Carbohydrates in your body
Lately the sugar is increasingly used as a scapegoat for America’s and Europe’s obesity problem. But what about fructose? Is there a valid base for the fructose-bashing trend? Let’s see first how carbohydrates are processed in the body.
At first point you eat or drink something that is high in carbohydrates. The source could be “complex” like oats, quinoa, beans or even vegetables. The carbohydrates from these sources will be broken down in the gut and absorbed as the simple sugar glucose. Table sugar, syrups and some natural foods contain “simple” carbohydrates, like sugars. For example, milk contains the sugar lactose, which breaks down into glucose and galactose. Table sugar (sucrose), breaks down into glucose and fructose (50/50). Foods like fruit and high fructose corn syrup (1) also provide fructose and glucose, but fructose is slightly higher (about 55/45). But note that HFCS and the table sugar usually have comparable sugar composition and metabolic consequences in the body. Specifically speaking, when you eat carbohydrates, they are absorbed as simple sugars and sent to the liver first. For processing glucose, the liver has an orderly and controlled system and, based on body’s needs, it will do different things with the amount of glucose you have consumed. For example, after an intense lifting session, your liver may decide to use glucose to replace depleted carbohydrate storage (the so called glycogen) in the muscles. However, if your liver is doing a lot of processing and needs energy for itself, the glucose may be used to make more energy molecules (ATP). In contrast, if you have been sitting and munching all day, the liver will turn glucose into fat for long-term storage. Fructose is also sent straight to the liver, but it is dealt with in a more chaotic process once it gets there. Because the structure of fructose is a bit more complicated, to state it simply, the easier thing for the liver to do with fructose is to make fat from it. Scientists have speculated for a long time that the liver does this, but there was little research to support this notion, until recently.
The latest research-based information on fructose
In the literature, there is evidence that fructose turns to fat much easier than other sugars. Specifically, a recent research from the University of Texas Southwestern Medical Center, fed healthy people breakfast drinks containing three different “sugar combinations” followed by a carefully controlled lunch. They did this over several weeks. In one test the drink contained 100% glucose, in another the drink was half glucose half fructose (which is what you will find in ordinary sugar). In the third condition, the subjects got a drink that was 25% glucose and 75% fructose. The researchers were interested in how fast the sugars in the drink turned to fat in the liver as well as how this morning meal will influence the metabolism of foods eaten later in the day (the subsequent lunch for example). In the findings, the researchers found that fructose got “made” into fat more quickly than other sugars. And for the second thing, they found that when fructose was eaten with fat (junk food), the fat was much more likely to be stored rather burned.
The theory behind all this notion, that fructose increase appetite, is that fructose does not elicit an insulin & leptin response, and thus does not blunt appetite. But, hold on a second. People do forget that most fructose in both the commercial and natural domain has an equal amount of glucose attached to it. You would have to go out of your way to obtain fructose without accompanying glucose. Thus in studies directly comparing the effect of fructose and glucose preloads on subsequent food intake, one showed no difference (2), while a recent literature review on fructose’s effect on satiety found no compelling case for the idea that fructose is less satiating than glucose, or that HFCS is less satiating than sucrose (3). In general, both compounds have substantial research showing not just their ability to elicit an insulin response, but also their suppressive effect on appetite (4-5). In the only study that fructose was linked to a greater next-day appetite in humans, 30% of total daily energy intake was in the form of free fructose (6). The amount of free fructose provided to the subjects was 135g, which is the equivalent of 6-7 soft drinks (Just for the record, a medium banana has 7g of fructose, an apple 12g of fructose and 2 cups of strawberries 4g of fructose). Is it really that ground-breaking to think that consuming a half-dozen soft drinks per day is not a good idea? In my opinion, demonizing fructose without mentioning the dose-dependent nature of its effects is misleading and dishonest. Like everything else, fructose consumed in gross chronic excess can lead to problems, while moderate amounts are neutral, or beneficial in some cases (7).
Summing up
Taking everything into consideration, although examples of pure fructose causing metabolic upset at high concentrations abound, especially when fed as the sole source of carbohydrate, there is no evidence that the ordinary fructose-glucose sweeteners do the same. Thus studies using extreme carbohydrate diets may be useful for probing biochemical pathways, but they have no relevance to the human normal diet. In general, the upper safe limit of fructose per day (all sources considered) depends on a number of variables, such as an individual’s physical activity level and lean body mass. Although there is research suggesting that up to 90g per day has not negative effect in fasting triacylglycerol or body weight, there is some other research suggesting that the safe range is between 25-40g per day (8). If we think that both sides are biased, the middle figure between the two supporters, is approximately 50g for active adults. Once again, it is important to mention that the big picture solution is managing the total caloric balance with a diet that contains minimally refined foods and sufficient physical activity.
References:
1) http://en.wikipedia….tose_corn_syrup
2) Spitzer L, Rodin J. Effects of fructose and glucose preloads on subsequent food intake. Appetite. 1987 Apr;8(2):135-45.
3) Moran TH. Fructose and satiety. J Nutr. 2009 Jun;139(6):1253S-1256S. Epub 2009 Apr 29.
4) Melanson KJ, et al. High-fructose corn syrup, energy intake, and appetite regulation. Am J Clin Nutr. 2008 Dec;88(6):1738S-1744S.
5) Soenen S, Westerterp-Plantenga MS. No differences in satiety or energy intake after high-fructose corn syrup, sucrose, or milk preloads. Am J Clin Nutr. 2007 Dec;86(6):1586-94
6) Teff KL, et al. Dietary fructose reduces circulating insulin and leptin, attenuates postprandial suppression of ghrelin, and increases triglycerides in women. J Clin Endocrinol Metab. 2004 Jun;89(6):2963-72
7) Dolan LC, et al. Evidence-based review on the effect of normal dietary consumption of fructose on development of hyperlipidemia and obesity in healthy, normal weight individuals. Crit Rev Food Sci Nutr. 2010 Jan;50(1):53-84
8) Sánchez-Lozada LG, et al. How safe is fructose for persons with or without diabetes? Am J Clin Nutr. 2008 Nov;88(5):1189-90
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