The consumption of sugar, primarily in the form of sweetened beverages, has significantly increased recently, and along with it, the frequency of obesity, diabetes, and cardiovascular diseases has risen.
Two main factors are decisive in this: the increase in the caloric content of the diet primarily due to simple carbohydrates (sugars) and the high consumption of fructose.
Fructose is perhaps the most well-known sugar substitute.
It is believed that consuming fructose instead of sugar is associated with a lower risk of harm to health. Its main proponents are people with diabetes and advocates of a healthy lifestyle.
At the same time, a large number of experts in dietetics assert that the benefits of fructose are a myth, and the harm from its consumption significantly exceeds the harm from sugar…
In this article, we will discuss what fructose is, the difference between sugar and fructose, and how beneficial/harmful fructose really is compared to sugar.
Main thoughts:
With regular exercise, especially aerobic exercises, the harm from fructose is significantly lower
What is fructose? Difference from sugar
Fructose or “fruit sugar” is a type of sugar that gives a sweet taste to fruits, honey, and some vegetables.
Few people know, but table sugar is 50% fructose. The other 50% is glucose, a basic simple carbohydrate that is the main source of energy for our body.
Glucose is a structural component of almost all types of carbohydrates, both simple and complex: all carbohydrates differ in the number of glucose molecules or their isomers (slightly altered at the molecular level) in their molecular structure. It is glucose that is the end product of the carbohydrate digestion process.
Fructose, after absorption, also needs to be converted into glucose. This occurs in the liver.
The glycemic index of fructose is lower than that of sugar and glucose 21. This means that it does not cause a sharp increase in blood sugar and insulin levels. This is its main beneficial property.
| Glycemic index of sugars | |
|---|---|
| Glucose | 100 |
| Sucrose (table sugar) | 65 |
| Fructose | 20 |
Fructose is one of the most popular sugar substitutes. Table sugar is 50% fructose. Fructose has a low glycemic index and, unlike sugar, does not cause sharp spikes in blood insulin levels
We recommend: Glucose, fructose, sucrose: what are the differences? Which is more harmful?
What are the harms of fructose?
Many scientists today are convinced that consuming fructose in large amounts is a cause of many serious diseases, including the development of “fatty liver,” obesity, type 2 diabetes, cardiovascular diseases, and even cancer 2.
Fructose is metabolized in our body differently than glucose. The liver plays an active role in this process. Accordingly, the more fructose, the greater the burden on the liver.
It is the peculiarities of fructose metabolism involving the liver that explain the reasons for fructose being more harmful compared to sugar (and glucose).
Scientific studies indicate that excessive consumption of fructose:
- disrupts the lipid composition of the blood, resulting in an increased concentration of bad low-density cholesterol in the blood, an increase in fat around internal organs, and a higher risk of cardiovascular diseases;
- increases the amount of fat in the liver, leading to the formation of “fatty liver”;
- is not as beneficial for weight loss as commonly believed, since fructose, compared to glucose (and sugar), does not suppress appetite, does not provide satiety, stimulating the consumption of excess calories.
Reasons for the Harm of Fructose
Several features of the fructose metabolism process make it particularly dangerous for health 8:
- it is delivered to the liver in very large quantities, significantly exceeding concentrations in other tissues;
- it increases the level of enzymes necessary for the formation of triglycerides – fat particles, high concentrations of which in the blood are a recognized factor increasing the risk of cardiovascular diseases;
- fructose does not require insulin for its metabolism, i.e., insulin does not control its level in the blood, as is the case with glucose; therefore, even in conditions of insulin resistance, characteristic of diabetes, fructose actively participates in the process of fat formation;
- fructose suppresses the process of fat oxidation within cells for energy, hindering weight loss;
- fructose stimulates the production of uric acid, which contributes to fat formation in the liver;
- in the process of fructose metabolism, free radicals are formed, enhancing the process of fat formation in the liver.
Next, we will take a closer look at some of the mentioned properties of fructose based on scientific research.
Overall, it should be noted that scientific studies are quite contradictory.
