A number of fruit are known to contain high concentrations of protein digesting enzymes. These fruit include the pineapple, papaya and kiwifruit. The role of the proteases in the fruit is likely a ripening aid, as protease levels rise during maturation of the fruit and this causes the tissues to become softer and more appealing for consumption by animals. This then increases the likelihood that the fruit will be eaten and the seeds dispersed. Proteases are also present in the stomach of animals where they are used to break down dietary proteins. Such animal proteases include trypsin and pepsin. The proteases found in various fruits at high concentrations are cysteine proteases and are structurally different to the proteases found in animals. However, evidence shows they are active at the low pH of the stomach and therefore they may be able to increase protein digestion rates in animals when consumed alongside dietary proteins. Evidence from animal studies also suggests they increase the gastric emptying rate.
For example, in one study1 the effects of proteases from the green kiwifruit (Actinidia deliciosa cv. Hayward) were investigated. Kiwifruit is rich in a particular protease called actinidin, which has previously been shown to be able to digest a number of animals tissues including gelatin and beef muscle as well as the plant protein gluten. In the study rats were fed a number of protein sources including beef muscle, gelatin, gluten, whey protein, zein and soy protein isolate. These proteins were selected because they showed a range of digestion rates. To these proteins was added 76.5 IU per gram dry matter freeze dried kiwifruit. The kiwifruit was in two forms, either Actinidia chinensis cv. Hort16A which is devoid of actinidin or Actinidia deliciosa cv. Hayward, which contain actinidin. The authors then measured the gastric emptying rates and protein digestion rate of the rats in response to test meals. Dietary actinidin increased the digestion rates of beef muscle 0.6 fold, gluten 3.2 fold and soy protein isolate 0.6 fold.
Therefore actinidin appears able to increase the protein digestion rates of a number of dietary proteins in the rat. This is nutritionally interesting because the presence of undigested proteins in the small intestine is able to stimulate the release of cholecystokinin from the I cells of the duodenum. Cholecystokinin then activates receptors in the stomach causing contraction of the muscles in the pyloric sphincter. In this way undigested protein is inhibited from leaving the primary site of digestion, the stomach, before it has been denatured and hydrolysed. The ability of actinidin to increase protein digestion rates in the rats would suggest that the inhibition caused by cholecystokinin would be removed earlier than would have been the case in the absence of actinidin. And this is exactly what the researchers observed. Actinidin increased gastric emptying rates of the test meals containing both beef muscle and zein. Using kiwifruit, or extracted actinidin, as a dietary digestive aid therefore appears to show some merit, at least in rats.
RdB
