Lecture 5B: Lipids

FN 225: Nutrition
Tamberly Powell, M.S., R.D.
Health Professions Division
Lane Community College
Eugene, Oregon

Additional FORUM topics for Week 5:

1. What are your thoughts about the following articles? An NPR article, "The Full-Fat Paradox: Whole Milk May Keep Us Lean", "Is Whole Milk Healthier than Fat-Free and Low-Fat?" and a newsletter written by Ellyn Satter, RD, "Should you put your child on skim milk?"

How does the information in these articles support or conflict with the information you have learned in this class so far? Does this information make you think differently about the type of milk you are drinking?

LECTURE 5B: Chapter 5: The Lipids: Fats, Oils, Phospholipids and Sterols

First look through the major headings of this Lecture Outline in your packet:

Covered in Lecture 5A:

I The THREE types (kinds) of LIPIDS

II Where do lipids come from?
III Lipoproteins
IV Recommendations regarding lipids & heart health

Now we're ready for:

V Digestion and Absorption

VI Usefulness of Fats

IV Digestion and Absorption of Triglycerides
(As mentioned before, triglycerides are the major type of lipid in food and in the body.)

A. Mouth- digestion of triglycerides begins in the mouth. The tongue produces an enzyme that acts on triglycerides to free fatty acids from glycerol (especially short chain fatty acids like you find in milk). Name of enzyme- lingual lipase.

B. Stomach- little or no digestion happens in the stomach, but gastric lipase digests some triglycerides.

C. Small Intestine- here is where most enzymatic digestion and absorption occurs. First fat must be emulsified, which takes place thanks to bile. Without bile, fat would simple separate out from the watery digestive juices in the small intestine.

Thanks to bile, fat is able to mix with the watery digestive juices where enzymes can help break fat down into glycerol and fatty acids.

Remember bile is an emulsifier that is made in the liver and stored in the gallbladder. Bile functions in the small intestine, not the stomach. And bile is a chemical that is making a physical change. It is the enzymes that chemically break down fat.

Once triglycerides are emulsified, then triglycerides can be enzymatically digested in the small intestine. Enzymes that are made by the pancreas and small intestine are lipase enzymes. These enzymes act to split fatty acids from glycerol leaving glycerol, fatty acids, and monoglycerides to be absorbed.

Small products of lipid digestion (glycerol and shorter-chain fatty acids) can be absorbed into the intestinal cells and go directly to the blood.

Larger products of lipid digestion (monoglycerides and long chain fatty acids) need help. Once inside intestinal cells they are reformed into triglycerides, then incorporated into chylomicrons. Chylomicrons are absorbed into lymph, but eventually enter the blood and deliver triglycerides to cells.

Below is a simple animation that demonstrates how triglycerides are enzymatically digested. The animation mentions micelles (a term that is not defined or described in your text). You don't need to know about micelles for this course.

Here is the link to the above video: https://www.youtube.com/watch?v=siitsmRtMKM

Enzymatic digestion of triglycerides is complex! Try not to get too hung up on all the little details. Keep the big picture in mind. I want you to know:

where we find triglycerides in foods
that triglycerides need to be enzymatically digested
after enzymatic digestion glycerol, fatty acids, and monoglycerides are absorbed into intestinal cells.

Below is an example of the type of application question you will see on this material both within study questions and exams.

Q: If you drank a glass of whole milk. What carbohydrates and lipids would need to be enzymatically digested, and what would be absorbed into the cells lining the small intestine after digestion?

A: The carbohydrate in milk is lactose. Lactose is enzymatically digested down to its building blocks, glucose and galactose.

The lipid in whole milk that needs to be enzymatically digested is triglycerides. These are broken down into glycerol, fatty acids, and monoglycerides.

There is also cholesterol in whole milk (since this is an animal fat) but cholesterol is not enzymatically digested.

