Thursday, September 01, 2011

Why You Should Not Eat Too Many Fruits at One Time

To answer this question, you need to appreciate that the absorption of the frustose molecules in your body is not straightforward, it requires some help from a fructose transmembrane transporter called GLUT5, which is specialized in this task.

Let's begin with our DNA program. The red line in the diagram below marks the location of the SLC2A5 gene in Human Chromosome 1.


The SLC2A5 is the genetic program which carries the instruction to manufacture the GLUT5 in your small intestine. The size of the SLC2A5 program is 53,371 base-pair, approximately 6.5 kilobytes if stored in binary codes (quite a small program!).

The purpose of the GLUT5 transporter is to carry and transfer fructose in intestinal fluid to the blood line across the intestinal epithelial membrane. You can imagine GLUT5 as a truck specially designed to carry fructose molecules. The number of GLUT5 transporter in your intestine is finite and if you eat too many fruits, you will not have enough GLUT5 transporter to carry the fructose molecules and put them into the blood line. The average absorption threshold for one-time fructose consumption is 0.15 mol (about 4 apples)

When this happens, the intestinal osmotic pressure increases and water molecules in the blood line will diffuse into intestine. The fructose molecules will then get flushed to colon and consumed and fermented by bacteria living there.

Fermentation of fructose releases gases like methane, hydrogen and carbon dioxide and propels intestinal peristalsis and, lo and behold, it's toilet time!

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