Despite all of the negative press about sugar, our bodies need sugar to survive. It is the main source of energy used to maintain cellular function and growth. However, the form or chemical structure of the sugar consumed can be important – sucrose, fructose and glucose are the main types that I’ll discuss here. Also of importance is the source of the sugar – is it freely added as in added table sugar or consumed as the natural parts of fruits and vegetables or added to processed food as either a preservative or flavour enhancer. Another issue is the amount consumed – how much is too much?
Sugar and Science
It's well known that most of us in the western world are consuming much more sugar than we used to and more than the recommended daily amounts. However, finding conclusive evidence linking sugar as a cause of increasing cardiovascular disease (CVD) and other health problems is complicated. One reason is the difficulty in conducting research in human nutrition. Many studies are epidemiological in design, meaning that they are essentially surveys involving many participants. This type of design can show that higher sugar consumption may occur with increased incidence of CVD but not that it caused it – a detail mostly overlooked in the media. Studies that can demonstrate a causal relationship – randomized control studies are an example – are expensive to conduct and may be limited in scope because of the smaller number of participants involved. Additionally, the complexity of the human metabolism makes it difficult to determine the mechanisms of control or how a certain substance (i.e. sugar) causes an effect. Another factor is that sugar comes in different forms and they are not metabolized in the same way. So, different sugars – different effects.
Current dietary guidelines for reducing the risk of cardiovascular disease (CVD) have focused on decreasing the amount of saturated fats we consume and replacing them with more carbohydrates (sugars and starches). However, there is increasing interest in the effects of carbohydrate on the risk factors for metabolic syndrome, type II diabetes, and CVD. In fact, you don’t need to go too far in either conventional media or social media to hear all sorts of advice/opinion on the evil effects of sugar on our health.
Main Types of Sugar
Sucrose is the fancy name for table sugar and is usually made from sugar cane or sugar beets. It is a disaccharide, which, when metabolized by the body, produces the simpler sugars glucose and fructose.
Glucose, also known as dextrose or grape sugar, is a simple sugar found in plants and honey. Glucose is also produced from the metabolism of carbohydrates by the body. It is the form of sugar that is necessary for maintaining cellular function and growth.
Fructose is found naturally in honey, tree and vine fruits, flowers, berries, and most root vegetables. Commercially available fructose (used in processed foods and beverages) is made from sugar cane, sugar beets, and most commonly, high fructose corn syrup (HFCS). Unlike glucose, dietary fructose is not necessary for any human biochemical reaction.
Regardless of the type of sugar consumed, the energy content or calories is generally identical. However, the Glycemic Index for glucose and fructose is different. Fructose has a much lower GI, leading many people to believe that it is a healthier option. This isn’t necessarily the case as the next section will explain.
Metabolic pathways (or what happens when sugar is digested)
Although the energy content of the different sugars is similar, how they are metabolized and where the energy is stored is different.
When we consume glucose, approximately 20% enters the liver. The remaining 80% enters via the blood circulatory system raising the blood glucose levels which results in insulin release. A series of biochemical reactions sets in motion three results: a lowering of the blood glucose levels; the conversion of glucose to glycogen for storage and as a source of energy for cells; and conversion of the remaining small amount into VLDL cholesterol. VLDL contains the highest amount of triglyceride (a fat found in the blood), so having a high VLDL level means you may have an increased risk of coronary artery disease, which can lead to a heart attack or stroke. Higher amounts and large VLDL particles are also associated with an increased risk of high blood pressure and stroke. However, since only 2% of ingested glucose becomes VLDL, glucose contributes extremely slowly to these potential health problems.
Fructose is metabolized entirely in the liver. At high consumption levels fructose exerts 3 different negative impacts on human metabolism, each of which is exclusive of its calories (so it’s not just empty calories).
- It converts to fat (VLDL) at a much higher rate than glucose, which can result in the conditions ‘fatty liver’ or hepatic steatosis, and insulin resistance (a precursor to metabolic syndrome).
- It can produce excess levels of free radicals which increase the risk of cellular damage if not quenched by antioxidants (i.e. Vit C, Vit E).
- It can affect how much we eat. It is routinely added to food to increase palatability thereby encouraging us to eat more. Additionally, it can undermine the normal satiety signals the body produces when we eat, by inducing other biochemical changes.
What it all means
- First, sugar is not the enemy – we need sugar (or more specifically, glucose) in our diet to maintain good cellular health. But this sugar generally refers to sugar occurring naturally in fruits and vegetables, not the sugar added to processed food and beverages.
- Secondly, we consume too much sugar. Current average consumption of sugar is about 130 lbs/year! The recommended daily consumption is 6 – 9 teaspoons. To put this in perspective, one can of cola or two pop tarts exceeds this daily recommendation.
- Thirdly, the type of sugar that we consume (fructose vs glucose) is probably only important if we consume more than recommended daily amounts of added sugar. The research on fructose suggests that it is a health risk, but only at higher levels of consumption. It is the fructose added to our food (typically HFCS) that is the problem, not the fructose which naturally occurs in fruit and veggies (unless you’re eating copious amounts of honey, dates, etc).
Is honey, agave or other sugar substitutes healthier than table sugar? No, all contain glucose and fructose. There is no reliable scientific evidence that the body can tell the source of the glucose/fructose. Again, it’s the amount that is the critical factor.
Akram, M. And A. Hamid, ‘Mini review on fructose metabolism’, Obesity Research & Clinical Practice, 7: 89-94, 2013.
Lustig, Robert H., ‘Fructose: It’s “Alcohol without the Buzz”’, American Society for Nutrition, Advances in Nutrition. 4:226-235, 2013.