Is Fructose Making Us Fat? by Syrone Burr
The increase in obesity has become more pronounced in recent decades. The reason for this increase is often attributed to basic energy imbalance i.e. we are eating more calories than we are burning off through activity, and this may be true for many of us, but could there be more to it than that? Are all calories really equal? Well the answer to the question, really depends on its interpretation. From a metabolic perspective, many researchers studying carbohydrate metabolism, in particular the metabolism of fructose, believe the answer may be no.
Obesity studies in the USA have found significant correlation between increases in the intake of High Fructose Corn Syrup (HFCS) and the rise in obesity. HFCS was introduced into the US food supply in 1970 as a cheaper and sweeter substitute to sucrose, and its use increased one hundred fold between 1970 and 1990 and currently represents 40 percent of all caloric sweeteners. This increase far exceeds changes in intake of any other food or food group and also happened to precede the rapid increase in the prevalence of obesity. For this reason many researchers have been interested in the potential link between increased HFCS consumption and the epidemic of obesity.
What’s so bad about fructose?
Fructose is a naturally occurring sugar found in fruits, vegetables and honey and has been part of the human diet for hundreds of years. HFCS is produced by enzymatic isomerization and contains both glucose and fructose. So how is the fructose found in HFCS different from fructose naturally found in food? The answer is that there is no difference. The fructose is the same whether it is found naturally or is produced industrially by enzymatic isomerization, however the concern with HFCS is in part due to the sheer volume that has been introduced to food products and the fact that it is so pervasive that it can be difficult to find food products in which it is not contained.
Fructose metabolism and satiety
Glucose and fructose are metabolized in very different ways, with fructose mainly being metabolized in the liver. Of particular concern is the fact that fructose is able to bypass the main regulatory step in glycolysis, facilitated by the enzyme phosphofructokinase, and in doing so effectively ‘forces’ glycolysis, resulting in de novo lipogenesis.
Fructose also does not stimulate the release of insulin, which acts in the body as a satiety signal to the brain, releasing hormones into the circulation, which inhibits further food intake. Insulin is also instrumental in the release of leptin, a long-term regulator of food intake. Leptin feedback to the hypothalamus, signals available energy stores, while low leptin feedback signals starvation. Without insulin, and without leptin the brain believes the body to be starving and not only fails to release a satiety signal in response to the fructose intake but instead sends out hunger signals causing an increased intake of calories.
So is fructose making us fat? Research suggests that a diet high in fructose could, not only increase the amount of fat synthesized in the body but could also cause overconsumption. Further research is needed but until then, if you are concerned about overweight it may be wise to reduce your consumption of added sugars and sugar sweetened beverages. Eating a healthy and balanced diet in conjunction with regular exercise will help to establish and maintain a healthy weight.
Syrone Burr is a year 2 BSc Nutrition and Dietetics student interested in the role of nutritional factors in managing chronic illness, in particular the condition cystic fibrosis and I am a (volunteer) dietetics research assistant at Royal Hampshire County Hospital.