For example, here are the results of one scientific review of studies dedicated to the topic of fructose’s harm to health, where the authors, based on the analysis of 20 studies involving 344 people, draw the following conclusion:
“Fructose increases the levels of cholesterol in the blood, lactic acid, and triglycerides (fats), however, it does not seem to be more harmful than glucose in terms of its effect on insulin secretion, the formation of fatty liver, and the disruption of the lipid composition of the blood.
The benefit of fructose lies in its lack of impact on insulin levels in the blood and blood pressure. It may also be beneficial for weight control during weight loss.” 16
One of the controversial points is the claim that fructose is no more harmful than glucose in its effect on the formation of fatty liver and the disruption of the lipid composition of the blood.
As will be shown below, it is fructose, not glucose or sugar, that is largely responsible for these two important health indicators.
The question of the benefits of fructose for weight loss is also quite controversial and will be discussed below.
The reason for the greater harm of fructose compared to glucose (or sugar) lies in the differences in their metabolism. Through several mechanisms, fructose contributes to an increase in the amount of fat in the body, around internal organs, and in the blood, and hinders its burning for energy. This increases the risk of cardiovascular diseases, the formation of “fatty liver,” and other metabolic disorders.
Features of Fructose Metabolism
In the process of fructose absorption, a large portion of it is converted into glucose (29-54%), part of which is stored as glycogen in the liver and muscles, while another part is converted into lactic acid (about 28%). A very small percentage (~1%) of fructose is converted into fats 5,10.
Despite the fact that a significant portion of fructose is converted into glucose, the levels of sugar and insulin in the blood rise only slightly. However, as noted above, the level of triglycerides in the blood increases 7.
About 31-59% of fructose is oxidized, forming carbon dioxide within 3-6 hours after consumption 7.
This suggests that a possible explanation for the mechanism of increased triglyceride concentration in the blood when consuming large amounts of fructose may not be the stimulation of greater fat formation, but rather a shift in the priority of burning carbohydrates for energy instead of fats, which leads to an increase in the concentration of the latter in the blood 10.
The Harm of Fructose to the Liver, Cardiovascular System, and Metabolic Health
The accumulation of fat in the liver or the formation of so-called “fatty liver” indicates serious disruptions in the functioning of the body or “metabolic dysfunction,” more precisely – its final stage.
Known factors contributing to this process include the consumption of large amounts of fats, sugars, and alcohol.
The excess of fructose in the diet plays a greater role in the formation of fatty liver than a fatty diet.
Some scientists argue that fructose is harmful to the liver only when consumed in very large amounts 11. There is also an opinion that the degree of this harm does not exceed the degree of harm from consuming regular sugar 9.
However, the results of recent scientific studies indicate that the harm of fructose to health is dose-dependent, i.e., the greater the amount of fructose in the diet, the greater the harm.
Recommended: Milk: benefits and harms for the human body. Scientific research
Experiment: The more fructose in the diet, the more harm
In one of the experiments, scientists fed a group of young healthy individuals (without obesity) for 13 days with different doses of fructose: no fructose, 10%, 17.5%, and 25% of daily caloric intake from fructose.
The results were as follows 15:
- the young individuals gained body mass proportional to the amount of fructose in their diet;
- the concentration of triglycerides (fats associated with an increased risk of cardiovascular diseases) in the blood was proportional to the dose of fructose, meaning that even small amounts of fructose had a negative effect on health.
How can such an effect be explained?
Fructose, along with sugar and other carbohydrates, as well as fats, is a source of energy (= calories) for our body and therefore contributes to fat formation whenever the amount of calories consumed exceeds the need for them.
Moreover, decades of research indicate that regular consumption of fructose trains our body to absorb it more efficiently and convert it into triglycerides (fats), as the ability of intestinal cells to absorb it increases 11.
Scientific research indicates a dose-dependent harm of fructose: the more it is in the diet, the more harm; even small doses matter
Experiment: Fructose instead of glucose: impact on metabolic health
The results of the following experiment, which studied the effect of replacing the simplest sugar – glucose, with fructose, are indicative.
