Surely you know that smoothies are better for you than juices because they retain the fiber content of the fruit/vegetable, but have you asked yourself if a fruit smoothie has the same effect on your body as whole fruit? The answer may surprise you!
This question came up today because a friend of mine has been on Weight Watchers for the past few months, and she was recently switched from PointsPlus to the new SmartPoints program. Under PointsPlus, most fruits and vegetables counted as zero point foods. With the updated points system, whole fruit is still zero points, but blended fruit (i.e. fruit in smoothies) has a SmartPoints value.
My friend said she and everyone else at her Weight Watchers meeting were very frustrated at this change. After all, fruit in a smoothie still technically contains the whole fruit! What gives?
If you’ve been wondering the same thing, I tried to dig up some science to explain it. It surprised me that there isn’t more evidence available documenting the effect of blending a food on glycemic index, satiety, and nutrient absorption. Here’s what I found, though:
- A 2007 study published in the International Journal of Obesity studied the effect of liquid meals vs. solid meals on satiety. The results showed that participants consumed on average 12% more calories per day when drinking a carbohydrate-rich drink. This suggests that drinking a blended fruit smoothie is less filling than eating a calorically equivalent portion of fruit. This is likely attributed to the fact that eating fruit takes longer than drinking it. Satiety signals are released as a hormones from the gut, and these must circulate through the bloodstream to your brain before you realize you’re full. That’s why waiting 15-20 minutes after eating a meal before deciding to eat a second helping is a good practice to adopt to avoid overeating.
- Chewing increases satiety. This has been documented in a study published in The British Journal of Nutrition in 2013. More chewing led to an increase in secretion of CCK in the gut (hormone that decreases appetite) and decreased secretion of ghrelin (hormone that increases appetite).
- Blending fruit changes the insoluble fiber contained in it. While it’s not accurate to say that blending fruit “destroys” the fiber, it does “change” it by breaking it into smaller particles. When you break the fiber compounds into smaller pieces, different things can happen, as explained in The Journal of Food Science article here. This is where the science gets sparse. Robert Lustig, MD, professor of pediatrics at University of California, suggests that blending fruit fiber has a negative effect on the glycemic index of foods. “The blades destroy the insoluble fiber, which means that the ‘gel’ that forms on the inside of the intestine has no structure. The sugar is absorbed at a maximal rate, overwhelming the liver’s capacity to metabolize the sugar, and the excess sugar is turned into liver fat which is the precursor to metabolic syndrome.” (Quote from Time magazine article April, 2015) That sounds reasonable to me, but I would much prefer a peer reviewed article cited to back that claim up!
**If you want to completely nerd out over the food science of stuff that affects glycemic index, read this as well.
These factors combined help to explain why eating the whole, unprocessed version of a food is more filling than eating a blended, juiced, or puffed version of it. This also helps to explain why pureed or blended fruits have SmartPoints under the new Weight Watchers program. I hope that helps clear things up! Hit me up in the comments if you’d like to see more questions like this answered in the future.
International Journal of Obesity (2007) 31, 1688–1695; doi:10.1038/sj.ijo.0803667; published online 19 June 2007
Zhu Y, Hsu WH & Hollis JH (2013). Increasing the number of masticatory cycles is associated with reduced appetite and altered postprandial plasma concentrations of gut hormones, insulin and glucose. British Journal of Nutrition 110: 384–390.