A magical journey to the sweet kingdom

Figure 1 “Too Much Sugar” (Source: WeChat official account)

On July 14, 2023, the International Agency for Research on Cancer released an assessment report on the health effects of the sugar-free sweetener aspartame, which classified aspartame as a possible human carcinogen, sparking heated discussions online. [[i]] Prior to this, artificial sweeteners such as saccharin and sodium cyclamate had also been controversial. [[ii]]

Why do people love sweets so much?

From the perspective of evolutionary psychology, humans’ preference for sweets can be explained from three perspectives: energy source, biological evolution, and emotional comfort . [[i]] First, sweet foods are usually high-calorie foods. Early humans lived in an environment where food was scarce, so they had a natural preference for foods that could quickly provide energy. Such foods can quickly replenish physical energy and increase the chance of survival. Second, in the natural environment, poisonous plants usually have a bitter taste, while sweet foods are mostly non-toxic.

Therefore, the preference for sweetness may be a natural selection process that helps humans avoid toxic foods and thus increase their chances of survival. In addition, studies have found that sweets may stimulate areas of the brain associated with happiness and comfort, especially activating the dopamine system responsible for motivation and reward. Therefore, in the long process of evolution, we gradually formed a preference for sweets, and the brain evolved a reward mechanism for the pursuit of sweetness, which helps people maintain emotional balance in stressful environments.

What exactly is sugar?

First of all, we need to understand the relationship between carbohydrates and sugars, which are commonly known as carbohydrates. Most carbohydrate substances are composed of three elements: C, H, and O. The chemical formula can be written as (CH2O)n, where the atomic ratio of H and O is 2:1, which is exactly the same as the ratio in water molecules. In the past, people thought that this type of substance was a hydrate of carbon, so they called it carbohydrates. But later, with in-depth research, people found that there are many other sugar substances whose atomic ratio of H and O is not 2:1, such as deoxyribose (C5H10O3) that makes up the DNA molecule; there are also many substances whose atomic ratio of H and O is 2:1 but are not sugars, such as the gas formaldehyde (CH2O) that everyone hates. However, because the name carbohydrate has been used for a long time, people still use it widely today. Therefore, carbohydrates and sugars actually refer to the same type of substance .

Sugar can be divided into monosaccharides, disaccharides and polysaccharides . The monosaccharides that people are familiar with are glucose and fructose; disaccharides are maltose and sucrose; polysaccharides are starch, cellulose and so on.

From a chemical point of view, sugar is a polyhydroxy ( -OH ) aldehyde or polyhydroxy ketone . Glucose molecules contain both aldehyde and aldehyde groups, which can form a ring within the molecule to become cyclic glucose. [[ii]]

Figure 3 Glucose (left) aldose; fructose (right) ketose

Figure 4 Circular glucose molecule

Disaccharides and polysaccharides are formed by the dehydration condensation of two or more monosaccharides.

Figure 5 Polysaccharide structure diagram (Image source: Baidu)

The discovery of saccharin

Sweets are delicious, but eating too much can cause tooth decay; modern people are increasingly concerned about keeping in shape, and consuming too much sweets may also cause fat accumulation and lead to obesity. As a result, people began to look for sugar substitutes , which are called " non-nutritive sweeteners " in academia. They can bring sweet enjoyment to people's taste buds, but have lower calories than ordinary white sugar. The most familiar sugar substitute should be saccharin.

In 1879, two researchers from Johns Hopkins University in the United States, Lemson and Fahlberg, jointly published an article on a substance called "o-benzoylsulfonyl imide" and its synthesis method. Another name for this substance is "saccharin".

Figure 6 Saccharin structure

In fact, the discovery of saccharin was purely accidental. One day after finishing an experiment, Fahrberg went to eat without washing his hands. He suddenly felt a distinct sweetness in his mouth, so he went back to the laboratory to search and found the residual sweet substance. Thus, the world's first artificial sweetener was "accidentally" developed. (PS: Everyone should maintain good experimental and hygienic habits when doing experiments, otherwise, they may really sacrifice themselves for the cause of science......)

Saccharin is 300-500 times sweeter than sucrose, but costs only 1/10 of it. It is almost not involved in human metabolism and has low calories, so it quickly became popular. The discovery of saccharin had a huge impact on the food industry, and more and more sweeteners came into being. In 1937, cyclamate was invented, which was 30-50 times sweeter than sucrose; in 1965, aspartame was invented, which was about 200 times sweeter than sucrose; in 1967, acesulfame potassium was invented, which was about 200 times sweeter than sucrose; in 1976, sucralose was invented, which was about 600 times sweeter than sucrose...

