As I’ve explored previously, drinking sugar water is bad for you. If you have people drink a glass of water with three tablespoons of table sugar in it, which is like a can of soda, this is the big spike in blood sugar they get within the first hour. The body freaks out, and releases so much insulin we actually overshoot, and 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 blood stream as if we’re starving, because our blood sugar just dropped so suddenly. And the same thing happens after drinking apple juice.

Here’s what happens to your blood sugar in the three hours after eating four and a half cups of apple slices: it goes up and comes down. But if you eat 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 stuck those four and a half cups of sliced apples in a blender with some water, and puréed them into an apple smoothie? It would still have all its fiber, yet still cause that hypoglycemic dip. The rebound fall in blood sugars, which occurred during the second and third hours after juice and purée, was in striking contrast to the practically steady level after apples. This finding not only indicates how important the presence of fiber is, but also, perhaps whether or not the fiber is physically disrupted, as happens in the blender.

Let’s play devil’s advocate, though. Eating four and a half cups of apples took 17 minutes, but to drink four and a half cups of apples in smoothie form took only about six minutes, and you can down two cups of juice in like 90 seconds. So maybe these dramatic differences have more to do with how fast the fruit entered in our system, rather than its physical form. If it’s just the speed we could just sip the smoothie over 17 minutes and the result would be the same, so they put it to the test. Fast juice was drinking it in 90 seconds, but what if you instead sipped the juice over 17 minutes? Same problem—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 whole apple eating? A little better, but not as good as just eating the apple. So eating apples is better than drinking apple smoothies, but who drinks apple smoothies? What about bananas, mangoes, or berries?

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 they were exercising. Bananas in general, though, may actually improve blood sugars over time. The same thing with mangoes—and this was with powdered mango—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 puréed in a blender. Add blended berries in addition to the sugar water, and you don’t get the hypoglycemic dip; you don’t get that burst of fat in the blood. Drinking blended berries isn’t just neutral, but improves blood sugar control. Again, 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 Mayo’s basic green smoothie recipe, packed with berries and greens, would be expected to deliver the best of both worlds: maximum nutrient absorption without risking overly rapid sugar absorption.

To see any graphs, charts, graphics, images, and quotes to which Dr. Greger may be referring, watch the above video. This is just an approximation of the audio contributed by Katie Schloer.

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