Many people consider fruit juice to be beneficial to the health. This is based on the evidence showing the health benefits of whole fruit. However, evidence shows fruit juices to be no better for he health than sugar sweetened soft drinks. Fructose is the primary driver of mammalian insulin resistance, a metabolic abnormality that desensitises the cellular response to insulin. Insulin resistance is the likely cause of metabolic syndrome, a cluster of diseases that causes metabolic dysfunction and the accumulation of adipose tissue in the abdomen. Foods that contain fructose, including fruit juices, are therefore implicated as a primary cause of obesity. While whole fruit also contains fructose it is not considered detrimental to body composition, and this suggests that fruit juices have a quite different composition to their original whole fruits. In particular, fruit also contains fibre which has been shown to be protective of insulin resistance.
Fibre is beneficial because it alters the milieu of the intestinal tract and changes the digestion rate of carbohydrates such as the fructose in fruit. Because the liver has a finite capacity to convert fructose to glycogen, ingestion of large quantities of fructose above a threshold causes increased flux through an alternative pathway that increase production of lipids. This de novo lipogenesis pathway results in increased plasma levels of triglycerides, which are converted to free fatty acids by the action of lipoprotein lipase. The free fatty acids interfere with nitric oxide synthesis and cause endothelial dysfunction which can increase blood pressure, and also accumulate in skeletal muscle and hepatic tissues where they interfere with insulin signalling, leading to insulin resistance. By slowing the rate of absorption of fructose, fibre may limit de novo lipogenesis by preventing liver overload. The fibre in fruit also decreases the amount of fruit consumed.
The benefits of a high fibre to carbohydrate ratio has been demonstrated many times in the nutritional literature. For example in one study1, researchers fed healthy subjects isocaloric snack consisting of processed foods (sugar sweetened soft drink and a potato snack or a chocolate coated snack bar), or whole foods (raisins with peanuts or banana and peanuts) and then measured their plasma glucose responses. The results showed that the area under the curve for glucose was 70% higher following the processed snack, when compared to the whole food snacks. In addition, plasma glucose rose higher and then fell further following the processed foods. The authors reported that one subject had a pathological insulin response to the processed snacks, but responded normally to the whole food snacks. These results therefore show that the fibre to carbohydrate ratio is the determining factor in glycaemic response.
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