed them. A great thing, to be sure. But, in a case of throwing the baby out with the bathwater, we also eliminated many of the really good microorganisms that are not only important for digestive health, but a host of other physical effects as well. So many people in the modern era are without some of the probiotics that make for optimum health.

According to the American Gastroentological Association, this lack of probiotics effects digestive health — and may have further reaching implications.

"If this balance is disturbed, it might result in a number of disorders, including functional bowel disorders, inflammatory bowel diseases and other immune mediated diseases, such as celiac disease and certain allergies. Also, metabolic conditions, such as type 2 diabetes, and perhaps even behavioral disorders, such as autism and depression, can be linked to gut microbial imbalances," says microbiota expert and professor Francisco Guarner of the University Hospital Valld'Hebron, Barcelona, Spain.

This is because much of our immune function is tied to digestive balance; in short, when our gut is healthy and populated with lots of good bacteria, our bodies are stronger and more able to kick out the bad, including viruses. As Guarner elaborates, "The mechanisms underlying the beneficial outcome of probiotics are becoming increasingly clear. Through different molecules, probiotics interact with the host via various mechanisms and pathways. Some probiotics, for example, can hold pathogens at bay: by improving the intestinal barrier function, they defend the host against disease-causing microorganisms trying to invade."

While taking antibiotics can disturb gut health (they kill good and bad bacteria alike), avoiding them when they are needed isn't always a good idea — though making sure taking antibiotics are truly necessary is always a good idea. But it is also the choice of what to eat on a regular basis that has an impact on the number and variety of beneficial gut organisms (it's healthy to have more than 400 different kinds of good bacteria, and most of us have far fewer than we should). Regularly consuming whole grains, fruits, vegetables and fermented foods like kimchi, pickled veggies, kombucha, and dairy like kefir and yogurt (look at the label to see what kinds of organisms it contains, then choose the one with the most variety and number) is a delicious place to start. Avoiding greasy and fried foods, as well as those with high levels of animal fat — gut bacteria don't thrive on a diet with too many fries-and-burgers combos — will make your digestion easier and give good bacteria a chance to thrive.

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Starre Vartan ( @ecochickie ) covers conscious consumption, health and science as she travels the world exploring new cultures and ideas.

Probiotics improve digestive health—and more
Gut health (or lack of it) can influence almost every function in your body, including the immune system.