Division of Endocrinology and Diabetes, Department of Medicine, University of Minnesota, MN, USA
Fructose is a monosaccharide which is abundant in nature. It is the sweetest naturally occurring carbohydrate. The availability of fructose increased substantially when it became possible in the 1960s to economically produce high fructose syrups from corn starch and other starches. Such high fructose syrups are now used to sweeten soft drinks, fruit drinks, baked goods, jams, syrups and candies. The most recent data available suggest that fructose consumption is increasing worldwide. Fructose presently accounts for about 10% of average total energy intake in the United States. Studies in both healthy and diabetic subjects demonstrated that fructose produced a smaller postprandial rise in plasma glucose and serum insulin than other common carbohydrates. Substitution of dietary fructose for other carbohydrates produced a 13% reduction in mean plasma glucose in a study of type-1 and type-2 diabetic subjects. However, there is concern that fructose may aggravate lipemia, particularly in men. In one study, daylong plasma triglycerides (estimated by determining the area under response curves) in healthy men was 32% greater during a high fructose diet than during a high glucose diet. There is also concern that fructose may be a factor contributing to the growing worldwide prevalence of obesity. Increasing fructose consumption is temporally associated with the increase in obesity. Moreover, on theoretical grounds, dietary fructose might increase energy intake. Fructose stimulates insulin secretion less than does glucose and glucose-containing carbohydrates. Since insulin increases leptin release, lower circulating insulin and leptin after fructose ingestion might inhibit appetite less than consumption of other carbohydrates and lead to increased energy intake. However, there is not yet any convincing experimental evidence that dietary fructose does increase energy intake. Although evidence that fructose has adverse effects is limited, adding fructose in large amounts to the diet may be undesirable, particularly for men. Fructose that occurs naturally in fruits and vegetables is a modest component of energy intake and should not be of concern.
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Fructose is a six-carbon monosaccharide which is abundant in nature. Free fructose is present in honey, dates, figs, apples, grapes and most berries. An additional, important quantitative natural source of fructose is the disac-charide sucrose which is composed of equimolar quantities of fructose and glucose. When ingested by humans, fructose is absorbed by an active transport system but at a slower rate than is glucose . Co-ingestion of glucose increases intestinal absorptive capacity for fructose. In the absence of glucose, human intestinal capacity to absorb fructose appears to be quite variable with some people unable to completely absorb 30- to 40-gram quantities . Those individuals unable to completely absorb ingested fructose are at risk for diarrhea and other gastrointestinal side effects.
The first several steps in fructose and glucose metabolism differ significantly. Fructose stimulates only modest insulin secretion and does not require the presence of insulin to enter cells . Avidly taken up by hepatic cells, fructose is rapidly converted to fructose-1-phosphate and bypasses the early rate-limiting steps of glucose metabolism. Fructose-1-phosphate is mainly converted to lactate, glucose and glycogen . Gluconeogenesis from fructose is increased by starvation and poorly controlled diabetes. Fructose may also form acetyl CoA which is used in fatty acid synthesis. Enhanced activity of lipogenic enzymes with chronic fructose feeding may promote hepatic triglyceride production and output of VLDL particles. Presumably, energy intake must be excessive for fructose to stimulate lipogenesis.
Fructose is the sweetest tasting naturally occurring carbohydrate (table 1). Advances in technology in the 1960s made possible the production of inexpensive high fructose syrups from corn starch . Corn is an abundant worldwide source of starch. To make high fructose syrups, corn starch is first separated from other corn by-products by wet milling. Next the starch is digested with mineral acid and amylase to form glucose. The enzyme glucose isomerase is then used to convert glucose to fructose. A syrup containing 42% fructose is the first product of this process. Through chromatographic enrichment, 55 and 90% high fructose syrups can then be produced. The 55% high fructose syrup has taste and sweetness equivalent to sucrose. Because of sweetness and low cost, high fructose syrups found commercial application. In the mid 1980s, 55% high fructose syrup was adopted by the carbonated-beverage industry and became the predominant sweetener in soft drinks.
The United States has from the beginning been the world's largest producer of high fructose corn syrups but Japan, Canada, South Korea, China, Argentina, and other countries are also significant producers . In Asia, tapioca starch and broken rice are used in production of high fructose syrups. High fructose syrups are widely used in soft drinks, fruit drinks, baked goods, jams, syrups and candies. In 1977-1978, average fructose intake was estimated to be 37 g per day, accounting for ~8% of total energy intake in the
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