Sugar broken down: Glucose vs Fructose and does it really matter?
For the month of March, our resident nutritionist Melinda Bakker is sharing her thoughts on sugar and some of the new research emerging on the dangers of the sweet stuff. This is part one in a two-part series on sugar. Also, for more of Melinda’s health insights, be sure to check out our Twitter feed where she shares news and handy tips and facts!
In my last post I talked about how it is deceptive to think of added sugar as just ‘empty calories’. This week, I will talk about what happens in your body when you consume high levels of added sugar and the health risks associated with increased sugar intake.
In order to understand the effects of added sugar, it’s important to first consider its composition. Refined sugar comes in two primary forms: there’s sucrose, commonly known as table sugar, and high fructose corn syrup (HCFS, also called glucose-fructose on food labels). Sucrose is made up of one glucose plus one fructose. Most of the high fructose corn syrup that we consume is made up of 42% glucose and 55% fructose, although another form of HFCS contains 53% glucose and 42% fructose. So both sucrose and HFCS have roughly equal proportions of glucose to fructose. Fructose and glucose are metabolized very differently by your body and I will discuss the metabolism of each in turn.
Glucose can be used for energy by every cell in the body. Following digestion and absorption of sugar, only a fraction of the ingested glucose (~20%) is taken up by the liver, while the rest enters the peripheral circulation6. The glucose from sugar raises your blood sugar levels and causes insulin to be released. Insulin helps bring down your blood sugar by stimulating the muscle and fat tissue to take up glucose and it also promotes storage of glucose -as glycogen- in the liver. Over time, high glycemic loads can lead to the development of insulin resistance and inflammation7.
Fructose is metabolized quite differently. Fructose does not increase blood sugar or generate the release of insulin, however it has other damaging effects which are thought to be more harmful than those of glucose at high intake levels. The vast majority of fructose is quickly processed by the liver, where it promotes synthesis of fats5. As a result fat droplets accumulate in the liver which is linked to the development of insulin resistance over time5,6. Some of the fats leave the liver, resulting in high levels of triglycerides and “bad” cholesterol in your blood8. These fats tend to be taken up by adipose tissue around the abdominal area, increasing belly fat5. High levels of fructose consumption have also been found to raise blood pressure9,10.
It’s important to note that these metabolic pathways can vary depending on the individual and their energy state6. For example, for someone who is thin, insulin sensitive and fasting, very little fructose has been found to be converted to fat. However, if someone is obese, insulin resistant, and recently fed, then added fructose is converted to fat at a high rate (~30%). For the majority of us who stand to lose a little extra weight and reduce our caloric intake, we are probably closer to the second situation, although it’s likely a graded response.
There is also some evidence that fructose causes us to overeat. In one study that directly compared the effects of fructose to glucose, drinking a sugar beverage containing 100% fructose was found to alter blood flow to areas of the brain involved in appetite regulation and drinking the fructose beverage did not increase feelings of fullness, unlike the glucose drink11. Furthermore, animal studies repeatedly show that fructose increases food intake and body weight more than glucose12.
While the adverse effects of isolated fructose compared to glucose are a hot topic for scientific research and provide some interesting insights into their differences in metabolism, the bottom line is that the sugars typically added to foods (HFCS, sucrose) contain roughly equal amounts of glucose and fructose, so we are simultaneously exposed to both. What do research studies tell us about the health effects of added sugars on risk of chronic disease?
- Many studies have linked increased sugar consumption to weight gain13. A review commissioned by the World Health Organization (WHO) found that decreased intake of sugars was associated with a reduced weight while increased sugar intake was associated with higher weight14. Furthermore, a review of observational studies in children found that those with higher intakes of sugar sweetened beverages had a 55% increased risk of being overweight or obese compared to those with the lowest intake.
- Regular consumption of sugar sweetened beverages was associated with higher blood pressure15, which may be due to increased levels of uric acid in the serum16,17.
- Consumption of increased sugar sweetened beverages (most often 1-2 servings/day) was associated with a 26 % increase in risk for developing type 2 diabetes as well as a 20% greater risk for metabolic syndrome (a group of conditions including high blood pressure, increased blood sugar, excess abdominal fat and abnormal cholesterol levels)18.
- Increased intake of sugar sweetened beverages was linked to increased risk of coronary heart disease. Just one serving per day increases the risk by 16%19.
In summary, consumption of added sugar is a major contributor to weight gain as well as other chronic health conditions. The effects of sugar go beyond the ‘empty calories’. It’s time that we cut out added sugars from our diet, switching from sugar laden processed foods to natural, whole foods.