Do smoothies actually release extra nutritional value?

by Jennifer Bowden / 11 March, 2018

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Smoothie hype includes claims that pulverising plants boosts their nutrient value.

In marketing speak, high-powered blenders offer “the highest degree of nutrition your food has to offer”, according to one manufacturer; another, meanwhile, says its machine produces “a smooth, cool ‘super juice’ filled with fibre, vitamins and natural goodness”. They sound great, but are the claims supported by science?

Modern blenders cut and pulverise fruit and vegetables at high speed, turning them into a fine, smooth, creamy liquid. In further sales speak, they “break down the cell walls of fibrous plant foods, releasing important vitamins and minerals contained within. At the same time, they reduce beneficial fibre, pulp, seeds and skins to a smooth-as-silk texture, delivering food to your body in an easily digestible, easily absorbable form.”

Scientifically speaking, pulverising fruit and vegetables into a smooth liquid will indeed disrupt the natural matrix of the plant cells, which will influence the release and absorption of some nutrients in the digestive system.

However, it’s not as cut and dried as blender manufacturers claim. Even if more nutrients and antioxidants are released from the food matrix, they might interact with other food components in the gut, binding to macromolecules such as fibre to form a molecular complex that might reduce or improve their ability to be absorbed into the body’s circulation.

Other factors also influence nutrient absorption. Carotenoids, for example, a class of mainly yellow, orange and red pigments found in carrots, oranges, squashes and tomatoes, must be dissolved in dietary fats within the gut to be absorbed. There are many different carotenoids, many with antioxidant properties, and some that are converted into vitamin A. They are our main source of the vitamin.

Although carotenoids are more easily absorbed from cooked, puréed or finely chopped vegetables, they are only slowly taken up by the body. Blending carrots into a morning smoothie may seem a great way to consume carotenoids, but absorption of the extra molecules will be limited without the presence of dietary fat. Adding oil to a smoothie is one answer.

Every vitamin, mineral and phytochemical released from pulverised fruit and vegetables has its own absorption mechanism. More research is needed before anyone can say that blending plant foods improves overall nutrient uptake. It’s also unclear what the health effect might be if there was an increase in uptake.

Even if we knew all that, we would need to balance any possible benefit against what we already know about rapid sugar absorption from fruit and vegetable juices and smoothies, consumption of which causes an unhealthy spike in blood glucose levels.

What’s more, as noted by Jim Mann, a professor of human nutrition and medicine at the University of Otago, we still don’t fully understand how pulverising dietary fibre affects gut health. “Is some of the benefit of dietary fibre a result of it not being too ground up or too refined or too processed?” he said in a column on this subject last year  (“Crushing it”, December 2).

As manufacturers themselves note, blenders “reduce beneficial fibre, pulp, seeds and skins to a smooth-as-silk texture”. Is that desirable, or will more fibre end up being digested in the upper gut, effectively cutting off the supply of health-promoting pulp to the microflora living in the lower gut? Will a lack of mechanical brushing from dietary fibre affect bowel health?

Although these questions remain unanswered, one thing that can be said is that any claim that blenders produce a drink of the highest nutrition value is pure speculation. That evidence does not exist.

This article was first published in the February 17, 2018 issue of the New Zealand Listener.

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