The unseen health risks of smoothies

by Jennifer Bowden / 03 February, 2018
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Smoothie operator. Photo/Getty Images

Sugars are absorbed more quickly into the blood system when fruit is liquefied in smoothies – and that’s not a good thing. 

For people wanting to limit their sugar intake, smoothies and fruit juice are to be treated with caution, says researcher Barry Popkin. A professor of nutrition at the University of North Carolina, Popkin highlighted soft-drink consumption as a major public health issue in 2004. More recently, in an interview with the Guardian, he pointed to the growing appetite for smoothies and fruit juice, which he sees as the next battleground in the struggle between the beverage industry and the health sector.

On the face of it, smoothies are marvellous, blending healthy fruit and/or vegetables into delicious drinks. But there are unseen risks in their sugar content, acidity, effect on appetite and potential to raise blood-glucose levels.

So, is it possible for a smoothie love affair to remain a healthy relationship? Fruit is rich in sugars and complex carbohydrates. The sugars are contained within the fibrous cell walls of the flesh, and the fibre slows the digestion and absorption of the sugar. There are no reports of any adverse health effects resulting from these types of intrinsic sugars.

In contrast, so-called “free sugars” contribute to weight gain and dental caries and are linked to poorer dietary quality, obesity and the risk of noncommunicable illnesses, such as cancer and cardiovascular disease. The World Health Organisation defines free sugars as all monosaccharides and disaccharides added to foods by the manufacturer, cook or consumer, as well as sugars naturally present in honey, syrups and fruit juices.

Although smoothies aren’t named in the sugar hit list, they’re a close relative of fruit juices. A typical smoothie includes fruit pulp, but because it is liquefied, the fruit sugars are absorbed more quickly into the blood system, which is not desirable.

Another downside of smoothies is that the volume of fruit they contain is often greater than the amount we would eat as whole fruit. What’s more, studies have found we consume more energy overall in a day if we have fruit in a liquid rather than solid form. In other words, we don’t feel as full after a smoothie. 

So, here are some rules of thumb when making a smoothie: don’t add sweeteners, and use only the amount of whole fruit you could reasonably eat in one sitting. A smoothie with half an apple and half a banana is reasonable, whereas having three oranges and three apples, say, is excessive. Whether you add another small piece of fruit really comes down to your appetite and energy needs.

Kiwifruit is a good addition. Plant & Food Research scientists recently discovered that when kiwifruit is eaten at the same time as other carbohydrate-containing foods (wheat-biscuit cereal), it slows the absorption of sugars from the breakfast cereal into the bloodstream of healthy adults.

Using a simulated gut for testing, the researchers found fibre from the kiwifruit distributes itself widely through the intestinal contents. That could explain how kiwifruit slows our overall sugar absorption from both the kiwifruit and other foods in the gut.

Leafy green vegetables and nutritious nuts are also healthy smoothie ingredients.

Although whole fruit is best, a home-made smoothie is better than consuming no fruit at all. So, balance your personal preferences with the health effects to find a compromise that suits you.

This article was first published in the January 20, 2018 issue of the New Zealand Listener.

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