For many years fructose was touted as a safer and healthier alternative to other sweeteners because it is low on the glycemic index. The glycemic index is a numerical index given to carbohydrate-rich foods that is based on the average increase in blood glucose levels after consumption. Fructose does not enter the blood stream as glucose, but is metabolized through the liver; therefore, it has negligible impact on blood glucose levels. But because fructose does not enter the blood stream as glucose, the glycemic index is not an accurate method to evaluate whether fructose is healthier or safer than other sweeteners. To evaluate the health effects of any sweetener it is important to have a basic understanding of sugars and how they are metabolized in the human body.
Sugaris a term for a class of edible crystalline carbohydrates—mainly sucrose, lactose, and fructose. Sucrose, also known as table sugar, is produced from sugar cane or sugar beets. Lactose is milk sugar and fructose is known as fruit sugar. Sugars are classified based on the number of monosaccharide units present. Mono means one and saccharide means sugar; thus a monosaccharide is the simplest form of the carbohydrate-sugar. Fructose is a monosaccharide and it is found naturally in many foods. Glucose and galactose are also dietary monosaccharides. Sucrose is a disaccharide containing one unit of glucose and one unit of fructose. Lactose is also a disaccharide, containing one unit of galactose and one unit of glucose. Honey, agave, tree fruits, berries, melons, and some root vegetables contain significant amounts of fructose, usually in combination with glucose. Most carbohydrate-sugars are a combination with varying amounts of fructose and glucose. Sucrose is made up of 50% glucose and 50% fructose. Honey is 55% fructose to 42% glucose. High-fructose corn syrup refers to a family of mixtures of varying amounts of fructose and glucose. In most commercially processed foods HFCS- 55 is used (containing 55% fructose and 42% glucose). Agave nectar is 92% fructose and 8% glucose. Crystalline fructose, which is produced from a fructose-enriched corn syrup, is the monosaccharide fructose, it contains no glucose. Crystalline fructose and high-fructose corn syrup are often confused as the same product, but they are not.
Fructose digestion, absorption, and metabolism
In food, fructose is found either as a monosaccharide (free fructose) or as a unit of a disaccharide (sucrose—the combination of glucose and fructose). Free fructose is absorbed directly by the intestine, then it enters the hepatic portal vein and is directed to the liver. When fructose is consumed in the form of a disaccharide (i.e., sucrose, honey, etc.) digestion occurs in the upper small intestine as the enzyme sucrase breaks the bond to yield one glucose unit and one fructose unit. The fructose is directed to the liver and the glucose directly enters the blood stream where the hormone insulin is required for cellular uptake. Glucose is either used for immediate energy needs or is stored for future energy (in the liver, muscle, or fat cells). Fructose metabolism in the liver is a complex multi-step process. In simple terms the liver converts the fructose to fat (triglycerides) and then transfers it out to lipoproteins for storage. Research has shown that the more fructose in the diet the higher the subsequent triglycerides levels in the blood. Fructose is referred to as the most lipogenic (fat producing) carbohydrate because it does not enter the blood stream as glucose but instead must be converted to fat (triglycerides) by the liver. Elevated triglycerides are a risk factor for insulin resistance, vascular disease, and diabetes. When the liver is metabolizing fructose it appears to cause a traffic jam, blocking the metabolism of glucose and the synthesis of glycogen (the storage form of glucose that is made by the liver). As a result, blood glucose levels remain higher for longer, requiring the pancreas to secrete more insulin. This too has negative effects on health as both elevated insulin and glucose are associated with some of the same health risks as elevated triglycerides.
All sugars pose potential health risks
Sucrose and HFCS-55 are both effectively half glucose and half fructose. In digestion and metabolism, the fructose will stimulate the liver to produce triglycerides while the glucose will stimulate insulin secretion. High blood levels of insulin increase body fat storage, block the ability to burn fat for energy, and increases the risk for all modern-day degenerative diseases (i.e., cancer, Alzheimer’s, diabetes, heart disease, high blood pressure, etc.). Both glucose and fructose cause the formation of advanced glycation end-products—AGEs, the glomming together of proteins in cells and tissues. AGEs accelerate the aging process in the human body by damaging collagen and connective tissues.
According to United States Department of Agriculture (USDA) the average American now consumes 20 teaspoons of added sugar per day. It is estimated that currently in the United States people consume 135 lbs of sugar per person per year, whereas in the early 1900’s the average consumption was only 5 lbs per person per year. Most of this added sugar comes in the form of HFCS found in soda and processed foods. Over the past 35 years we have seen a continual rise in obesity, heart disease, and diabetes.
Is fructose the cause of obesity, heart disease, and diabetes?
Fructose is the sweetest of all naturally-occurring carbohydrates—it has a high relative sweetness compared to glucose, which is one reason it is used so frequently in food processing. The other reason is its low cost. There is a correlation between eating a diet high in refined sugars (especially HFCS) and carbohydrates and the development of obesity, heart disease, and diabetes, but over consuming sugar in any form is bad for human health. We are just not meant to consume sugars in high quantities; both fructose and glucose can have negative health effects.
A healthy diet rich in vegetables, fruit, quality protein, and healthy fat supports good health. Sugar in all its forms should be limited and consumed only occasionally as a special treat. Eliminating refined carbohydrates and processed sugars will improve your health and help prevent modern-day degenerative diseases.