Smoothies vs Whole Fruit: What Does the Science Say?

by Dr. Michael Greger; Care2

As you might expect, drinking sugar water is bad for you. According to research, if you have people fast and then drink a glass of water with three tablespoons of sugar in it (which is about the amount in a can of soda) you get a big spike in blood sugar within the first hour.

Our body freaks out and releases so much insulin that we actually overshoot. By the second hour, we’re relatively hypoglycemic, dropping our blood sugar below where it was when we started out fasting. In response, our body dumps fat into our bloodstream as if we’re starving because our blood sugars just dropped so low. The same thing happens even after drinking apple juice.

In the three hours after eating four and a half cups of apple slices, your blood sugar goes up and comes down. (You can see all of these spikes and drops in my video Green Smoothies: What Does the Science Say? below.) What happens if you take in the same amount of sugar in apple juice form—about two cups? Your body overreacts, releasing too much insulin, and you end up dipping below where you started. The removal of fiber in the production of fruit juice can enhance the insulin response and result in this “rebound hypoglycemia.”
What would happen, though, if you put those four and a half cups of sliced apples in a blender with some water and pureed them into an apple smoothie? Despite still having all the fiber, it still caused that hypoglycemic dip. The rebound fall in blood sugars, which occurred during the second and third hours after drinking juice and puree, “was in striking contrast to the practically steady level after eating apples.” This finding not only indicates how important the presence of fiber is, but also perhaps whether or not the fiber is physically disrupted, like in a blender.

Let’s play devil’s advocate. Eating four and a half cups of apples took 17 minutes, but drinking four and a half cups of apples in smoothie form only took about 6 minutes, and you can down two cups of juice in about 90 seconds. So, maybe these dramatic differences have more to do with how fast the fruit entered our system rather than its physical form. If it’s just the speed, we could simply sip the juice over 17 minutes and it should be the same, right? Researchers put it to the test. They found this had the same results as drinking quickly. So, it wasn’t the speed—it was the lack of fiber. What if you disrupt that fiber with blending but sip it as slowly as the apple eating? The results were a little better, but not as good as just eating the apple. The take-away? Eating apples is better than drinking apple smoothies.

What about more common smoothie ingredients like bananas, mangoes or berries? The results are a little different, surprisingly. There was a study that compared whole bananas to blended bananas and didn’t see any difference, but they only looked for an hour and it was while participants were exercising. Bananas in general, though, may actually improve blood sugars over time. The same goes for mangoes, as demonstrated with powdered mango, and you can’t get any more fiber-disrupted than that. It may be due to a phytonutrient called mangiferin, which may slow sugar absorption through the intestinal wall.

Berries help control blood sugar so well they can counter the effects of sugar water even when they’re pureed in a blender. By adding blended berries to sugar water, you don’t get the hypoglycemic dip and you don’t get that burst of fat in the blood. Drinking blended berries isn’t just neutral—it improves blood sugar control. Again, this is thought to be due to special phytonutrients that may slow sugar uptake into the bloodstream. Indeed, six weeks of blueberry smoothie consumption may actually improve whole body insulin sensitivity.

So, while apple smoothies may be questionable, a recipe like The Mayo Clinic’s basic green smoothie recipe, which is packed with berries and greens, would be expected to deliver the best of both worlds—maximum nutrient absorption of greens without risking overly rapid sugar absorption.

I have a whole series of videos on green smoothies. In Are Green Smoothies Good for You?, I talk about the enhanced nutrient availability absorption. In Are Green Smoothies Bad for You?, I raise the questions about teary-eyed gut flora and intact grains, beans, and nuts. For even more, see Liquid Calories: Do Smoothies Lead to Weight Gain? and The Downside of Green Smoothies.

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