Is a glass of juice a day really healthy? Beware of the "sweet" trap of juice

Tuchong Creative

According to the recommendations of the Dietary Guidelines for Chinese Residents (2022 edition), children, adults and the elderly should eat fruit in moderation. In the past, many people liked to squeeze fruit into juice or buy ready-made juice, thinking that this not only replenished water but also counted as eating fruit. However, in recent years, more and more professional groups have recommended that fruit juice should not be used to replace fruit, and some studies even believe that long-term consumption of fruit juice can be detrimental to health. On one side is fresh fruit, and on the other side is juice squeezed from fruit. How can the health results be so different?

In fact, the international attitude towards fruit juice is a bit ambiguous. How to understand this ambiguity? The positive role of fruit in health is beyond doubt, but whether fruit juice is harmful is actually controversial. Specifically, the dietary guidelines of various countries have not reached a consensus on whether drinking fruit juice is good or bad. For example, the dietary guidelines of the United States and the United Kingdom believe that fruit juice has certain benefits and recommend that daily fruit intake can be partially replaced by fruit juice. However, countries such as Australia hold a more cautious attitude and only recommend drinking fruit juice occasionally. Moreover, even in the same country, different academic groups have different views. Take the United States as an example. The American Academy of Pediatrics has always recommended not to give fruit juice to babies under 6 months old, and in 2017 it raised the age to 1 year old. my country's diet and nutrition community has not completely denied the nutritional value of fruit juice, but it does not recommend replacing fruit with fruit juice.

So why is there a saying that drinking fruit juice may be harmful? What is the relationship between fruit juice and health?

At present, more and more studies have found that there may be a correlation between the tendency to drink large amounts of fruit juice and the occurrence of certain diseases, but it is not yet known whether this correlation is causal.

In 2008, a study that tracked more than 70,000 nurses for 15 years found that, with the same weight and lifestyle, people who drank at least 3 cups of apple juice per month had a 15% increased risk of diabetes compared to those who did not drink juice. A study from Harvard University published in the journal Diabetes Care also found similar results. Drinking 100 ml of sugary drinks a day increases the risk of diabetes by 16%. This study specifically mentioned that sugary drinks include 100% pure fruit juice. In 2019, a study published in the Journal of the American Medical Association found that drinking 350 ml of juice a day was associated with a 24% increase in all-cause mortality. In the same year, a study in the British Medical Journal also found that drinking 100 ml of pure juice a day was associated with a 12% increase in the overall risk of cancer and a 15% increase in the risk of breast cancer.

Does an increased risk of illness and death mean that it will definitely occur? In other words, how should we view these results?

On the one hand, the increase in the prevalence is usually related to a variety of factors, including the extension of the course of the disease, the extension of the life expectancy of uncured people, the increase in new cases, the immigration of cases, the migration of healthy people, the immigration of susceptible people, the improvement of diagnostic level and the increase in reporting rate. Of course, more important here are certain exposure factors, which are risk factors that may cause the disease, such as obesity is a risk factor for diabetes.

On the other hand, the scientists involved also admit that these findings cannot rule out the problem of "reverse causation," that is, in the observed association, the result is actually the cause of the exposure, rather than the exposure causing the result.

Therefore, the current evidence is not sufficient to define fruit juice as a risk factor or even a cause of these diseases.

Although the evidence is weak, there are indeed health problems with fruit juice. Let's analyze it in detail.

First of all, we eat fruit as a snack to supplement nutrition, and juice has lower nutritional value. Although juice can also provide a certain amount of vitamins and minerals, which is a good thing for those who do not eat enough fruits and vegetables. However, the loss in the juicing process is huge.

The first thing to be affected is the loss of cellulose, especially insoluble dietary fiber. Dietary fiber can not only increase satiety, but also provide food for intestinal flora, regulate blood lipids, and prevent constipation.

Secondly, a certain amount of antioxidants, such as vitamin C, beta-carotene and polyphenols, will be lost during the juicing process. These substances will oxidize during the juicing process and lose their nutritional value. This phenomenon can be observed when cut apples turn brown. So how much will the loss be? Taking vitamin C as an example, the experimental determination of the vitamin C loss rate of cucumber, mango and orange after fresh juice is 84.11%, 58.92% and 32.76% respectively. And the longer the storage time, the greater the loss.

What is more terrible than the loss is the relatively increased sugar content. Many scientists even directly equate fruit juice with sugary drinks because it contains more free sugar. When the World Health Organization defines free sugar in the "Guidelines for Sugar Intake for Adults and Children", it does not include sugar in whole fresh fruits, but includes sugar in fruit juice. This is mainly because whole fresh fruits also contain healthy ingredients such as dietary fiber and plant compounds, which can relatively "offset" the negative effects of sugar in fruits. However, after becoming juice, the sugar is free. For example, the sugar content of orange juice is about 10%, pomegranate juice and yellow peach are 10-11%, grape juice and apple juice are 12-13%, and the sugar content of prune juice can reach 18%. A normal-sized bottle of juice often contains 20-30 grams of sugar, which is almost the same as the sugar content of a bottle of cola. Moreover, the change in shape also brings about changes in the glycemic index. Taking oranges as an example, the glycemic index (GI) has risen from 43 to 57, changing from a low-GI food to a medium-GI food.

In addition, the problem of fructose in sugar is also very prominent. The fructose content in fruit juice is about 10% to 18%. In addition to affecting uric acid metabolism and aggravating gout attacks, excessive fructose has also been found to be related to immune damage. A study in Nature Communications found that an increase in fructose content will accelerate the body's oxidation and promote the secretion of cytokines by cells. These cytokines will further stimulate the body's immune cells, resulting in various chronic inflammatory symptoms, which will further damage the body's immune system.

With the development of modern kitchen technology, more and more juicers choose to retain the pomace. Does this kind of juice not have these problems?

Although more fiber is retained, the problems of nutrient loss and free sugars still exist. Moreover, small appliances are sometimes difficult to clean thoroughly, and the residual pomace and juice may increase the risk of bacterial contamination.

So does this mean that we can no longer drink fruit juice, especially for the elderly who have poor chewing ability?

In fact, it is not that absolute. If you really cannot eat fruit directly, it is okay to drink some fruit juice. Just don't be obsessed with fruit juice, and don't use it to replace fruit. Also, be sure to clean your mouth after drinking it.

This article is a work supported by the Science Popularization China Creation Cultivation Program

Author: Zhang Yu

Reviewer: Zhang Na, Associate Researcher, School of Public Health, Peking University

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.