Roger Lentle and Allan Hardacre of Massey University, New Zealand, describe a new test for measuring the rate of digestion and release of sugars from foods in the gut.
Food manufacturers are well aware of the link between the rise in blood sugar levels after eating a particular food, generally measured as glycaemic index, and the onset and management of diseases, such as diabetes. This relationship has led to a clamour for foods that liberate glucose slowly, particularly by health professionals and the burgeoning number of people with diabetes. However, whilst the results of glycaemic index testing are a prerequisite for the marketing of these foods, the index provides little information on the process of digestion within the gut lumen i.e. the rate at which food disintegrates and is digested to release glucose. Such information could provide critical insight as to how the changes in the composition and physical properties of food matrices can slow the digestion of the food and hence the rate of glucose liberation.
Scientists at Massey University in New Zealand have adapted a test that is commonly used by doctors to identify disorders in the permeability of the gut wall. The new test measures the rates at which the solid matrices of foods disintegrate and the contained nutrients are liberated and absorbed by the gut [1]. It relies on two metabolically inert and harmless sugars, mannitol and lactulose, which are passively absorbed through the gut wall and are promptly excreted unchanged in the urine.
Test procedure
The test was conducted by doping a batch of food with small quantities of the two tracer sugars prior to manufacture. Known quantities of the doped manufactured product were then fed to volunteers and their urine sampled at intervals over six hours. The rates of appearance of the two sugars in the urine reflected the rate at which the food product was disintegrating in the gut. Further, the fact that the mannitol was more readily absorbed in the small intestine and the lactulose in the large intestine, allowed the site at which the food was digested to be identified. Hence mannitol appeared first in the urine whilst lactulose appeared when the bulk of the digesta containing the food had passed through the small intestine into the large intestine.
The efficacy of the test was measured by feeding each of 22 healthy female volunteers aged between 20 and 40 years (mean age 27) either a powdered or a pelleted pasta meal containing the tracer sugars lactulose and mannitol and assessing changes in blood glucose and the rates of excretion of the tracer sugars in the urine. Only female participants were used as it is known that the permeability of the intestine varies slightly with gender. The participants were also given a dummy meal consisting of a solution containing lactulose and mannitol, which acted as a control and was similarly tested. Fifty grams of the pellets, powder or the dummy meal were mixed with 150ml of a fruit flavoured sauce immediately prior to consumption, to make the meals more palatable. After 1 hour, participants were given 100mL of water followed by a further 200mL 3 hours after the meal. Hence each subject ingested a total of 500g of fluid containing 50g of solids during the 6-hour monitoring period.
All meals were consumed within 10 minutes. Urine was collected and blood samples were taken immediately prior to consuming the meal and at 30 minute intervals for 6 hours. The synchronous sampling of urine and blood allowed the results from the new test to be compared with blood glucose analysis used as the basis for glycemic index. The participants were allowed to consume no other food during the 6-hour sampling period following the consumption of each of the meals.
The dry pasta pellets were around 0.38mm in diameter and the dry powdered pellet particles were between 100 and 500 microns in diameter. Both contained 82% starch, 10% lactulose, 5% mannitol and 3% canola oil. The larger pellets were expected to delay digestion as they had a surface to volume ratio about 10 times lower than that of the powdered pellets for enzymatic attack. No solid matter was present in the dummy meal.
The new test measures the rates at which the solid matrices of foods disintegrate and the contained nutrients are liberated and absorbed by the gut.
Release of sugars
The results showed that the excretion of mannitol peaked about 2 hours after the consumption of both the dummy liquid meal and the powdered pasta, whilst the digestion of the pellets took significantly longer, with mannitol excretion peaking at 3.5 hours after the meal, suggesting that the integrity of the pellets was more or less maintained in the small intestine The ratio of the appearance of the peaks of lactulose and mannitol in all three meals indicated that the foods had remained in the small intestine for similar periods before entering the large intestine.
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The test should be able to identify the effects of changes in the structure and properties of pasta and other foods to determine which foods are more resistant to digestion. Hence the test could be useful to food technologists who wish to identify processing techniques that slow down the rate of food breakdown during digestion and as a result lower the glycaemic index.
Interestingly, the tests all showed that whilst absorption of mannitol continued for around 3.5 hours, the levels of glucose measured in blood taken at the same time as the urine samples fell to their baseline values within 2 hours of consuming the meals. Hence the time period over which nutrients are absorbed is considerably longer than that measured in the glycaemic index, the ongoing digestion and liberation of glucose being masked by the high rates at which it is taken up from the blood by the liver and other tissues. This result highlights one of the major draw backs of measuring glycaemic index values because assessment is often limited to the first 120 minutes of a process that takes up to four hours.
The new test allows the entire process of digestion to be assessed. A particular advantage of the test is that it is non-invasive, requiring only regular urine samples, which need little further processing prior to quantification by HPLC. The test can be further simplified by omitting the lactulose component without losing precision in estimating rates of small intestine digestion. However, caution is necessary when the test is used for composite foods, such as those containing whole grains or other components which cannot readily be doped with the tracer sugars.
Massey University plans to produce a pasta ‘standard’ doped with known quantities of the two probe sugars, which can be used by the industry for comparison with other foods.
Roger Lentle and Allan Hardacre, Institute of Food Nutrition and Human Health, Massey University, SH 57, Palmerston North, New Zealand
Tel: 06 3569099 xn 83018 Email:a.hardacre@massey.ac.nz
References
1. Lentle RG Sequira IR, Hardacre AK Reynolds G (2016) A method for assessing real time rates of dissolution and absorption of carbohydrate and other food matrices Food and Function DOI 10.1039/c6fo00406g.
