OFFICE OF SCIENCE OUTREACH
DEPARTMENT OF BIOLOGY
Obesity, Type 2 Diabetes, and Fructose
Part 4: The Sources of Fructose
We showed earlier that High Fructose Corn Syrup was invented in the early 1970s, and began to replace regular sugar in prepared foods around 1975. Thus, the onset of the obesity epidemic coincides with the use of HFCS. This correlation has led many people to conclude that HFCS is responsible for the obesity epidemic and the consequent diabetes epidemic.
The food industry has responded with an advertising campaign that tells us that HFCS is fine; it is equivalent to our old friend, regular sugar. This is true. But, rather than conclude that HFCS is good, it might be more appropriate to conclude that sugar is also bad. We will show below that any and all natural sweeteners -- sugars -- are nutritionally equivalent.
Regular table sugar is sucrose. It is a double-sugar, or disaccharide. It is composed of one molecule of glucose bonded to one molecule of fructose. Our intestinal cells are unable to absorb sucrose, however. They can absorb glucose and fructose, but not the disaccharide, sucrose. Therefore, our digestive system produces the enzyme, sucrase, which digests sucrose into glucose and fructose.
Digestion occurs quite rapidly in the intestine. Therefore, when we eat regular sugar ("table sugar," sucrose), we are essentially eating a 1:1 mix of glucose and fructose.
Sugar cane and sugar beets are the two most economically-important sources of sucrose. They produce sucrose in large enough quantities that it is relatively easy to purify it from them. But sucrose production is not limited to sugar cane and sugar beets. Plants produce sucrose in their leaves, from glucose made during photosynthesis. The leaf cells then export the sucrose to the plant sap (analogous to the plant's bloodstream), through which the sucrose is transported to the other parts of the plant. In particular, sucrose is essential for root growth and the growth of new shoots and leaves, and of flowers and seeds. Roots are underground; not being exposed to light, they cannot perform photosynthesis. Therefore, the leaves feed them. Flowers and seeds lack chlorophyll, and also need to be fed. At the growing tips of the shoots, cells are immature and lack functional chloroplasts. They too need food from the leaves.
Sucrose, then, is common in plants. It is usually fairly dilute. To obtain maple syrup from the sucrose-containing sap of maple trees, we must collect very large quantities of sap and boil away most of the water to produce a small amount of concentrated sucrose solution.
We should not be fooled by the extent of purification. Refined sugar, raw sugar, cane juice, corn syrup, rice syrup, and agave nectar, are all sucrose. It's the same chemical whether it's been left in the form of concentrated syrup, or has been partially-purified (raw sugar), or whether it has been re-crystallized (refined sugar).
It is, perhaps, an oversimplification to say that all of the "natural sweeteners" mentioned above are sucrose. Plants also contain sucrase, and can break sucrose down to glucose and fructose. Seeds, in particular, use this enzyme to convert the sucrose they obtain from the sap into the glucose they use to build starch. Starch is the concentrated, tasteless, storage-form of carbohydrates in plants, and is the major nutrient in seeds.
The corn-on-the-cob that we eat in the late summer is immature corn. The seeds have not quite reached their full size, and they have not yet converted all of the sucrose into starch. In fact, sweet corn carries a mutation that inactivates one of the two enzymes that build starch from glucose. Consequently, its sucrose, glucose, and fructose concentrations are higher than in normal corn ("field corn.") Super-sweet corn carries this mutation plus a second mutation that inactivates the other enzyme that builds starch from glucose. Consequently, it is even sweeter. But neither of these is the "real" corn that is grown over thousands of square miles for commercial use. That is field corn, which converts all of the sucrose to starch. The dry, mature seeds of field corn are nice and plump, packed full of starch. The seeds of sweet corn shrivel somewhat during maturation; those of super-sweet corn shrivel tremendously. As water is drawn out of the seeds during maturation, the starch remains trapped in the seeds; most of the sugars flow out with the water, and the seeds wrinkle. This is analogous to the round vs wrinkled pea seeds that Gregor Mendel studied.
Therefore, to produce corn syrup, manufacturers break down the starch into glucose — essentially the same digestion reaction that we use in our digestive systems when we eat starch.
But glucose is not particularly sweet. The particular sugar that our sweet-sensing taste buds recognize is fructose. Sucrose tastes sweet because the molecule contains a fructose unit. Pure fructose is even sweeter, because it can interact with our taste buds more strongly. Therefore, manufacturers treat the glucose syrup with an enzyme, to produce fructose. [Glucose and fructose can inter-convert readily, even without the enzyme. However, the enzyme speeds up the reaction tremendously.]
At equilibrium, this reaction produces a mixture of glucose and fructose that is about 55% fructose and 45% glucose. This is the same ratio that is in honey and fruit, though these also contain some sucrose and other sugars. To produce high-fructose corn syrup, manufacturers separate the fructose from the glucose by chromatography. The fructose is now at about 90%. The glucose can be re-treated with the enzyme to convert more of it to fructose.
Mixing the purified fructose with glucose, or using the initial equilibrium mixture, gives HFCS42 (42% fructose), HFCS55 (55% fructose), and HFCS90 (90% fructose). The sweetness of these mixtures increases with the percentage of fructose.
Because of corn subsidies by the government, and import tariffs on Brazilian sucrose, HFCS is much less expensive than normal sugar. It is also much sweeter. Consequently, HFCS has replaced sugar in most processed, sweetened foods and drinks.
With the exception of artificial sweeteners like aspartame, acesulfame, sucralose, and saccharin, and the natural sweet compounds from stevia, all sweetened foods contain either sucrose or HFCS. As described above, even normal sugar is digested to a glucose/fructose mixture that we could call HFCS50 (that is, if we wanted to refer to our intestinal contents in such terms.)
Food Manufacturers add Sugars to Many Foods
The number of processed foods containing HFCS, sucrose, or one of the less-highly-refined preparations of sucrose (cane syrup, rice syrup, agave nectar, etc) is astonishingly great. It ranges from sweet drinks to sports supplements to yogurt to ketchup to barbecue sauce, and even to a surprising number of products intended as the main course of a meal -- sweetened meat mixtures. Many of these products have been "reformulated" to reduce the amount of fat. Of course, removing the fat makes the material unpalatable, so manufacturers add sweeteners to make them attractive to us.
The 2001–04 National Health and Nutrition Examination Survey reports that Americans eat an average of 22.2 teaspoons of added sugars per day. The American Heart Association recommends no more than 6 teaspoons for women and 9 for men (men, on average, being larger). This recommendation comes from the types of analysis described here: sugar, and particularly the fructose component, are a major source of weight gain and a major contributor to metabolic syndrome.
Most of us don't get out the sugar bowl (or HFCS bottle) when we sit down to dinner. How can we cut back on added sugars, if we don't add them ourselves? The key is to read the list of ingredients of any processed foods you purchase. If the product contains any of the fructose-sources mentioned here, think very hard about whether you want to buy it. In addition, it is important to recognize that soft drinks are the primary source of sugar for Americans. Fruit juices are also contributors, since they are either sweetened with one of the products mentioned here, or mixed with white grape juice, which is rich in sugar on its own (the glucose/fructose mix of natural fruits.) It seems boring to replace those yummy, sweet drinks with ordinary water, but with a slice of lemon or lime, water is mighty refreshing.
The simplest solution, of course, is to avoid processed foods altogether, and prepare your own meals.
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last updated: August 24, 2010