How does fiber reduce blood sugar spikes?

When you eat foods that contain digestible carbs — like fructose in fruit or starch in pasta — your gut breaks these carbs down into sugar, also known as glucose.

At the same time, your pancreas releases insulin, a hormone that encourages your cells to take in glucose.

As a result, the level of sugar in your blood goes down.

It’s personal

ZOE’s own research has found that blood sugar responses can vary significantly from person to person.

This means that people can have wildly different blood sugar responses even after eating the same meal. 

If more erratic blood sugar patterns last for months and years, they can increase the risk of health conditions, such as cardiovascular disease. 

So, for people with more pronounced responses, understanding how to minimize these might benefit their long-term health. 

Why fiber matters

Fibers are carbs — they’re chains of single sugars. But unlike digestible carbs, your body can’t break fibers down.

Your enzymes can’t digest fiber, so it travels down into your lower intestines, where the bulk of your gut bacteria live. 

So, having a fiber-fueled diet helps keep your gut healthy. It also lowers cholesterol, protects your heart, and has a slew of other benefits.

Plus, fiber can help improve your blood sugar responses to food, and that’s what we’ll focus on today. So, how does fiber do it?

Fiber and blood sugar

Fiber comes in many forms, and they all have different characteristics. For instance, some fibers dissolve readily, some form a gel. Some are easy for gut microbes to break down, and others not so much.

These characteristics determine how these fibers work in your body and what they do when they reach your gut. 

For simplicity's sake, experts sometimes split fiber into two types: insoluble and soluble. Generally speaking, insoluble fiber doesn’t dissolve well in water, and soluble fiber does. 

When soluble fiber dissolves in water, it forms a gel-like substance in your stomach. This makes your stomach contents more viscous — thicker and less runny. 

And this, it seems, is the source of fiber’s blood sugar-reducing powers.

The importance of viscosity

For decades, scientists have understood that the relationship between soluble fiber and viscosity has an important effect on blood sugar responses.

A study from 1978 is a good example. The researchers gave participants 50 grams of glucose and different forms of fiber.

One of these fibers was guar gum, which readily forms a viscous gel. 

However, when the scientists chemically changed the guar gum so it didn’t form a gel, it no longer reduced participants’ blood sugar responses.

So, viscosity is essential. But why?

Why viscosity matters

When your stomach contents are more viscous, they stay in your stomach for longer. And they move into your small intestine more slowly. This is where most of the nutrient absorption happens.

Plus, evidence from lab studies suggests that when your gut contents are more viscous, digestible carbs (sugars and starches) break down into glucose at a slower rate. 

And because carbs can’t pass into your blood until your body breaks them down into glucose, your blood glucose levels also rise more gradually.

Viscous fibers include beta-glucans from oats and pectin in citrus fruits. We’ll direct you to more examples of foods that contain these fibers later on.

But first, why does increased viscosity mean that carbs are broken down more slowly? 

But if your gut contents are less fluid, enzymes find it harder to mix in, so they break carbs down more slowly.

It might help to picture the difference between stirring milk into coffee and stirring it into something thicker, like dough.

Meanwhile, viscosity also has effects further along your food’s journey. 

The ileal break

Because viscosity prevents digestible carbs from being broken down early on, these carbs can travel farther into your bowel.

Sugar and other macronutrients, like fat and protein, don’t make it as far so often. 

This effect is driven by the release of two hormones: glucagon-like peptide-1 (GLP-1) and peptide YY.

When your body releases these hormones, it makes your stomach empty more slowly and causes food to move through your intestines less rapidly. 

It doesn’t affect how many nutrients you absorb, but it slows the process so that glucose reaches your blood at a more leisurely pace.

More about GLP-1

As we’ve seen, when macronutrients — like carbs — reach your distal ileum, it triggers the release of the hormone GLP-1 into your blood.

One of GLP-1’s effects is to release insulin. And as we mentioned earlier, insulin helps your cells take up glucose, which reduces the amount of glucose in your blood. 

Also, GLP-1 decreases glucagon secretion. Glucagon is like the opposite of insulin. 

While insulin helps your body move glucose out of your bloodstream for storage, glucagon helps release stored glucose for use as energy when you need it. 

So, decreasing glucagon secretion also reduces the amount of glucose in your blood.

According to the authors of a review, “Improved glycemic control is proportionate to the viscosity of a hydrated fiber.” 

In other words, types of fiber that are best at increasing viscosity have the greatest effect on your blood sugar response. 

The second-meal effect

The effects we’ve covered so far are short term and happen relatively soon after you eat.

This means: If you had a high-fiber breakfast, your blood sugar response to lunch may also be reduced. In other words, the effect of fiber lasts all the way to your next meal. 

How does that work? Although the research is in its infancy, scientists do have some theories.

As you may recall, fiber reaches your large intestine without being digested.

Once it's there, gut bacteria chow down on it. As they ferment this fiber, some species of “good” bacteria produce short-chain fatty acids (SCFAs).

For instance, certain SCFAs can bind to receptors in your intestines and trigger the release of GLP-1 and peptide YY.

And we’ve seen what these hormones can do — their combined activity helps reduce your blood sugar response.

Fiber takes some time to get to your large intestine, where most of your gut bacteria live, and fermentation is a relatively slow process, so the release of SCFAs continues until your next meal. 

What should you do?

Experts agree that most people in the Western world don’t eat enough fiber. For example, in the United States, only around 5% of people eat the recommended 25–34 g of fiber daily. 

And in the United Kingdom, the recommended fiber intake is 30 g a day, but on average, people only eat around 20 g.

Above, we’ve focused on soluble, viscous fiber and its ability to reduce your blood sugar response. But all forms of fiber support your gut health and overall health.

For more information about fiber and where to find it, we have a handy list of high-fiber foods to add to your diet.

Finally, if you'd like to know more about your internal workings, we can help. When you join ZOE, we show you how your blood sugar and blood fat levels respond to food.

We also analyze your gut microbiome and provide ongoing nutrition support tailored to your body. Start by taking our free quiz.