Crystalline fructose devoid of its natural fibre, and ethanol (alcohol) are metabolic poisons that have deleterious effects on the liver (here). Because both are almost exclusively metabolised in hepatic tissue, large influxes over a short period of time result in liver overload syndrome. Overloading the liver like this increase flux through de novo lipogenesis, the end products of which are fatty acids. These lipids deposit themselves in the hepatic tissue and cause insulin resistance and mitochondrial dysfunction. Fructose and ethanol can therefore lead to the formation of non-alcoholic and alcoholic fatty liver disease, respectively. Over time the development of such changes leads to widespread metabolic aberrations that can be termed the metabolic syndrome. Crystalline fructose and ethanol therefore have deleterious effects when consumed in large concentrations, and such intakes should be avoided if health is a concern. However, both substances do have useful properties that may make ingestion advantageous, if tightly controlled.
The beneficial effects of ethanol have been extensively reported in the nutritional literature. In particular, ethanol can reduce the risk of cardiovascular disease through unknown mechanisms. Studies investigating the physiological effects of ethanol have shown that alcoholic drinks possess beneficial health effects that cannot be attributed to their polyphenol content. For example, experiments with dealcoholised wine and grape juice show that polyphenols may confer protection from cardiovascular disease. However, other studies show that the alcohol component may also be protective. A number of mechanisms for the cardioprotective effects of alcohol have been proposed, but the research to date has focused on the ability of wine to increase fluidity in cell membranes. This may occur through the membrane disruptive effects that alcohols possess, or may be due to the ability of ethanol to increase the essential fatty acid content of blood cells. Irrespective of the mechanisms ethanol in the form of alcoholic drinks when moderated, is protective of disease.
Crystalline fructose is controversial because its positive effects are more limited compared to ethanol. However, the characteristic that makes it a metabolic poison, that of being metabolised exclusively by the liver, also makes it somewhat useful to athletes. Liver glycogen can become depleted with exercise because it is used to maintain blood sugar levels during periods of net glucose oxidation. Depletion of liver glycogen is therefore a limiting factor during physical exercise and repletion of hepatic glycogen stores is a pivotal factor in recovery. Following exercise, crystalline fructose is metabolised in the liver not through the de novo lipogenesis pathway, but instead via the glycogen synthesis pathway. Studies show that dietary fructose is more beneficial following physical activity with respect to synthesising glycogen, when compared to glucose. Therefore, fructose consumption can be beneficial to athletes if they are willing to moderate intakes to those periods immediately following exercise.
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