The absorption of glucose causes a release of insulin from the pancreas. This insulin is required to allow the uptake of glucose to cells and therefore has a blood sugar lowering effect. The rise in blood sugar seen following ingestion of carbohydrate foods is a reflection of the chemical composition of the starch or sugars in the food, the amount of other macronutrient (protein and fat), the amount of fibre, the amount of total carbohydrate ingested, as well as the water content of the food. Foods that cause rapid and large increases in blood sugar and a concomitant large increase in insulin release are increasingly being linked to disease. Reducing the glycaemic response following carbohydrate ingestion is associated with improvements in long term health and a number of modifying factors have been investigated. In particular, refining and processing foods is believed to cause detrimental effects to postprandial glycaemia because the fibre and water content of the foods can be altered from the original plant material.
Simple sugars tend to elicit larger postprandial glycaemic effects compared to polysaccharides. Fruit contains high amounts of simple sugars, but generally the glycaemic response from such foods is low. However, processing the fruit to produce juices removes the fibre and decreases the water to sugar ratio, and both these things can modify the glycaemic effects. For example, in one study1 the effect of whole oranges (South African Navel Oranges) or their juice was investigated with regard changes in the postprandial blood glucose and insulin levels of healthy subjects. The results showed that the whole oranges produced a significantly lower postprandial rise in blood sugar and a significantly lower insulin response, compared to the orange juice. This was reflected in a pronounced hypoglycaemia experienced by the subjects consuming the orange juice, a phenomenon not experienced by those consuming the whole oranges. Satiety was also reported to be induced by the consumption of whole oranges but not orange juice.
The authors commented that their previous findings showed similar beneficial glycaemic effects for whole apples versus apple juice. This suggests that refining the juice from oranges and apples has a detrimental effect on glucose absorption rates and postabsorptive blood sugar levels, and that consumption of orange and apple juice over the long term could increase the risk of insulin resistance significantly. Interestingly the same study also investigated the glycaemic effects of grapes versus grape juice, and found that ingestion of grapes caused a larger insulin response compared to grape juice, while the postabsorptive glucose profiles were similar. The authors concluded that the high osmolarity of the grape juice may have inhibited gastric emptying rates and slowed the passage of glucose through the gut and thus decreased the subsequent glucose absorption rate, thereby producing a pronounced reduction in the the stimulation of insulin. However this may also have related to the sugar content of grapes (which is different to oranges).
The fibre content of the oranges was 2.5 % of the edible content. The oranges were crushed to remove their juice and were depectinised enzymatically (had the fibre removed) to produced a juice with 8 grams of carbohydrate per 100 mL composed of 2.3 % glucose, 2.6 % fructose and 3.3 % sucrose. Analysis of the whole oranges showed large inter fruit variation and large inter segment variation in the same orange. The grapes used were black Spanish Napoleon grapes and juice was made by crushing the fruit followed by depectinisation. The fibre content of the whole grapes was reported to be 2.7 % of which 49 % was contributed by the pips. The grapes contained 17.7 grams of total carbohydrate per 100 mL which was made up of 8.8 % glucose and 8.9 % fructose. The juice contained 18.6 grams per 100 mL total carbohydrate with 9.3 % fructose and 9.3 % glucose. The test meals were all standardised to contain the same amount of carbohydrate (50 or 60 grams monosaccharides for oranges and grapes, respectively).
Dr Robert Barrington’s Nutritional Recommendation: Grapes, grape juice and orange juice elicited a pronounced hypoglycaemia following the rapid rise in blood sugar. However, for oranges this was not evident. The higher insulin response to grapes compared with grape juice may have been as a result of the high osmolarity of the juice. In fact when the authors diluted the juice the insulin response increased suggesting that this explanation may have been correct. While the insulin response to the grapes was higher than for grapes juice, like the oranges (and apples tested in a previous study), the grapes did elicit a greater feeling of satiety compared to the juice and did delay the return of appetite. The high glycaemic responses to fruit juices as well as their failure to stimulate satiety suggests that regular consumption may be detrimental to the health. Therefore fruit juice should be avoided, with exceptions being the period after a workout, or as an adjunct to creatine supplementation, both of which necessitate sugar consumption.
RdB
