Fruit, Fiber and Blood Glucose


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As I have been working in the health and nutrition space for a few years now, I have learned many things I had never had exposure to in college. Something we all can agree on, and have been engrained to know from early on, is that fruits and vegetables are some of the healthiest things to include in our diet. Packed with vitamins, minerals, polyphenols, and fiber, you would be wrong to not include them in your diet.

It recently came to my attention that there are a lot of people, often those with type II diabetes, who exclude fruit from their diet in order to prevent unhealthy blood sugar spikes. Considering all the health benefits of fruits, I decided to do some digging into the literature on the effects of fruit consumption – both as whole fruits and in the form of fruit juice on blood glucose levels.

Some Background

Simple carbohydrates or dietary sugars are turned into glucose in the body for cells to use as energy. These carbohydrates are primarily the monosaccharides glucose and fructose, the disaccharide sucrose (consisting of equal amounts of glucose and fructose), and digestible starches, all of which are processed and rapidly absorbed in the small intestine. In order to get these absorbed sugars rapidly into the cell as fuel, the body produces insulin. Any carbohydrates not used up by the body will be converted into glycogen and stored in muscle cells and the liver (approx. 600g will be stored as glycogen – any more will max out your stores and be converted to triglycerides/fat).

However, not all carbohydrates are created equal. Complex carbohydrates composed of a number of sugar molecules cannot be broken down and absorbed in the small intestine, and travel down into the end of the small intestine and into the large intestine, where they are broken down by the gut microbes into short chain fatty acids. They have also been referred to as Microbiome Accessible Carbohydrates or MACs. As opposed to the simple carbohydrates they are not converted into easily absorbable sugars by our small intestine, but into anti-inflammatory, health promoting molecules by our gut microbes.

As we have discussed many times in this blog, short chain fatty acids, in particular butyrate have a range of anti-inflammatory effects in the gut and throughout the body. Whole grains are very different from refined grains in that they have retained their undigestible carbohydrate component, e.g., fiber from lack of processing. Studies have shown that foods containing complex carbohydrates (fiber-rich) slow gastric emptying of food leading to a slower absorption of the associated sugars, maintain a steady blood glucose rather than causing sharp spikes.

Glycemic Index and Glycemic Load

If you are a diabetic or have tried the so-called “low glycemic index diet”, you’ve heard of the Glycemic Index (GI). The GI is a tool used to measure how much carbohydrate-containing foods increase blood glucose levels. Foods listed are classified as low, medium, or high glycemic foods and ranked 0-100; low being 0-55, medium being 56-69, and high being 70-100.

While this is a very helpful tool, telling you how rapidly 50g of the food turns into glucose, it doesn’t tell you how much of that glucose is in a serving of that food. This is where the Glycemic Load (GL) comes in. GL estimates how much the food being eaten will raise a person’s blood glucose level after eating it, where one unit of GL approximates the effect of eating one gram of glucose.

Similar to GI values, GL values breakdown into three ranges; 0-10 is low, 11-19 is medium, 20+ is high. The GL is based on the GI, but it is calculated by multiplying the grams of available carbohydrate in the food by the foods GI, then dividing by 100. The GI of a watermelon is 72 (high), but the GL of a typical serving (120g) of watermelon is 4 (low).

Because of their high content of nonabsorbable carbohydrates, e.g. fiber, most fruits and vegetables, beans and legumes, and whole grain products fall within the low to medium-low GI values, whereas refined grains (white rice, white bread), potatoes, and anything processed to have its fiber removed, including both added-sugar as well as pulp-less fruit juice, all have high GI values. This is because fiber slows the gastric emptying and therefor the absorption of these foods, so if you remove said fiber, you’re going to get a rapid absorption of sugar, thereby raising your blood glucose levels.

Juicing – Good or Bad?

It would seem that fiber plays a huge role in whether or not a food causes your blood sugar to rise. Focusing in on fruits, it has been shown that a fruit’s GI value can be affected by a number of factors, including variety, ripeness, processing, cooking, and storage.

A meta-analysis found that 100% fruit juice (unprocessed, no added sugar) hasn’t been shown to alter blood glucose significantly more than consuming the whole fruit itself. That being said, fruit juice that has its fiber removed (pulp-less) or has sugar added to it (commercial juices can contain high amounts of high fructose corn syrup), is a different story. Both added sugar, as well as removing the pulp causes quick absorption, leading to rapid blood sugar spikes.

If you are a fan of fruit juice, and want to be conscious about your blood glucose, it seems the best option is to blend the whole fruit yourself, avoiding any processed fruit juices with added sugar, and to ensure the pulp is mixed fully with the juice to slow absorption.

Fruit vs. Fruit Juice

Coming back to people who say you should avoid fruit if you’re diabetic or worried about spiking your blood glucose, you have been misled. We now know that the natural fiber in fruits will slow the absorption of sugar, preventing your blood glucose spiking.

Not only is fruit okay to eat, but it actually has also been shown to reduce risk of type II diabetes and other metabolic disorders. A study looking at 7,675 Australians found that total fruit intake was inversely associated with serum insulin, insulin resistance of β-cell function, and positively associated with insulin sensitivity. Interestingly this study states that fruit intake, but not fruit juice intake, may reduce the risk of type-2 diabetes by increasing the consumptions of dietary fiber (being converted into anti-inflammatory short chain fatty acids), and phytochemicals (flavonoids).

Another study looking at 66,105 women from the Nurses’ Health Study (1984-2008), 85,104 women from the Nurses’ Health Study II (1991-2009), and 36,173 men from the Health Professionals Follow-Up Study (1986-2008) who were all healthy, found that fruit consumption was associated with lower risk of type-2 diabetes. They found that greater consumption of specific whole fruits, particularly blueberries, grapes, and apples (all of which contain high amounts of flavonoids), was significantly associated with a lower risk of type-2 diabetes, while greater fruit juice consumption was associated with an increased risk of type-2 diabetes.

It is unclear whether these juices were 100% fruit juice or not, but they reference this study from 1981 which looked at blood glucose when consuming oranges vs. orange juice, as well as grapes vs. grape juice. The researchers found that with oranges, as previously reported with apples, there was a significantly smaller insulin response to fruit than to juice, and less postabsorptive fall in plasma glucose.

In summary, when consuming a healthy, largely plant-based diet with a high variety of fruits and vegetables, you don’t need to be worried about consuming too much sugar and about the negative health effects, including type II diabetes and excessive insulin spikes. The complex carbohydrates and the polyphenols enriched in such a diet, slow gastric emptying and provide your gut microbes with the raw materials to generate health promoting anti-inflammatory molecules, which in turn reduce your risk for metabolic disease, including type II diabetes.

E. Dylan Mayer is a graduate from the University of Colorado at Boulder, with a major in Neuroscience and minor in Business. He is currently completing his master’s degree in Human Nutrition from Columbia University. Dylan is fascinated by the close interactions between nutrition, exercise and human health, especially with regard to the brain-gut-microbiome system – and regularly posts his content on his Instagram (@mayerwellness).