Is juicing actually good for you?
Freshly juiced fruit has become a basic dietary item—especially for busy, health-conscious consumers who can save time on making (and perhaps even chewing) their food without missing out on nutrients. Fruit juice also has been linked to claims that it can help you both lose weight and stay healthy. All of which, of course, has made it highly profitable. The global fruit and vegetable juices market was valued at $154 billion in 2016 and is expected to grow.
But is juicing really as healthy as we think? Most foods containing fructose—a naturally occurring sugar found in all fruits and fruit juices—don't seem to have harmful effects as long as they're not contributing to excess daily calories. This is because the fibre found in whole fruits is intact, and the sugar is contained within the fruit's cells. It takes our digestive system a while to break these cells down and for fructose to enter the bloodstream.
But this is not the case with fruit juice. Unlike whole fruit, fruit juice has most of the fibre removed. This is why the fructose in fruit juices counts as 'free sugars'—which also include honey and the sugars added to foods. It's recommended by the World Health Organisation that adults have no more than 30g of added sugar, the equivalent of 150ml of fruit juice, per day. The problem is that, with the fibre removed, fruit juice's fructose is absorbed more quickly. Sudden increases in blood sugar cause the release of insulin(胰岛素) to bring it back down to a stable level. Over time, this mechanism can wear out, increasing the risk of developing type 2 diabetes. In 2013, researchers analysed the health data of 100,000 people collected between 1986 and 2009 and found that fruit juice consumption was linked with increased risk of type 2 diabetes. Another study found a relationship between fruit juice and type 2 diabetes after following the diets and diabetes status of more than 70,000 nurses over 18 years. The researchers explain that the possible reason for this may partly have been the lack of the other components found in whole fruit, like fibre.
Juices containing vegetables can provide more nutrients and less sugar than juices made up of just fruit—but they still lack valuable fibre. Diets high in fibre have been linked to a lower risk of developing heart disease, stroke, high blood pressure and diabetes, and it's recommended adults consume 30g per day.
Aside from the link to type 2 diabetes, many studies agree that fruit juice is harmful if it contributes to an excess in calories. In an analysis of 155 studies, John Sievenpiper, associate professor at the University of Toronto's Department of Nutritional Sciences, set out to understand if the associations between sugary soft drinks and health—including risk of diabetes and heart disease—were applicable to the food and drink we generally consume as part of a healthy diet. He compared studies looking into the effects of fructose-containing sugars with control diets free from or lower in these sugars. His aim? To isolate the effects of consuming too many calories from the effects of foods containing different sugars.
He found negative effects on blood sugar and insulin levels when food provided excess calories from sugars, including fruit juice. However, when the calories weren't in excess overall, there were some advantages to consuming whole fruit—and even fruit juice. Sievenpiper concludes that the recommended 150ml of fruit juice per day, which is an average serving, is reasonable. The paper concluded that foods containing fructose may have some small benefits for long-term blood sugar control when they don't lead to over-consumption of calories. But when they do lead to too many calories, they risk raising blood sugar and insulin levels.
So while we know that fruit juice can cause diabetes if it's part of a diet that has more calories overall, it's less clear how juice affects the long-term health of those who aren't overweight. But we have a higher risk of consuming more than the recommended daily number of calories (around 2,000 for women and 2,500 for men) on the days we drink juice, according to research. Numerous studies have shown that drinking fruit juice doesn't make us consume any less of other foods throughout the day.
As it is easy to consume large amounts of fruit juice quite quickly, extra calories are taken in. And extra calories can contribute to weight gain. One way we can improve the effects of fruit juice on our health is making sure we select ripe fruit to retain as much of its goodness as possible, according to Roger Clemens, professor of pharmaceutical(药物的) sciences at the University of Southern California.
Another reason for fruit juice's recent popularity is the argument that it can help to remove poisonous or harmful substances from the body. However, "the whole concept of a juice diet helping to remove poisonous or harmful substances from the body is completely false," says Clemens. "We consume compounds every day that may be poisonous and our bodies do a wonderful job of removing and eliminating anything harmful from our bodies."
And juice isn't a cure-all for getting your nutrients, either. "There are a lot of nutrients contained in the parts of the fruits, such as in apple skins, that are excluded when you juice them," says Heather Ferris, a professor of medicine. "You end up with sugary water with some vitamins."
Plus, she adds, it's not an ideal way to reach the advised 'five a day'. "People try to get five servings of fruit and vegetables per day and don't realise this isn't just about getting vitamins," she says. "It's also about reducing the amount of grain carbs(碳水化合物) and protein and fats in our diet, and increasing fibre." So while fruit juice is better than no fruit at all, there are limits. It particularly comes with risks when we consume more than 150ml of free sugars per day, or when it puts us over our recommended calorie intake. It provides us with vitamins—but fruit juice is far from a quick fix.