H

Glycerol

Figure

Fatty acids and glycerol result from fat digestion. Intestinal villi absorb them, and most are resynthesized into fat molecules before they enter the lacteals.

Chylomicrons are transported by the blood to capillaries of muscle and adipose tissues. A specific type of protein (apoprotein) associated with the surface of chylomicrons activates an enzyme (lipoprotein lipase) that is attached to the inner lining of such capillaries. As a result of the enzyme's action, fatty acids and monoglyc-erides are released from the chylomicrons, and they enter muscle or adipose cells for use as energy sources or to be stored. The remnants of the chylomicrons travel in the blood to the liver, where they bind to receptors on the surface of liver cells. The remnants quickly enter liver cells by receptor-mediated endocytosis (see chapter 3, p. 91), and lysosomes degrade them. Some fatty acids with short carbon chains may be absorbed directly into the blood capillary of the villus without being converted back into fat.

Be sure to read the labels on cakes claiming to be fat free. Often these products contain diglycerides, rather than the triglycerides found in dietary fats. Once diglycerides are broken down to their constituent fatty acids during digestion and enter the small intestine epithelium, they are reformed into fat. So, yes, these cakes are technically no-fat, but the result after digestion may be the same as eating the real thing.

In addition to absorbing the products of carbohydrate, protein, and fat digestion, the intestinal villi absorb electrolytes and water. Certain ions, such as those of sodium, potassium, chloride, nitrate, and bicarbonate, are readily absorbed; but others, including ions of calcium, magnesium, and sulfate, are poorly absorbed.

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