Lipids and Nutrition

Unit 5: Lipids - Nutrition

Learning Objectives

• Understand the functions of fats in the body and diet.
• Identify the three major types of lipids, describing their structure, sources, and functions.
• Describe saturated, polyunsaturated, monounsaturated, and trans fatty acids.
• Identify and define essential fatty acids and their functions.
• Describe fat digestion and absorption processes.
• Describe lipid transport and utilization, and interpret blood cholesterol values.
• Explain dietary recommendations for fats and their impact on heart health.

Functions of Fats in the Body

• Storing Energy
  • Large, calorically-dense, long-term energy storage.
• Insulating and Protecting
  • Visceral fat: Protects vital organs within the abdominal cavity.
  • Subcutaneous fat: Insulates and pads under the skin.
• Regulating and Signaling
  • Hormone production.
  • Reproductive health.
  • Regulation of cholesterol, blood clotting, and inflammation.
  • Nerve impulse transmission.
  • Cell membranes.
• These functions can be met with a small amount of dietary fat, as the body can synthesize most lipid molecules.

Functions of Fats in the Diet

• Aid Absorption and Increase Bioavailability
  • For fat-soluble vitamins and phytochemicals.
• Contribute to Taste, Smell, and Satiety Value of Foods
  • Satisfy appetite by adding flavor.
  • Satisfy hunger by slowing digestion and absorption.
• Provide Essential Fatty Acids
  • Fatty acids that cannot be synthesized in the body.

Main Types of Lipids

• Lipids are molecules that don't mix well with water.
• Triglycerides
  • Main form of lipid in the diet and body.
  • Often referred to as "fat" in the body and food.
• Phospholipids
  • Structure is both fat and water-soluble.
• Sterols
  • Include cholesterol.

Triglycerides

• Main type of lipid in food and the body.
• Composed of 3 fatty acids on a glycerol backbone.
• Can contain different types of fatty acids.

Phospholipids

• Similar in structure to triglycerides.
• Composed of 2 fatty acids + 1 phosphate group on a glycerol backbone.
• Amphiphilic: Both fat- and water-soluble.
• Functions
  • Cell membranes.
  • Lipid transport.
  • Emulsification of food.

Sterols

• Multi-ring organic molecules.
• Cholesterol is the main sterol in animals.
  • Not an essential nutrient because we can synthesize it.
  • Only found in foods from animals (meat, poultry, eggs, fish, butter, dairy).
• Functions of Cholesterol
  • Precursor to vitamin D, bile, and hormones.
  • Part of cell membrane structure.
• Plant sterols are found in plant-derived foods (vegetable oils, nuts, seeds, whole grains) and can inhibit cholesterol absorption.

Fatty Acid Types

• Fatty acids differ in:
  • Chain length (2 to 24 carbons).
  • Degree of saturation (number of double bonds).

Foods Contain a Mixture of Fatty Acids

• Triglycerides can contain different proportions of fatty acid types.
• A given food or fat source contains a mixture of all 3 categories.
• Foods are often classified based on their predominant fatty acid type.
  • Example: Coconut oil is referred to as a saturated fat.
  • Example: Olive oil is referred to as a monounsaturated fat.

Types of Fatty Acids

• Saturated
  • No C=C double bonds; all carbons are saturated with hydrogens.
  • Pack together well, solid at room temperature.
  • Food sources: Animal fats, dairy products, coconut oil, palm kernel oil.
  • Health effects: May increase the risk of heart disease if in excess.
• Monounsaturated
  • 1 C=C double bond; one point of unsaturation.
  • Bent structure, liquid at room temperature.
  • Food sources: Canola, olive, peanut oils; avocados, almonds, pecans, sesame seeds.
  • Health effects: May decrease the risk of heart disease.
• Polyunsaturated
  • 2+ C=C double bonds; multiple points of unsaturation.
  • Bent structure, liquid at room temperature.
  • Food sources: Sunflower, safflower, soybean, corn oils; fish, flax, and walnuts.
  • Health effects: May decrease the risk of heart disease.

Omega-3, Omega-6, and Essential Fatty Acids

• Polyunsaturated fatty acids are omega-3 or omega-6 depending on the position of the first double bond from the omega end.
• There are 2 essential fatty acids, which must be consumed in the diet.
  • Linoleic acid (omega-6), found in vegetable oils like corn and soybean oils.
  • Alpha-linolenic acid (omega-3), found in nuts, flax, whole grains, legumes, and dark green leafy veggies.

Digestion and Absorption of Lipids

• Triglycerides are large, water-insoluble molecules that require special handling for digestion and absorption.
• Enzymatic digestion by lipases in the mouth, stomach, and small intestine.
• Emulsification by bile in the small intestine to break large fat globules into smaller droplets.

Lipid Digestion and Absorption in the Small Intestine

1. Emulsification by bile: Large fat droplets are emulsified by bile salts.
2. Enzymatic digestion by pancreatic lipase: Emulsified fat droplets are broken down by lipase into fatty acids and monoglycerides.
3. Absorption of products of fat digestion
   • Short and medium-chain fatty acids and glycerol (small products) are absorbed into the blood via capillaries.
   • Long-chain fatty acids and monoglycerides (large products) form into triglycerides and are transported in chylomicrons into lymph vessels.

Chylomicrons Transport Lipids from Intestine to Bloodstream

• Chylomicrons are a type of lipoprotein – lipid transport vehicles

Structure of Lipoproteins

• Allows lipids to move through the aqueous environment of the body.
• Composed of water-soluble components and insoluble lipids.

Lipoprotein Comparison

Functions of Lipoproteins

• Chylomicrons transport lipid from the small intestine and deliver triglycerides to the body’s cells.
• $VLDL$ are large and low density and deliver triglycerides from the liver to the body’s cells.
• $LDL$ are formed from $VLDL$ after removal of most triglycerides; they deliver cholesterol to the body’s cells.
• $HDL$ are the smallest and most dense lipoproteins; they clean up cholesterol and deliver it to the liver for disposal.

Understanding Blood Cholesterol Values

• Blood cholesterol or lipid panel values report total cholesterol, $LDL$, $HDL$, and triglycerides.
• There is no “good cholesterol” or “bad cholesterol” in food; cholesterol is just cholesterol.
• In blood, $LDL$ is considered “bad cholesterol” because too much $LDL$ in the blood can cause formation of plaques in artery walls, leading to atherosclerosis and increased risk of cardiovascular disease.
• Increasing physical activity, lowering saturated fat intake, and stopping smoking can help lower $LDL$. Medications such as statins are also used to treat high $LDL$.

Lipid Recommendations and Heart Health

• Dietary Guidelines for Americans
  • Calories from fat should make up 20-35 percent of total calories, with most fats coming from polyunsaturated and monounsaturated fats (e.g., fish, nuts, and vegetable oils).
  • Consume fewer than 10 percent of calories from saturated fats.
  • Keep the consumption of trans fats as low as possible.
  • Choose more lean and low-fat products when selecting meat, poultry, milk, and dairy products.
• What we know about dietary fat and heart disease risk
  • This relationship is complex and not entirely understood; it is not just dietary fat but the whole diet that matters.
  • Reducing saturated fat intake and replacing it with refined carbohydrates increases the risk of heart disease. Replacing saturated fat with unsaturated fats decreases the risk of heart disease.
  • Dietary cholesterol has only a small impact on blood cholesterol levels.
  • Look at the big picture: Focus on choosing whole grains over refined grains and whole food sources of unsaturated fats (nuts, seeds, fish) and fiber.