Why fruit juices are little blood sugar bombs

Fruits and vegetables are good for your health, of course. Fruit juices are therefore often considered healthy products, even welcome allies in respecting the sacrosanct rule of 5 fruits and vegetables per day. But are they really so flawless? It's not obvious at all. Fruit juices may indeed promote weight gain in children and adults. Why ? How often is it safe to drink it? What is the difference between homemade juice and industrial juice? Le Figaro takes stock.

Each fruit provides essential nutrients for the proper functioning of the body. Sugar obviously, in the form of fructose, glucose or sucrose, but also vitamins (vitamins C, pro-vitamin A carotenoids), minerals and especially fiber, essential for good digestion. However, most industrial fruit juices come without pulp and contain very little fiber (0.3g compared to 2g for a whole fruit). In the same way, after opening, they are subject to a rapid depletion of vitamins and minerals because these elements oxidize within a few hours on contact with air.

Do the subtraction: all that's left in an already opened glass of fruit juice is a handful of nutrients drowned in water and a lot of sugar. Not to mention that when we drink a glass of juice, we actually swallow more than a whole fruit. “A glass of juice, or around 150 ml, contains 2 to 3 fruits if it is homemade but up to 5 for certain industrial juices,” explains Marie Behar, dietitian nutritionist at the Bichat-Claude Bernard hospital, in Paris, and doctor of public health. In addition, the nutritional composition of juices varies depending on the fruit, underlines Professor Marie-Josephe Amiot-Carlin, research director at Inrae in nutrition and public health. “Grape or apple juices are generally higher in sugar and have a lower vitamin content than some citrus or multifruit juices.” Finally, a glass of juice contains on average 20 grams (2 squares of sugar), almost as much as a soda.

All these elements combined, the risk of weight gain becomes obvious. First of all because of the high sugar content of juices coupled with a reduced fiber intake. “Fiber traps nutrients and thus slows down the process of digestion and absorption of carbohydrates, which contributes to a slower release of glucose into the blood,” explains Dr. Behar. Because juices have less fiber, the carbohydrates they contain are therefore absorbed more quickly. Concretely, when the body receives a dose of sugar greater than what it needs, this excessively stimulates the secretion of insulin, a hormone from the pancreas, which allows the storage of excess sugar in fats, lipids. This promotes weight gain,” adds the nutritionist.

The other pitfall is that fruit juices confuse satiety signals, which contributes to their addictive potential. Normally, chewing stimulates the release of hormones, such as ghrelin or leptin, which send satiety messages to the brain. Chewing longer gives the brain time to understand that you have eaten enough. When you drink fruit juice, this mechanism is suppressed. Consequence: the brain registers the absorption of food less quickly and the body has already accumulated too much sugar when you are no longer hungry. For these reasons, drinking fruit juice is not equivalent to consuming a whole fruit which, conversely, has a regulating effect. “We see it clearly: a child has much more difficulty chewing fruit and younger children often cannot eat it whole,” emphasizes Dr. Behar.

Thus in France, the recommendations of the national nutrition and health program (PNNS) indicate that children should not exceed half to one glass of fruit juice per day (i.e. 125 ml), compared to one glass for adults. “Fruit juices are not recommended before two years,” adds Professor Amiot-Carlin. According to a study published in 2024, which takes stock of 42 scientific studies, each additional portion (240 ml) of pure industrial fruit juice (or 100%) is associated with an increase in body mass index (BMI) of a child. As a reminder, BMI is the reference indicator for assessing body size based on height and weight. A child is considered overweight when their BMI is equal to or greater than the 97th percentile (this means that at least 97% of children have a weight lower than theirs).

Fruit juices should be consumed in moderation and never on an empty stomach in order to benefit from the fiber provided by other foods. Also be careful to distinguish the categories of juice among “pure juice (or 100%)”, “concentrates” and “nectars” because manufacturers are likely to add sugar to the recipe, in addition to the naturally present sugar. in fruit. “This only concerns nectars which are the sweetest, especially since their composition may include sweeteners,” indicates Dr. Behar. “Concentrated” juices contain only pressed fruit but the juice is heated to high temperature then frozen, to facilitate transport, before being re-diluted when placed on the shelves. Such processes can lead to a significant loss of minerals and vitamins.

When it comes to choosing, “pure juices” are the closest to a homemade juice. They are obtained by pressing the fruit, without added sugars. But here again, you have to be wary. Pure, unrefrigerated juices undergo, before being bottled, a pasteurization process which consists of heating the juice at high temperature, and therefore denaturing it in vitamins and minerals. To be stored in the refrigerated section, the pure “fresh” juices were subjected to “flash pasteurization”: the temperature of the juice is increased for a few minutes before bottling. “The advantage of flash is that the heating time is so rapid that it prevents the denaturation of the juice in vitamins and minerals,” emphasizes Professor. The best is still to favor fresh juices which have not undergone these temperature raising processes but have been stabilized cold. They then fully retain their minerals and vitamins.