Gastrointestinal Lecture 4 Part 2 Digestion and Absorption of dietary fats

Question 1

The majority of fat in the diet is in the form of ________

Answer 1

triglycerides

Question 2

fat in the diet

Answer 2

• The average diet contains 65-95 g of fat, accounting for ~35% of total caloric intake
• In a diet with moderate fat intake, 95% of ingested fat is absorbed

Question 3

A key factor driving the process of fat digestion and absorption

Answer 3

is its insolubility in water

Question 4

The primary goal of fat digestion

Answer 4

to break down triglycerides into fatty acids

Unique aspect of fat digestion is that ingested lipids are generally insoluble in water, forming large globules of fat. Thus, fat must be emulsified prior to digestion

Question 5

Digestion of dietary fat: emulsification

Answer 5

Emulsification requires two things…

1. Mechanical digestion
2. Emulsifying agents contained in bile: phospholipids and bile salts = amphipathic molecules

• Polar regions = Hydrophilic (water-loving)
• Nonpolar regions = Lipophilic (lipid-loving

The hydrophobic side sorrounds the smaller fats and So that they cannot reform the large fat globule

Question 6

Digestion of dietary fat: lipolysis

Answer 6

The process of fat breakdown is often referred to as lipolysis

Triglycerides in emulsion droplets are acted on by lipase enzymes secreted from the pancreas

Binding of the water-soluble pancreatic lipase to the surface of the droplet is facilitated by a colipase. This amphipathic protein embeds into the lipid droplet surface and holds pancreatic lipase in place.

The products of lipolysis are monoglycerides and free fatty acids

Question 7

Digestion of dietary fat: micelles

Answer 7

The products of lipolysis are poorly water-soluble, and will form micelles with the hydrophobic fatty acyl tails projecting into the center, surrounded by more hydrophilic bile salts, etc.

A small fraction of the fat-digestion products are dissolved in solution and are free to be absorbed

Question 8

Absorption of dietary fat

Answer 8

• The small fraction of solubilized fatty acids can be absorbed across the epithelial cell membrane
• Uptake of fatty acids occurs by simple diffusion across the cell membrane, likely through a process referred to as “flip-flop”
• Fatty acids and monoglycerides taken up into epithelial cells are repackaged into triglycerides for secretion into the circulation via chylomicrons

Question 9

Absorption of dietary fat

Answer 9

• The small fraction of solubilized fatty acids can be absorbed across the epithelial cell membrane
• Uptake of fatty acids occurs by simple diffusion across the cell membrane, likely through a process referred to as “flip-flop”
• Fatty acids and monoglycerides taken up into epithelial cells are repackaged into triglycerides for secretion into the circulation via chylomicrons

Question 10

Digestion and absorption of dietary fat: summary

Answer 10

Into lymphatic system and can then enter the general circulation

Question 11

dietary protein in diet

Answer 11

A healthy adult requires 40 to 50 g of protein to supply essential amino acids
The average diet contains 60 to 90 g of protein, providing ~1/6 of total caloric intake

In addition to dietary intake, the intestine also receives large amount of protein that is secreted into it in the form of mucus, enzymes and degraded epithelial cells

Regardless of the source, the majority of protein (>95%) entering the intestine is broken down into amino acids and absorbed.

Question 12

How are polypeptides broken down?

Answer 12

• protein digestion requires the breakdown of large complex molecules into their respective building blocks
• Protein digestion primarily mediated by proteolytic enzymes (proteases)
• Because of their powerful activity, many of these enzymes are secreted as inactive precursors, thereby protecting the cells where they are manufactured
• Secreted precursor proteins are activated in the lumen by specific proteins