So, glucose, galactose, glycerol, fatty acids, monoglycerides, and cholesterol are absorbed into the cells lining the small intestine after enzymatic digestion of the carbohydrates and lipids in whole milk.

D. Absorption & Transport-

Small products of lipid digestion (glycerol and shorter-chain fatty acids) can be absorbed into the intestinal cells and go directly to the blood.

Below is an image of a typical lipoprotein, like a chylomicron. You can see that the triglycerides and cholesterol are tucked in the center hidden away from the watery body fluids, and protein and phospholipids are on the outside, helping to emulsify and stabilize the structure.

VI Usefulness of Fats

A. Value of Fats in the Body

1. FUNCTION of triglycerides and fatty acids in the human body

cushion (since triglycerides are stored under skin and around vital organs)
insulation layer (since triglycerides are stored under skin)
energy reserve (Body's own oil well.)
You don't need fat in the diet for these first 3 functions (cushion, insulation, energy) because you can make that fat from eating extra calories from carbohydrate, or protein.
specific functions of fatty acids in those triglycerides

Omega 6 Essential Fatty Acid (linoleic, shown below) are used in production of cell membranes, including skin, nerve & brain cell membranes.

The source of the slide photo above is unknown. In pencil, the slide says that the child on the left had a deficiency of essential fatty acids.

Perhaps he (or she?) was mistakenly given a fat-free formula or fat-free milk.

Perhaps he had an intestinal disease preventing fat absorption.

The slide also says in pencil that it's the same child on the right, once he got essential fatty acids.

Omega 3 Essential Fatty Acid (linolenic, shown below) are used to make hormone-like compounds (called eicosanoids) that regulate blood pressure, blood clots and the immune response. Notice the similarity between the structure of linoleic acid and the structure of linolenic acid

other important fatty acids like DHA & AA (they are NOT essential fatty acids and they are shown below)- are involved in visual acuity & brain development in infants.

DHA (docosahexaenoic acid) and AA (arachidonic acid; also called ARA)

2. FUNCTION of cholesterol in human body

used to make bile, which is an emulsifier (a substance that allows fat droplets to stay dispersed in water).
used to make vitamin D (Later you'll see a comparison of the structure of cholesterol and the structure of vitamin D.)
used to make reproductive hormones, like estrogen

You don't need cholesterol in the diet for these 3 functions cholesterol has in the body because your body can synthesize all the cholesterol it needs.

3. FUNCTION of phospholipids in human body.

Phospholipids are part of the lipoproteins. They are EMULSIFIERS in cell membranes as well as for lipoproteins.

As with cholesterol, you don't need phospholipids in the diet for these functions of phospholipids in the body because your body can synthesize the phospholipids it needs .

B. Value of Fats in the Diet

Fats & oils satisfy appetite (the desire to eat) because they satisfy our desire for flavor. Many flavor compounds are fat soluble so when the fat is removed from a food, much of the flavor is lost.

Consider fat-free cream cheese. When the fat is removed from the cream, much of the flavor goes also. As a result, many additives are used in an attempt to replace the lost flavor.
Fats & oils satisfy hunger (the need to eat) because fats are slow to be digested and absorbed. That means they may slow your need to eat again.

Here is a NPR story that demonstrates how fat impacts appetite and hunger.
The Full-Fat Paradox: Whole Milk May Keep Us Lean. (This is the same article that was listed above in the forum question).
Fats & oils carry the fat soluble vitamins, which are

vitamin A
vitamin E
vitamin K
vitamin D

You need at least 10% of your calories coming from fat to get the fat soluble vitamins in your diet. Vitamin E is very hard to get enough of with a low-fat diet since you find it in plant oils and products made with plant oils. Good sources of vitamin E are nuts like almonds, seeds like sunflower seeds, also whole grains, and even salad dressing.

High-fat fast foods are usually not great sources of Vitamin E, because Vitamin E is destroyed with heat. So your best bet are minimally processed whole foods.

Cholesterol (below)	Vitamin D (below)