It appears that this is an area of dispute amongst the experts, and I don't believe the research supports any bold claims.
For example, in 2016 Mother Jones interviewed Robert Lustig, who has been the subject of many Skeptics.SE questions here, and who provided limited evidence to the journalist.
Then the journalist followed up with David Katz, director of the Yale-Griffin Prevention Research Center at Yale University:
In an emailed note, he wrote that while the blending process “certainly [has] an effect” on fiber, there has been little research documenting precisely how much it breaks down insoluble fiber and reduces the benefits of fruit. He added, “Let’s face it: Chewing grinds up fiber to some extent, too.” That said, “we have a fairly solid basis for saying: Whole food is best,” he wrote.
So, while he believes the claim might be true, he admits the research isn't there yet.
Katz goes on to point out that liquidizing food has other effects which may have an even more important role that the claim about fiber:
For one thing, “blenderizing takes away chewing, which reduces the time spent eating” and may inspire you to take in more. Also, “fluids are less filling than solids.” Finally, he added, turning foods into liquids markedly raises their glycemic load, which is a measure of how much a particular amount of food affects blood sugar and insulin levels. Indeed, a whole apple has a glycemic load of 6, while a serving of apple juice clocks in at 30—higher even than Coca-Cola, at 16.
Meanwhile, a 2017 article in the Atlantic take another tack, talking to Robin Spiller, director of biomedical research at the Nottingham Digestive Diseases Centre in the U.K.
He looked studied the consumption of soup, and found blending it stopped consumers from feeling hungry for longer! However, his explanation for this is that when foods in the stomach "crack" - split into layers, the water can be quickly absorbed by the body. If the water remains in emulsion, it takes longer to consume.
On the other hand:
Though it’s not clear that all smoothies would do exactly the same thing as his chicken slurry soup, Spiller explains
So perhaps this argument isn't particularly strong for fruit smoothies, where the water may be easily absorbed.
Spiller goes on to warn of other effects:
“When you eat an apple, there’s a lot of crunching,” said Spiller. “That's a strong stimulus for the gut to secrete fluid. A lot of the behavior of your intestine is anticipatory –– it has to work ahead of what's happening. It’s no good producing enzymes to digest stuff when the material arrives. That’s too late. You’ve got to produce it ahead of time.”
So there are other effects that affect hunger unrelated to the specific claim which is about fiber.
Stiller weighs in here too:
“Blending won’t have a significant negative impact on fiber,” Spiller reassured me. “Fiber is what’s responsible for the viscosity of a smoothie and its impact on the bacteria of the large bowel. Mashing fiber up into small pieces should only enhance its availability for the bacteria. Its prebiotic effect is definitely unimpaired––it might be enhanced, even.”
So, the tally is:
- one guy who is known for controversial claims says it has an effect,
- one expert says he thinks it does (but has no research to support it) and points out other more significant effects.
- one expert says he thinks it doesn't (but doesn't quote any research), and points out other more significant effects.
What does the actual research say?
I have to note that I was hampered by the number of papers that matched my search because as part of the method for measuring the fiber in food, many experimentalists blended it first. It seems that they are not concerned about the effect of blending.
I found some research in hamsters showing that particle size affected digestion, but they were talking about "micronization", and I don't think blenders count.
The best reference I found was:
This review paper quoted a couple of experimental studies:
Two studies have shown that fibre material is retained in smoothies.
Scientists from the University of Leeds found that that cell wall structure
of smoothies remained intact after they had been manufactured
and exposed to simulated digestion for up to 16 h [14]. It was also observed that smoothie mixing led to a 68% reduction in viscosity, 30%
reduction in total dietary fibre and 10% increase in soluble dietary fibre
[14]. Equally, another trial found that fibre material is still present in
the smoothies after processing-16.9% and 17.5% fruit cellular material
by weight in the two smoothies tested [15]. These findings revealed that
the fibre content retained in the smoothies resembled that similar to
the process of chewing fruit, with this potentially having its own health
effects.
So, yes, the blender had an effect, but the author likened it to chewing.
My conclusion is that the research isn't there to support the claim that a blender affects the fibre more than chewing fruit might. However, there are a plethora of other reasons which may cause similar effects - to feel hungrier sooner or to consume more fruit faster.
