Does fructose make us fat? Really?

After yet another inaccurate description of fructose and its role in the body appeared in the Sunday papers this weekend, I thought it timely to provide a slightly more scientific view of the role of fructose in the diet and its proposed link to weight gain and obesity.

Fructose is one of the simple sugars, found primarily in fruit and vegetables – fructose is also one of the two simple sugars that combine to give sucrose or table sugar. It is argued that as fructose is metabolized by the liver, excessive consumption promotes insulin resistance and weight gain more readily than other types of carbohydrates.

This simplistic view of metabolism and food unfortunately barely touches the surface when it comes to understanding the complexities of food intake, digestion and metabolism. It also must be remembered that much of this hypothesis stems from the USA’s use of high fructose corn syrup, particularly in soda drinks – an ingredient rarely used here in Australia – and yet we remain, like the US very fat. (http://www.nytimes.com/2011/04/17/magazine/mag-17Sugar-t.html?pagewanted=1&_r=3)

Perhaps the argument would hold if we only ate fructose, but we do not. We eat complex mixes of carbohydrates in the diet which makes it extremely difficult to isolate the specific role of fructose as a potential promoter of obesity. In fact, food intake analysis suggests that fructose intake in Australia has not increased yet obesity rates continue to.

A more likely explanation in the increase in processed high GI foods including white breads, breakfast cereals, snack foods, juices and soft drinks which are all high GI – high GI foods as a whole result in an increased insulin response, which over time is linked to obesity.

Fructose of course is found primarily in fresh fruits and vegetables, most of which have a low GI and are all foods which have the highest nutrient density and lowest energy density of most of the foods we routinely consume. Based on this, it could be argued that it is somewhat irresponsible to declare them as foods that contribute directly to the obesity crisis. You do not need a nutrition degree to work to work out that sugary, processed foods are not good for us, but to blame fructose is jumping on a band wagon of something you know nothing about.

So, to end, a couple of comments from a dietitian mate who actually specializes in this area after she responded to an initial article published in a Sunday paper a few months back that blamed high fructose corn syrup for Australia’s weight issues:

Dear Editor,

I read the piece "Not so sweet" with disbelief at the incorrect information stated.
As a dietitian who works with gastroenterology and liver disease, I was amazed to hear that the "poor old liver" can't cope with fructose, or the caffeine, alcohol and medications our "toxic" modern lifestyle provides it. The liver, despite common perception, does not require "cleansing", rather, it has specific metabolic mechanisms to cope with cleansing itself.

I think some misleading points need to be clarified:

1. Fructose is NOT used in artificially sweetened beverages. Fructose contributes to total calories so cannot be used in "diet" products that usually contain aspartame or phenylalanine as sweeteners.
2. High fructose corn syrup is 55% fructose and 45% glucose and does not significantly increase blood glucose or insulin levels when compared to sucrose (table sugar, which is 50% fructose and 50% glucose) [Melanson, K.; et al. (2006). "Eating Rate and Satiation.". Obesity Society (NAASO) 2006 Annual Meeting, October 20–24,Hynes Convention Center, Boston, Massachusetts]

I am not in any way advocating a high fructose diet, or a high sugar diet, but I think nutrition experts would agree that a diet containing large amounts of added sugar (whether this be sucrose, glucose, fructose or any others) is a contributor to excess calories, which is essentially what causes weight gain when coupled with the sedentary lifestyle we lead. This could be said of any component of food when consumed in excess - too much protein, fat, alcohol or complex carbohydrates can do the same. Personally, I think sugar has a place in the diet (however, most of us would easily eat enough sugar without adding extras to our coffees, eating sweet treats after dinner, or curing our 3:30-itis!), and many foods that are high in fructose are perfectly safe to include in a nutritionally balanced diet.

Scientific aspects of nutrition and metabolism took several semesters of university level biochemistry for me to gain even a partial understanding. Pathways are complex and intertwined, and it is rarely such a two-dimensional process that was presented in this article