Sugar alcohol nutritional synthetic sweeteners

As mentioned above, sugar is a polyhydroxy aldehyde or polyhydroxy ketone. Under appropriate reducing conditions, sugar is reduced to a polyol, called a sugar alcohol . Take the familiar xylitol as an example. Xylitol is a naturally occurring five-carbon sugar alcohol that is widely found in many fruits and vegetables. Many of the health and medical functions of xylitol have been confirmed by scientific research, such as lowering blood sugar and preventing tooth decay. Currently, xylitol is widely used in the food industry as a substitute for sucrose, such as chewing gum, chocolate, various candies, beverages, milk, yogurt, etc. [[i]]

Figure 7 Xylitol chewing gum (Source: WeChat official account)

Can sugar substitutes really prevent weight gain?

We need to first understand the mechanism of weight gain. According to the law of conservation of energy, if the energy we take in from food equals the energy we consume in our daily activities, then our weight will be maintained; if the intake is less than the consumption, we will lose weight; if the intake is greater than the consumption, we will gain weight. Therefore, if a person consumes only a small amount of high-calorie food and exercises a lot to consume it, he will not gain weight; and if the so-called low-calorie, "zero-calorie" food is consumed in large quantities and the person does not exercise or consume it, it will also cause weight gain.

Studies have shown that when consuming foods or beverages containing functional sweeteners, the sweetness receptors in the mouth are activated, resulting in a sweet taste sensation. These functional sweeteners contain no or less energy and cannot be metabolized by the body. Therefore, when they reach the intestines, they change the sugar metabolism pathway in the intestines, leading to a compensatory increase in appetite. Compared with sucrose, after consuming sucralose, women and obese people are more sensitive to food stimulation and are more likely to overeat at buffets. Therefore, consuming sweeteners may increase people's food intake to a certain extent .

However, there are also studies showing that natural functional sweeteners such as steviol glycosides can not only reduce the energy intake of rats, but also do not increase their body weight when consumed for a long time (18 weeks of treatment); compared with sucralose, steviol glycoside treatment can also reduce the body weight of male mice.

Figure 8 Physiological parameter data of different experimental groups

(ASP, aspartame; CON, control group; SCA, sucralose; STV, stevia; SUC, sucrose; XYL, xylitol)

Regarding the effects of functional sweeteners on obesity, research results are inconsistent. Therefore, it is still impossible to draw a clear conclusion on the effects of functional sweeteners on obesity. In addition, existing studies have shown that the relationship between functional sweeteners and obesity is complex and is affected by various factors, including the type and amount of sweeteners consumed, individual differences, gender differences, and differences in intestinal microorganisms.

How to look at sweeteners scientifically

We cannot give a definite judgment on whether sweeteners are good or bad. However, it is meaningless to talk about good or bad without considering the dosage . We should study the effect of functional sweeteners on obesity in a dose-related manner. No matter how good something is, excessive intake is unhealthy. At the same time, we should fully consider individual differences, such as genetics, intestinal microbiota, and metabolic health, which may affect the body's response to sweeteners and personal obesity risk. In addition, we should also pay attention to the long-term effects of functional sweeteners on human obesity or health, so as to use sweeteners more reasonably.

[[i]] Gan Liping, Zhao Yifeng, Hong Tu, et al. Effects of functional sweeteners on obesity and related mechanisms [J/OL]. Food Science: 1-15 [2024-01-13]. http://kns.cnki.net/kcms/detail/11.2206.TS.20230620.1755.024.html.

[[i]] Feng Yongqiang, Wang Jiangxing. Characteristics of xylitol and its application in food[J]. Food Science, 2004(11):379-381.

[[i]] Lin Wen. Is sweet tooth related to the brain?[J]. Encyclopedia Knowledge, 2023, (16): 34.

[[ii]] Wang Jingyan. Biochemistry 3rd Edition. Higher Education Press, 2007:1-31.

[[i]] Guancha.com. July 14, 2023. WHO report says aspartame “may cause cancer”, US FDA protests.

https://baijiahao.baidu.com/s?id=1771364398173532662&wfr=spider&for=pc

[[ii]] Jia Yueyang. A scientific view on sweeteners[N]. Guangming Daily, 2023-11-05(007). DOI:10.28273/n.cnki.ngmrb.2023.005286.