Who knows about Fructose?

Lucas Cranach Adam & Eve

Lucas Cranach (1526). The Courtauld Gallery (extract)

Was it a sweet or sexy temptation?

The apple that Eve pluckedSweeter than honey from the tree was sweeter than honey, or to be perfectly accurate it had relatively more of the sweeter stuff than in honey. Apples contain more fructose than glucose, and fructose is much sweeter than either glucose or sucrose (the sugar in our pantry).

Aside from the creation story, the other theory predicts that some plants acquired an advantage by evolving biochemical pathways that generated more flavorsome sugar, like fructose, in their ripening fruit. Being more attractive to eaters, helped to disperse seeds to places where they could thrive. Eve is in the apple, as fructose.

‘Fructose’ has a healthy ring, from its Latin root for ‘fruit.’ We were advised to eat an apple a day to keep the doctor away, and fructose was recommended for people with diabetes because it has a low glycemic index. But it may not be such a good prescription, and has been losing its salubrious cachet ever since high fructose corn syrup was introduced forty years ago. Glucose still glitters, which is a differentiation that seems odd at first sight because the two molecules have the same number of carbon, hydrogen and oxygen atoms.

sugar

Glucose + Fructose = Sucrose

Having identical chemical formulas does not make them the same. They are no more the same substance than genetically identical twins are the same person. Glucose and fructose are monosaccharide sugars forming in equal amounts when their parent molecule, sucrose, is digested by enzymes breaking its oxygen bridge apart. The two ‘baby’ sugars have different shapes, but when it is their turn to be digested they generate the same number of calories. But not all calories are equal.

Shape matters. Consider another sugar-lover. When yeast cells feed on the sugary mixture in unfermented grape ‘must’ they mostly prefer glucose, and as alcohol increases in the liquor more undigested fructose is left behind. That can make a wine too sweet for taste. Vintners are therefore choosy because some strains of yeast have a greater affinity for fructose which makes a drier product. Not all yeasts are equal.

Of these twins, only glucose is essential for life, although we don’t need to eat it raw because it is created by digestion of more complex carbohydrates and fats. We manage perfectly well without fructose in our diet too. Even sperm, which are nourished in fructose-rich semen, are equally satisfied when provided with only glucose.

More interestingly, our body handles the two sugars very differently. Glucose is rapidly absorbed into the bloodstream from the small intestine via a special transporter (GLUT4). It then races through the liver for delivery to every corner of the body. Under strict control by the hormone, insulin, glucose provides fuel for metabolism and growth in cells, and any surplus is converted to fat and stored in adipose tissue.

On the other hand, fructose is absorbed slowly from the gut and by a different transporter (GLUT5), and even more poorly in infants where it can cause diarrhea. Once it reaches the hepatic portal vein it stops at the liver where it is taken up into cells by a process that doesn’t involve insulin. Once inside, it is handled differently to glucose, forming glycerol as a backbone for making triglycerides.  Several studies have shown that fructose-rich diets raise both triglyceride levels and LDL small particles while lowering HDL-cholesterol, which are thought to be unhealthy signs. Since fructose is lipidinous, it potentially contributes to metabolic syndrome, including diabetes and obesity, and there is additional evidence of a role in fatty liver, gout, and inflammatory conditions. Eve was wise to offer Adam only one apple.

Until a little over a century ago, dietary sugar could only be afforded by the wealthy and powerful. No longer. Growing up with cheap and omnipresent sugar, it is easy to equate abundance with beneficence. The longest aisles in supermarkets are stacked with sodas and sports drinks full of sugars, and rows of ‘healthy’ apple juice contain a whopping 100 grams of fructose per liter. Those are the most visited aisles in stores, and conveniently located near cash registers.

sweet drinks aisle

Longest aisle in the supermarket

At home, grocery products lining pantry and fridge shelves are also saturated with sugars. But it’s hard to know how much sugar we consume by reading labels. How can we accurately compare it in different foods or brands when labels describe amounts as ‘serving sizes’ instead of ounces or grams? How much sugar is added versus natural? How much is glucose versus fructose versus sucrose? As consumers buy more processed foods instead of using raw ingredients for home cooking we are locked into choices made by manufacturers. They spoon in the sugar to increase shelf-life and palatability (Eve’s temptation).

I know how hard it is to resist sweet stuff, and harder still to give up. Perhaps fructose is hardest of all, being so sweet and stealthy. Our gut absorbs it more rapidly as we become used to it, and it doesn’t suppress the hunger hormone (ghrelin) as glucose does. We get hooked, without being satisfied.

When we were introduced to tea and coffee as children our ration was a measly half-teaspoon of sugar to make them more palatable. As soon as we started savoring the beverages, mother weaned us off the sugar, and a few weeks later the sweetness was so repellent that if we sipped Dad’s tea by mistake it made us heave. It took much longer to adjust him to unsweetened drinks. In that age, when sweetness was well-regarded in every way, children could surprise visiting aunts and uncles by teasing them: “We are teatotal, and saving our sugar ration for chocolate!”

It is now cool to cut sugar and worry about fructose, and the stigma is sticking. Robert Lustig has over five million viewers for his YouTube diatribe against fructose, and Michelle Obama was quoted as saying,”Our kids don’t choose to make food products with tons of sugar and sodium in supersized portions, and then to have those products marketed to them everywhere they turn.” We must stop blaming the fat boys and girls in class for being gluttons. With choices made for them and sweet temptations at every turn, they are casualties of a food and drinks industry focused on annual financial performance. And since the FDA is unlikely to regulate sugar in foods, the sweet tide continues to rise.

High fructose corn syrup is gradually replacing sugar from cane and beet because it is so cheap. Produced by industrial hydrolysis of starch, HFCS55 and HFCS42 are the main products for foods and beverages, while HFCS90 finds specialty uses. The number represents the percentage, so the fructose hit in HFCS is similar to 50 in granulated pantry sugar. There are important differences, however, in the source, process, farm acreage, economic and environmental impacts of subsidized corn for HFCS.

While HFCS is regarded as a new dietary demon, we have been exposed to fiendish fructose for eons, although not in any quantity until recent generations. It is unavoidable, but lower exposure through smaller serving sizes, home cooking, dilution of fruit juices, etc. can be managed over time because there is no physical addiction to sugar. Perhaps it will become the new tobacco.

Animal studies show that it would take an impossibly large serving of fructose to kill us, but that doesn’t deny a toxicity that is cumulative and insidious over time. Even the most benign substances that life depends on, like water and oxygen, can harm us when we are overexposed to them.

I imagine a naïve man visiting the seaside for the first time. He sets up his deckchair on the beach at the low water mark so his toes are wetted by a gentle ripple at the ocean edge. Two hours later, he is enjoying the warm water washing around his calves while he is engrossed in an anthology of Robert Browning. When five more hours have gone by, the soothing water is up to his waist and the open book flops on his chest at the Paracelsus poem as he dozes off. But after ten hours, a wave splashing his face wakens him, but too late to swim back.

Perhaps the metaphor is too strong, but Paracelsus would argue otherwise. He was a sixteenth century doctor and the father of toxicology. A great deal of nonsense has been attributed to him, but he rightly pointed out that “the poison is in the dose.”

Next Post: Virginia Nature Journal for January

About Roger Gosden

British-born scientist specializing in human and animal reproduction & embryology. Academic career spanned from Cambridge and Edinburgh to McGill and Cornell's Weill Medical College in Manhattan where he was Professor & Research Director. Married to Lucinda Veeck Gosden, embryologist for the first successful IVF team in America, he retired early to Williamsburg, Virginia, to write and to recover from 'nature deficit disorder'
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