Participants included individuals with overweight and obesity who received 25% of their daily calories from sweet beverages: one group based on glucose, the other on fructose (112).
Both groups gained the same weight over the 10 weeks of the experiment; however, the impact on metabolic health was completely different.
- firstly, those who consumed fructose had a significantly greater increase in visceral fat mass (fat located between organs in the abdominal cavity).
- secondly, fructose consumption (but not glucose) led to a significant increase in triglyceride (fat) concentration in the blood.
- thirdly, the fructose group showed significantly higher concentrations of sugar and insulin in the blood after fasting, following a night’s sleep.
It is surprising how quickly metabolic disorders begin to manifest in the bodies of even healthy individuals when consuming excessive amounts of fructose: the rate of fat formation in the blood was 60% higher, and fat in the liver was 137% higher in a group of healthy men consuming fructose compared to those who received the same number of calories from complex carbohydrates 13.
The level of triglycerides in the blood remains elevated after consuming fructose for 24 hours 14.
Scientific studies indicate that consuming fructose instead of glucose increases the rate of fat formation around internal organs and the concentration of fats in the blood
We recommend: The harm of sugar to humans: sugar causes cancer. Scientific studies
Experiment: Exercise mitigates the negative effects of fructose
It is important to note that exercising, especially aerobic exercises, mitigates the negative effects of high doses of fructose. Aerobic exercises include activities such as running, cycling, swimming, and high-intensity interval training – one of the latest trends, exercises recognized as some of the most effective for weight loss and health.
One study showed that exercising four times a week did not result in the negative effects of excess fructose in the diet (amounting to 30% of daily calories). In particular, the level of triglycerides (fats) in the blood did not increase 11.
With regular exercise, especially aerobic exercises, the harm from fructose is significantly reduced
Experiment: Which is worse: excess fats in the diet or fructose?
It is interesting to compare diets high in fats on one hand, and fats and fructose on the other. The effects are largely similar, although there are significant differences 20.
A fatty diet leads to the development of obesity, insulin resistance, and some increase in liver fat, inflammatory processes in the body, but does not contribute to the development of fibrosis in the liver (replacement of liver tissue with connective tissue, resulting in the loss of liver-specific function).
A fast-food diet or a diet high in sugar (fructose) and fat stimulates the development of liver fibrosis, inflammatory processes in the body, and is even more effective in the development of obesity and fatty liver.
A diet rich in fats and sugars (including fructose) leads, among other negative consequences, to the development of liver fibrosis or loss of its function.
We recommend: Cholesterol in chicken eggs: how many eggs can an adult eat in a day?
The benefits of fructose. The effect of fructose on insulin levels
Insulin is a hormone, one of whose functions is to control blood sugar levels, i.e., glucose in the blood. But not fructose.
Unlike glucose, the effect of fructose on insulin levels is significantly weaker 17.
The benefits of fructose over sugar have been actively promoted due to its low glycemic index, which means that compared to sugar, it does not stimulate a sharp change in insulin levels in the blood.
This property is undoubtedly beneficial for people suffering from diabetes. A low insulin level is also recognized as one of the factors of metabolic health.
At the same time, as surprising as it may sound, some scientific data suggest that fructose in large amounts (just like sugars) can lead to the development of insulin resistance and impaired glucose tolerance – a condition that precedes diabetes 12.
Fructose, unlike sugar and glucose, does not affect insulin levels. However, just like sugar, it can lead to the development of insulin insensitivity.
Fructose instead of sugar in diabetes: is there any benefit?
In diabetes, fructose is recommended as a substitute for sugar to avoid a sharp increase in blood glucose levels. As noted above, fructose has a significantly lower glycemic index than sugar.
The question arises: how effective is fructose really?
Scientific research on this matter is also contradictory.
In one scientific review based on 11 studies involving about 280 people who consumed fructose in various doses (40-150 g per day) over different periods (from 2 to 10 weeks), it was noted that the effect of replacing sugar with fructose on blood sugar levels is very minimal 3.
