Sunday, July 3, 2011

Xylitol, a Low-Calorie Sweetener With Unknown Fat-Burning Side-Effects!? Replacing Cornstarch With the Five-Carbon Sugar, Xylitol, Ramps Up Fat Burning Enzymes in Rodent Model of Diet Induced Obesity.

Image 1: Xylitol christals under the micro-
scope (photo taken by Anders
Østergaard Madsen
, Denmark 2001)
From a health perspective, the five-carbon sugar alcohol Xylitol has hitherto principally been known for its beneficial effects on dental health. With a caloric value of 3kcal/g the naturally occurring ingredient of a wide variety of plants, citrus fruits (plums, strawberries, raspberries) and vegetables (cauliflower) does have less calories than sugar, nevertheless, according to the fundamentally flawed, but still widely held paradigm that "a calorie is a calorie, no matter where it comes from", the idea of replacing sugar with xylitol to reduce the overall caloric load would make little sense even on a diet that was relatively high in carbohydrates. So, if it were not for the highly marketable statement "sugar free" on chewing gums and candies, xylitol would probably not even have made it to the consumer market. A cursory glance at the abstract of a study (Ama. 2011) that is about to be published in the July issue of the Journal of Clinical Biochemestry and Nutrition (Vol. 49, No. 1) does yet suggest that these research results could renew the interest in a compound many of you may have considered to be nothing but another marketing scam from the (diet-)food industry - and, before this new data came out, you were 100% spot-on with your assessment ;o)

In the course of an 8-week feeding period, Kikoko Amu and his collegues from the University of Tokoshima Graduate School, the University of Shizuoka and the Food and Science Institute in Kanagawa, Japan, fed a group of 18 male Sprague-Dawley rats (initial body weight 290-310g) a high fat diet containing 312.3 g/kg cornstarch (control group), of which, in the experimental groups, X1 and X2, 16% (X1) or 29% (X2) were replaced with 1.0g/100kcal (X1) and 2.0g/100kcal (X2) of xylitol for group X1 and group X2, respectively. At the end of this period the scientists found that
long-term intake of xylitol supressed the accumulation of visceral fat [cf. figure 1, below] and the increase in plasma insulin and lipids concentrations in rats fed a high-fat diet.
Interestingly, at least part of the effects were mediated by xylitol-stimulated "expression of fatty acid oxidation genes in the liver, and lipid degradation and adiponectin genes in the adipose tissue." Furthermore, Amo et al.
found for the first time that xylitol ingestion lowered postprandial hyperglycemia [if administered at] a none-effective dose in causing diarrhea, and within the limits of orally administered physiological amounts (1-4g/kg body weight daily).
The human equivalent dose for this "non-effective dose in causing diarrhea" (nice, how these Japanese try to paraphrase that the dose they used did not cause "the runs", isn't it?) is 0.16-0.65g/kg per day, which would amount to somewhere between 13g and 52g of xylitol for a man who weighs 80kg. A pretty large dose considering the fact that some low-carbers out survive on hardly more than 40g of carbs a day.
Figure 1: Weight [in g/kg body weight] of retroperitoneal, epididymal and mesenteric part of visceral fat in high-fat (HFD) and high-fat xylitol-substituted (X1, X2) fed rats after 8 weeks of treatment (data adapted from Ama. 2011)
Despite being on a high-fat diet (I hope the readers of this blog have already gotten the message that low carb wont work in the absence of fat) its highly questionable, especially for "low-carbers", whether and to which extent healthy human beings would benefit from xylitol substitution. After all, the "high-fat" chow the rats were fat contained a whopping amount of 362.3g carbohydrates from corn and only 200g of fat from lard (75%) and soybean oil (25%). In view of the dietary 'quality' (I hardly dare to use this word in the context of what those poor critters were fed) of the grub the rats were given, it should not come as a surprise that despite having smaller visceral fat pads, the rats from the experimental groups (X1, X2) were just as overweight (X1: 525.1 +/- 9.6g; X2: 540.4 +/- 9,9g) as their mates (HFD: 543.2 +/- 10.2) who were crammed with what I would like to call a "high-fat corn diet". Furthermore, there were no significant differences in the amount of energie the rats from the different groups consumed and the lean mass of the soleus muscle, the only muscle variable measure in the study, was almost identical. Against that background the improvements in glucose management (cf. figure 2), the scientists observed, could simply be related to the decreased carbohydrate intake. 
Figure 1: Serum markers of glucose and lipid metabolism in high-fat (HFD) and high-fat xylitol-substituted (X1, X2) fed rats after 8 weeks of treatment (data adapted from Ama. 2011)
So, if it were not for the statistically significant increases in adipose gene expression of PPAR-gamma (key regulator of adipocyte differentiation) and the insulin sensitizing hormone adiponectin, as well as the increased lipolytic enzyme-activity (hormone sensitive lipase, HSL; adipose triglyceride lipase, ATGL) within the adipose tissue of the xylitol fed animals, all of you, who have already gotten off the westernized fast-food diet, the scientists were emulating in this study, could easily discard their results as meaningless. Until now, however you will have to wait for future studies on the effects of xylitol on metabolic health and body composition in animals (or humans) who are not fed a diet the detrimental health effects of which cannot be fought off by any supplement or drug on the market. And remember: As always, the SuppVersity is where you will hear about those studies first ;-)