When most people think of sugar, they likely imagine the tiny crystals that make our favorite desserts taste the way they do. However, there are many different categories of sugar, and we’re going to look at two of the most prominent, fructose and glucose, and dive into what they are and how they impact our physical health.
Fructose is a type of sugar that is common in naturally occurring foods like berries, apples, and honey. Like glucose and galactose (milk sugar), fructose is made of only one sugar group called a monosaccharide. Glucose, however, is metabolized by cells throughout the body and is essential as an energy source for human life. Fructose, meanwhile, is primarily handled by the liver, where when consumed in very large doses may cause harm by depositing fat in the liver and causing changes in glucose handling and insulin responses.
What is sucrose?
When a chemical bond unites glucose and fructose, they become another type of sugar called sucrose, better known as table sugar. When we eat this kind of sugar, the sucrose molecules need to be split into their component parts, fructose and glucose, before it can be absorbed. Sucrose is produced by plants, like sugarcane, and refined by crushing the plant and extracting the sugar crystals from the resulting liquid.
Sucrose is known as table sugar due to its common use as a household sweetener and consumer product. Unlike fructose and glucose, known as “monosaccharides,” which are each made of one sugar group, sucrose is a “disaccharide,” meaning two sugar groups. These exist within the broader category of “polysaccharides,” which are made of multiple sugar groups. Within the gut, sucrose is quickly split into fructose and glucose by an enzyme named sucrase, whose purpose is to free the monosaccharide subcomponents of sucrose so we can use them for energy. So, while fructose and glucose are very similar and are both monosaccharides, they have completely different metabolic pathways and are processed differently in the body.
What is fructose?
As mentioned above, fructose is a naturally occurring monosaccharide that can be found in natural food products like fruits and honey. Although most people can digest fructose without any problems, it is worth mentioning that some people can have fructose intolerance and fructose malabsorption. This would cause a significant difference between digesting fructose or sucrose instead of glucose. Even for normal people, however, the western and specifically American diet is full of fructose in the form of high-fructose corn syrup (HFCS) as found in processed foods. HFCS is a highly concentrated form of sugar that, like all sugars, can be harmful if consumed in excess. Consumption of any form of sugar in excess can, over the long run, do damage to vital organs such as the pancreas and kidneys.
Here is a shortlist of natural sources of fructose as well as list of commonly processed foods to which HFCS is typically added:
Natural sources of fructose
- Fruits and vegetables
- Tree sap
Foods to which HFCS is typically added
- Ice cream
- Snack bars
- Energy drinks
Today you can often find specialty products without HFCS, with artificial sweeteners, or without added sugars entirely. Different foods can use different blends of HFCS, and the most popular are made from 42% fructose (58% glucose/maltose) or 55% fructose (45% glucose/maltose), as measured by the proportion of each in terms of dry weight. For example, sucrose is a disaccharide made from glucose and fructose, so its proportion would be 50% of each.
What is the difference between fructose and glucose?
Turning towards how the body handles these sugars, the hormone insulin is responsible for transporting glucose from the blood into individual cells. Chronically high blood sugar can cause insulin resistance and other chronic conditions like diabetes. Fructose, however, is partially digested by the liver where it can be processed into glycogen, or stored glucose, and fats. The ingestion of fructose does not immediately trigger insulin release from the pancreas, so it may appear to be preferable as a sweetener in some populations. However, super-doses of fructose, as can be consumed if you drink sugary soda, for example, can overwhelm the body’s capacity to digest it. Since sucrose is made from both glucose and fructose, we can have a similar problem if large amounts of sucrose are consumed at once. So within the context of limiting your overall intake of sweeteners, it is also worth considering whether fructose and especially HFCS should be given particular scrutiny in your diet.
Why fructose may be bad for metabolic health
While natural sources of fructose, just like natural sources of glucose, are preferable to highly processed sources, a low overall proportion of sugar in naturally occurring foods is more important than the type of sugar. For example, HFCS, which includes fructose refined from corn, is no different from table sugar because of the overall quantity of glucose that will eventually make its way into the blood. Therefore, when we talk about the consumption of HFCS, we are talking about highly refined sugar and its effects on health.
What makes fructose so unique from a metabolic health viewpoint is that it bypasses the first metabolic step that glucose undergoes, and that first step serves as a regulatory checkpoint. So when this step gets bypassed, fructose readily gets taken up by the liver regardless of the quantities that are consumed. Therefore, diets that are high in fructose, such as the Standard American diet, can lead to excessive fatty acid synthesis in the liver. This can lead to excessive lipid accumulation in the liver, causing what is called non-alcoholic fatty liver disease (NAFLD). NAFLD reduces our insulin sensitivity, decreases glucose tolerances, and can lead to hepatic insulin resistance.
Additionally, there may also be a role played by both lower circulating insulin and leptin in response to fructose consumption. If you have less leptin in your blood, you will be more hungry and have more difficulty controlling your appetite. So any kind of food that doesn’t cause a strong response in leptin, like many snack foods with HFCS added to them, should be first on the chopping block when looking at ways to get the most metabolic bang for your buck.
Another large review of research papers comparing fructose to other forms of sugar concluded that fructose does not cause weight gain when substituted for other sugars. In that study, the authors also conclude that fructose that constitutes excessive feeding still leads to weight gain. For most people, moderate amounts of fructose are appropriate when consumed in the form of whole foods, or a few servings of fruit per day. However, you might want to limit fructose further if you show signs of insulin resistance and you should consult your doctor about calibrating the amount of fructose in your diet. Even healthy people should limit their intake of refined sugar, including fructose and especially in the form of sucrose and HFCS.
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