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Fructose - challenging the myths


In the past, the sugar component of most sweetened beverages was obtained either from sugar beet or cane sugar. For both sugar cane and sugar beet, the sugar found is called sucrose and it is one molecule of glucose linked to one molecule of fructose. In the late 1960s, an alternative to sugar was developed which was cheaper to use and less prone to the volatility of the global sugar market in terms of price and volume. This alternative is known as High Fructose Corn Syrup (HFCS) and it is produced in two stages. Ordinary cornstarch is first broken down to its basic constituent glucose and the glucose is then treated with a natural enzyme that converts it to fructose. The two sugars, glucose and fructose can now be blended yielding mixes with varying ratios. For the soft drinks industry, the ratio is 55% fructose and 45% glucose.

For some time now, various “experts” have come down heavily on fructose for its deleterious effects on health. Google the word ‘fructose and within the top three sites you find “Fructose-Sweet but dangerous” and “Sugar may be bad but fructose is far more deadly”. The latter has a link to a You Tube video by Robert Lustig, a professor of pediatrics at the University of California, San Francisco. The video is entitled “Sugar – the bitter truth”. In his video, he outlines his belief that our modern food supply is  “adulterated, contaminated, poisons and tainted” and the culprit is fructose. It really is difficult to understand the use of these terms by a medical professional. These are intended to get people scared and to grab their attention so that the learned doctor can give you the diagnosis and the cure. He goes on to ask the audience: ”What is in coke”. First he tells us that there is caffeine there, which, being a mild diuretic will make us thirsty and he then goes on to say that a can of coke has 55mg of sodium, which he says “is like drinking liquid pizza”. As far as I can tell from a 30 second search of the USDA food composition on-line database, an average serving of pizza from a fast food chain would contain about 800 mg of sodium. Why does the good doctor choose to distort the facts? He argues that the sodium and the caffeine diuretic are designed to make you thirsty. He then goes on to ask his audience why sugar is added to Coke. Why, of course, it’s to mask the salt content!

The good doctor goes on to explain that what fructose does to your blood proteins is what happens to your steak when it goes brown. Wrong again doc. When red meat is cooked, the iron containing protein in muscle, myoglobin, begins to lose its iron and as the temperature rises, the myoglobin loses more and more iron, thus turning brown. White meat such as chicken and pork do not brown in cooking because they contain much less myoglobin. What Prof Lustig probably meant was that the effect of fructose on blood proteins is, at a considerable stretch of the imagination, similar to the browning of toast where sugars react with proteins under the effect of strong heat.  Of course you don’t toast your blood so the analogy is a bit far fetched.
From the scientific point of view, a very recent systematic review of the literature of controlled human feeding studies, shows that among diabetics, where high sugar levels can react with proteins, causing undesirable clinical consequences, substituting fructose for other forms of carbohydrates actually lowers this process known as glycation[1]. This work was funded by the Canadian Institute of Health Research and completed by Professor David Jenkins, a world authority on diet and blood glucose. In a previous set of correspondence in the Journal of the American Dietetic Association, Jenkin’s challenges Lustig’s hypothesis that fructose promotes the accumulation of fat in the liver and adipose tissue in an article entitled “Is fructose a story of mice but not men?” The authors point out that studies in animals need to very carefully interpreted before the findings of such studies are extrapolated to man. Thus in mice and rats, the conversion of carbohydrates to fats can reach 70%. However, in man, the efficiency of conversion of carbohydrates to fat is much lower at about 5%. They point out that in studies with very high intakes of fructose in humans, this effect can double the rate of conversion of carbohydrate to fat, but the levels remain at 10%, a fraction of what is seen in experimental animals. Moreover, animal models often use very high levels of fructose well above (upwards of 6 times) that normally associated with human diets.

The idea that in man, carbohydrate conversion to fat is low is often greeted with surprise if not disbelief; so a little explanation is worthwhile. Many animals have the capacity to convert carbohydrate to fat but when physiologists started to look at this in humans using live-in calorimeters to study precise changes in the oxidation of fat and carbohydrate, they found that when excess calories were consumed as carbohydrate, the oxidation of body fat stores fell dramatically and that the excess carbohydrate calories were used for fuel thus sparing the fat. Thus if you require 2,500 calories a day from a typical mixed diet (fat 35%, carbohydrate 50% and protein 15%) and you consume an additional 200 calories of carbohydrate, instead of using all the ingested fat (35% of calories would be about 875 calories or 97 grams), you would only use 675 calories from fat, allowing 22 grams of fat to be spared. So, carbohydrate spares the burning of fat in man but it is poorly converted to fat in humans, even if it is as fructose and with high doses of fructose.

Sugar bashing is popular and fructose bashing is even more fun but basically built on very bad science. But, why let bad science get in the way of a good story or a nice video with 2.6 million viewings?




[1] Effect of fructose on glycemic control in diabetes. A systematic review and meta-analysis of controlled feeding trials. Diabetes Care Volume 35, July 2012

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