Blended Fruit Smoothies Versus Whole Fruit

Metabolic syndrome affects about ⅓ of the US adult population and includes hypertension, insulin resistance, abdominal obesity, and dyslipidemia. Insulin resistance is the culprit in type 2 diabetes where the body is no longer able to process sugar the same due to insulin no longer being effective. As opposed to type 1 diabetes where the body does not make enough insulin to properly store sugar. Type 2 diabetes often develops when the body too frequently has hyperinsulinemia after having hyperglycemia. This means that when a person eats a high amount of carbohydrates (which break down into sugars such as glucose) the amount of glucose in the blood spikes, this results in the body releasing insulin to facilitate the storage of glucose into cells and overall lower the blood glucose levels. The problem arises when the body is in a frequent state of releasing high amounts of insulin, so that the insulin becomes less effective and leads to insulin resistance, aka type 2 diabetes.

Fiber plays a very important role in preventing metabolic disease, especially in carbohydrate breakdown and blood sugar spikes. Fiber has been found to raise the viscosity of the stomach contents (makes the contents thicker or more gelatinous) which leads to slower gastric emptying. This means that glucose is being absorbed slower into the bloodstream and does not result in such a high insulin stimulation and release. Slower gastric emptying also means that someone will feel more full for longer resulting in less intake throughout the day which helps reduce excess calories and subsequently abdominal fat buildup (another metabolic syndrome factor!).

Not everyone finds it easy to eat the recommended 5 servings of fruits and vegetables every day, so people often look to smoothies as a quick on the go source of healthy calories. However, it is often stated that blending fruits changes the breakdown and could result in blood sugar spikes and high insulin release. So, are blended fruits as healthy as eating whole fruits?

The main study looking at whole versus blended fruits was published in 1977 and found that the type of fruit matters in the insulin response. They had 2 main findings:

Not significantly different- apples and mangos
Significantly lower- raspberries and passion fruit

However, they did not look into why the type of fruit matters.

A new study published in 2022 took a look at insulin levels in 20 healthy college students after eating whole fruit (apples and blackberries) versus blended fruits. The apples were whole with the skin still on, blackberries were whole, and participants drank water with the whole fruit to contain the same ingredients as the blended fruit. The blended smoothie was made with water and ice.

Interestingly, the study found that the blended fruit significantly reduced the glycemic response as opposed to the whole fruit. This was found in both males and females.

This study and another conducted on participants with and without obesity, both used a seeded fruit (blackberries, raspberries/kiwi) and a non-seeded fruit (apple, mango) and produced similar results. Therefore, it is believed that the grinding of the seeds releases fiber, polyphenols, fats, and proteins which result in reduced rate of gastric emptying and glucose absorption. The blending of these seeds breaks them down more than with just chewing, allowing for more nutrients to be utilized in the body. Grinding of insoluble fiber (found in many seeds) makes it more soluble which helps increase the viscosity of stomach contents, again slowing down stomach emptying and glucose absorption.

So what?

Smoothies can be a great nutrient dense option to increase your fruit and vegetable intake and be a quick snack/meal. However, what the smoothie is made with does matter. Here are our main tips:

Avoid using juices such as orange or apple juice. Instead use water, milk, or milk alternatives which have low sugar and some protein and fat (in the case of milks).
Be sure to use a seeded fruit (strawberry, blueberry, raspberry, blackberry, etc) in addition to your non-seeded fruits.
Other additions that can help add protein and slow gastric emptying could be nut butters, seeds, nuts.
Green vegetables can also be a great addition to smoothies such as kale, spinach, or beet greens. These don't change the flavor too much but do add another serving of vegetables!