Dietary Fats and Digestion

Dietary Fats

• Dietary fats, also known as lipids, are a group of compounds that are insoluble in water but dissolve in fat solvents. Primarily composed of carbon, hydrogen, and oxygen, fats contain a higher proportion of carbon and hydrogen than carbohydrates.
• Fats are a concentrated energy source, providing 9 kilocalories per gram.
• Fats play a crucial role in protein sparing, insulation, and organ protection.
• Fats are essential for the absorption of fat-soluble vitamins (A, D, E, K).
• Fats are necessary for hormone and cell membrane synthesis.
• Fats slow the emptying of the stomach.
• Fats affect satiety (fullness).

Digestion of Fats

• Fat digestion begins in the mouth, where chewing breaks fat into smaller particles.
• No significant chemical digestion occurs in the mouth.
• Gastric lipase in the stomach converts fat to emulsified form. Primary fat digestion occurs in the small intestine.
• Bile, secreted from the gallbladder, acts as an emulsifier, breaking down large fat particles into smaller ones, increasing the efficiency of enzyme action.
• Pancreatic lipase, from the pancreas, further digests fats in the small intestine.
• The alkaline nature of bile facilitates pancreatic lipase's ability to break down triglycerides into diglycerides, monoglycerides, fatty acids, and glycerol.
• Undigested fat is excreted through the feces.

Absorption of Fats

• Fat absorption occurs via two pathways:
  • Hepatic portal vein: This path is for medium-chain fatty acids and monoglycerides (chains of 10-12 carbon atoms).
  • Lymph system: Long-chain fatty acids and monoglycerides are resynthesized into triglycerides within the mucosal cells. These triglycerides are then packaged with phospholipids, cholesterol, and protein to form chylomicrons. Chylomicrons are transported via the lymphatic system and eventually enter the bloodstream.

Fat Metabolism

• After absorption, fats can be oxidized by body tissues to produce energy (CO2 + H2O) or stored in fat deposits.
• Unlike carbohydrates, fat storage can vary significantly.
• Fats are not a primary source of blood glucose but can contribute to ketone body production, which serve as an energy source for tissues when glucose availability is low.
• Glycerol, a byproduct of fat digestion, can enter the glycolytic pathway and potentially be converted into glucose (gluconeogenesis).
• Fatty acids are broken down into acetyl CoA, which can enter the citric acid cycle or be converted to acetoacetate (a precursor to ketone bodies).
• Most tissues (excluding the brain) can completely break down ketone bodies to produce CO2 + H2O and ATP.

Food Sources of Fats

• Most foods contain some level of lipids.
• Some fats are noticeable (e.g., added fats and oils).
• Many sources of fat are hidden (e.g. meat, milk, egg yolks), and must be considered in dietary analysis.
• Plant sources: oils, nuts, seeds.
• Animal sources: meat, fish, poultry, milk, dairy products
• Invisible sources: many items including desserts, pastries, and processed foods.

Deficiency of Fats

• A deficiency of essential fatty acids may cause problems with cell membranes and their permeability.
• Deficiency symptoms include eczema, dry scaly skin lesions, and issues with growth and weight.

Excessive Fat Intake

• Percentage varies regionally and by socioeconomic status.
• 20-25% of total calories should come from fats.
• Excessive fat intake can lead to obesity, high blood cholesterol, and cardiovascular diseases.
• Foods rich in omega-3 fatty acids may have a protective effect.

Recommended Dietary Allowances for Fats

• Daily fat intake should be limited to 30% of total calories.
• Saturated fat intake should not exceed 10% of total calories.
• Polyunsaturated fatty acids (PUFAs) should contribute at least 10% of total calories to ensure adequate intake of essential fatty acids.
• Proper ratio of omega-3 and omega-6 fats should be maintained.
• Cholesterol intake should not exceed 180 mg/day.

Fat Reduction Strategies

• Use skim milk and dairy products.
• Choose lean meats.
• Employ cooking methods like steaming, boiling, or poaching instead of frying.
• Limit salad dressings or use low-fat options.
• Replace baked goods and pastries with fruit-based desserts.
• Add herbs, spices, and lime juice for flavor instead of fat.

Interrelationship of Fat, Protein, and Carbohydrate Metabolism

• Fats, proteins, and carbohydrates are intertwined in metabolic processes.
• Glucose, and amino acids, can be converted to acetyl CoA, which can enter the citric acid cycle or be converted into other metabolites/energy stores.
• Fats, proteins, and carbohydrates are interchangeable to an extent.