Results from another review show different outcomes: replacing glucose or sugar in foods and beverages with fructose leads to a reduction in blood sugar and insulin levels, both in children and adults, with and without diabetes 4.
Scientific studies regarding the benefits of fructose in diabetes for controlling blood insulin levels are very contradictory.
Recommended: Which vitamins are better to take: synthetic or natural from food?
Is fructose beneficial for weight loss?
So, we have clarified the role of insulin. Its level in the blood is weakly dependent on the amount of fructose consumed.
Insulin plays an important role in weight loss as well as in gaining mass: the higher its level in the blood, the better the conditions for fat deposition, as it is responsible for controlling blood glucose levels and does this by sending excess glucose for storage in liver and muscle glycogen, as well as fat stores.
From this perspective, fructose is beneficial for weight loss.
But.
In addition to insulin, fructose does NOT affect the level of another important hormone – leptin, which plays a key role in the mechanism of obesity 17.
Leptin is a satiety hormone, secreted by adipose tissue, signaling the body about fullness and the need to stop eating. Disruption of its secretion results in uncontrolled appetite, as one of the mechanisms of its control is turned off, leading to obesity.
The levels of leptin and insulin in the blood are interconnected: insulin stimulates leptin secretion after a few hours. Therefore, when consuming sugar, which contains glucose, the appetite control mechanism works better.
The fact that fructose does not affect leptin concentration indicates that it more than sugar (or glucose) stimulates the consumption of excess calories, and thus leads to fat gain.
The role of leptin in weight loss or obesity is best observed in individuals whose leptin production is already disrupted, a condition medically referred to as leptin resistance: all of them have significant excess weight and body fat percentage 18,19.
Leptin resistance can develop as a result of consuming large amounts of sugar.
The benefits of fructose for weight loss are ambiguous. On one hand, it does not affect insulin levels, which is beneficial for weight loss; on the other hand, it disables the hormonal mechanism of appetite control, which is detrimental to weight loss.
Recommended: How to lose weight quickly and without harming your health? 4 principles based on modern scientific data
Fructose malabsorption
In some people, fructose is not fully absorbed. This condition is called malabsorption or fructose intolerance (from English fructose malabsorption) 1.
Interestingly, symptoms of malabsorption are most often not observed when fructose is consumed together with glucose (which we have in the case of table sugar).
The reasons for fructose intolerance are not fully understood by scientists. Among them are genetic changes that disrupt the process of creating transport molecules that facilitate its absorption 1.
Symptoms of fructose malabsorption include excessive gas production and discomfort in the digestive organs.
Additionally, chemical analysis of exhaled air shows an increased concentration of hydrogen, which is produced by bacteria in the intestines.
Fructose as a type of sugar is harmful, but not fructose found in natural fruits
The facts mentioned above regarding the harm of fructose primarily concern unnatural products with high fructose content. Natural fruits and vegetables are not included.
Why?
Primarily because the fructose content in them is very low. At the same time, they contain a significant amount of liquid and fiber, making it impossible to consume them in excessively large volumes, where the fructose concentration would reach a dangerous level.
Fructose from fruits is the smallest source of it in our diet. We get the most from refined products produced by humans, including in the form of table sugar.
We recommend: The most effective diets for weight loss: principles, myths, top 5 diets
Conclusion
The benefits and harms of fructose instead of sugar is a very controversial issue.
On one hand, fructose has an undeniable advantage as it does not cause sharp fluctuations in insulin levels, making it beneficial for people suffering from diabetes, as well as for anyone striving to be healthy.
The flip side is that due to the peculiarities of fructose metabolism, its consumption increases the concentration of fat particles in the blood, the percentage of fat around internal organs, and in the liver.
Moreover, it seems that the effect is dose-dependent, meaning the more fructose in the diet, the greater the harm.
A fairly popular opinion is that fructose is only dangerous in very large amounts and is no more harmful than sugar. But the concept of “very much” is always filled with a certain degree of relativity, and the characteristic effect of increased fat formation specifically associated with fructose is an indisputable scientific fact.
