Gut Check: Sugars and the Gut Microbiome

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Welcome to Gut Check, our running series where we explore how the food we eat is connected to our gut microbiome. We recently took a look at how low-calorie sweeteners impact the microbes living in our gut (I’ll summarize here: we still don’t know if, or how, they do). In this article we’re taking a closer look at simple and added sugars and the (limited) research on their connection to our microbial communities.

Although you’d never know it from the headlines, eating sugar can be part of a healthy diet. And that’s because sugar isn’t just in candy bars and doughnuts—it’s found naturally in fruits, grains, and dairy products, and even some vegetables. Sugars belong to the group of foods known as carbohydrates, one of the three major macronutrients that we need to sustain life (the other two being fat and protein).

When we think of sugar, it tends not to be the sugars found naturally in whole foods. Instead, what typically comes to mind is table sugar, known in science-y terms as sucrose. Sucrose is made up of two sugar molecules, glucose and fructose. We use sucrose in baking, to sweeten our coffee, preserve jams and jellies, and to fuel the yeast that helps us create a loaf of homemade bread.

Sucrose, glucose and fructose are classified as simple sugars because they’re not linked in long chains like starches and fiber are. Added sugars are the sugars and syrups that are put into foods during preparation or processing, or added at the table (like when you sprinkle brown sugar on your oatmeal). Since added sugars are usually simple sugars, we’ll lump them together just for today.

Unhealthy diets, including those with too many calories from added sugars, are associated with an increased risk for conditions like obesity, type 2 diabetes and cardiovascular disease. Sugars recently also have been subject to claims that they can “disrupt the gut” or give “bad bacteria” the fuel they need to proliferate. A quick Google search can lead to any number of blog posts on this topic, and it may be tempting to agree with these statements, given the heat that sugar’s been taking lately.

But does the evidence fit the crime? In this case, no.

The first conflict comes in thinking about how sugars are taken up from our digestive tract. Simple sugars are digested by enzymes in the small intestine, and to a small extent in the mouth right after we eat them. Then they’re immediately absorbed into the bloodstream. It’s unlikely that any more than the tiniest amount of sugar is making it to the very end of the digestive tract, where most of our gut microbes live. Our gut bacteria are much more partial to complex carbohydrates like fiber, which they use as a food source.

The real issue at play here is that, more often than not, people who eat a diet too high in added sugar might not be getting enough fiber—regularly eating white bread instead of whole grain, apple juice instead of an apple or candy instead of a carrot. A low-fiber diet can deprive our beneficial gut microbes of the food they need to flourish, which could offset the bacterial balance and reduce its diversity.

Research in humans on the topic of simple sugars and the gut microbiome remains sparse. Randomized controlled trials, the gold standard of research studies, have yet to appreciably venture into this territory.

Observational studies, which examine the relationship between an exposure (such as sugar intake) and an outcome (such as microbiome composition), are not able to provide evidence of cause and effect, and they can’t control for all of the other exposures or factors that could be causing or influencing the results.

Many of the studies that have been done on diet and the microbiome look at the overall Western dietary pattern (aka high-fat, high-sugar and low-fiber), which makes it impossible to separate out the specific effects of sugar.

As you can see, there’s a lot of confounding and confusing evidence at this point and no clear understanding of how simple sugars (natural or added) impact the gut microbiome, or if our health is affected as a result. Since most people make different food choices every day, and because no two gut microbiomes are the same, it’s challenging to make broad statements about how one specific nutrient affects gut health.

This is a developing area in the study of personalized nutrition, but that’s a whole separate article (that you can read here!). Until we have a clearer picture, we should be wary of these claims.

The reality is, we’re all bound to come across a sugary treat now and then that we just can’t resist (yes, even us dietitians). And that’s okay! As long as we’re aiming to follow a healthy eating pattern most of the time, which includes fiber-rich foods like fruits, vegetables and whole grains, we’re feeding both ourselves and our microbiome well.