The table sugar we are most familiar with is sucrose. This is a disaccharide, namely two simple monosaccharide sugar molecules (glucose and fructose) joined together. Disaccharides are split in two and absorbed in the small bowel with other free monosaccharides. When fructose, the “fruit sugar”, is absorbed into the blood stream it is carried to the liver and converted into glucose and fats that we use in every cell in the body for energy.
Fructose is absorbed via two mechanisms in the small bowel. The first major mechanism transports glucose in association with fructose across the gut wall. The second mechanism is dedicated to fructose alone and some people have low activity of the second transport mechanism. When fructose exceeds glucose intake in the diet these people cannot absorb the excess fructose in the small bowel and this sugar passes through to the large bowel where bacteria digest the fructose to short chain fatty acids (causing diarrhoea), carbon dioxide (gas, bloating, pain) and hydrogen.
Diagnosing Fructose Malabsorption
A fructose tolerance breath test is a useful test that allows diagnosis of
- Malabsorption of the fructose in the small bowel
- Bacterial overgrowth in the small bowel
An oral load of fructose is given and samples of breath are taken at baseline and at intervals for 2 hours afterwards. Rested, fasting people exhale only small amounts of hydrogen in each breath. Bacteria digesting a large fructose load will cause a spike of hydrogen detectable in the breath. The time of the hydrogen spike indicates the part of the bowel loaded with bacteria. A small number of people (<10%) do not carry bacteria that produce hydrogen and will not be suitable for this test. These people exhale methane that can be detected at baseline. Children (<16 years) or people who have had most of their large bowel removed are usually not suitable for this test.
Preparing for the test
4 weeks beforehand
- No antibiotics for 2 weeks before and during the test
- No colonoscopy or other faecal irrigation
3 days before the test
- No laxatives for 3 days before and during the test
Day before test
- No milk, fruit, fruit juices, onions, leeks, garlic, cabbage, beans
Evening before at 7pm
- Light meal of soup, and beverage
- Nothing further to eat or drink except water
- No smoking or chewing gum from now on
Morning of test
- 6-7 am: 1 litre of water to drink
- 7-8 am: brush teeth twice, no denture adhesive to be used
- Take other medicines as usual
- 8.30 am: Present to Suite 105 Medici Medical Centre for test
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