What role do lipids play in the body?

The Essential Roles of Lipids in the Human Body

Lipids serve as crucial components in the human body, functioning as energy storage, structural elements of cell membranes, signaling molecules, and precursors for essential compounds that directly impact mortality and quality of life through their effects on cardiovascular health, brain function, and overall metabolic processes. 1

Classification and Types of Lipids

Lipids can be broadly classified into two major categories:

Non-Hydrolyzable Lipids

• Fatty acids - Come in several forms:
  • Saturated (e.g., myristic acid, palmitic acid, stearic acid)
  • Monounsaturated (e.g., palmitoleic acid, oleic acid)
  • Polyunsaturated (e.g., linoleic acid, α-linolenic acid, DHA, EPA) 1
• Fatty acid derivatives - Including eicosanoids and specialized pro-resolving mediators (SPMs)
• Isopren derivatives - Including steroids like cholesterol, vitamin D, and steroid hormones 1

Hydrolyzable Lipids

• Acylglycerols - Including mono-, di-, and triacylglycerides (primary storage lipids)
• Phospholipids - Main structural components of cell membranes
• Sphingolipids - Including sphingomyelin and ceramide
• Cholesterol esters - Stored in intracellular lipid droplets 1

Key Functions of Lipids in the Body

1. Energy Storage and Provision

• Triglycerides (triacylglycerols) are the most abundant storage lipids in the body, serving as the major energy source 1
• When infused at 1-2 g/kg/day, lipid emulsions provide essential energy (approximately 10 kcal/g) 1
• Lipid oxidation increases by 15-24% during lipid infusion compared to baseline 1

2. Structural Components of Cell Membranes

• Phospholipids and cholesterol are the main structural lipids in cell membranes 1, 2
• Cholesterol serves as a structural component that maintains membrane integrity and fluidity 2
• Sphingomyelins are particularly rich in the myelin sheath of the central nervous system and play essential roles in axonal maturation and myelin integrity 1

3. Transport of Fat-Soluble Vitamins and Cholesterol

• Due to their hydrophobicity, lipids require specific transport vehicles (lipoproteins) for transportation in the blood 1, 2
• Cholesterol esters serve as the primary form for cholesterol transport in the bloodstream via lipoproteins, including chylomicrons, VLDL, LDL, and HDL 2
• HDL transports cholesterol from peripheral tissues to the liver, where it can be converted into bile acids and eliminated, providing a mechanism to withdraw cholesterol from circulation 1, 2

4. Brain Development and Function

• Long-chain polyunsaturated fatty acids (LC-PUFAs) like docosahexaenoic acid (DHA) and arachidonic acid (ARA) are critical for brain development 1
• Sphingolipids play essential roles in neuronal growth, migration, maturation, and myelination 1
• Rapid neurodevelopment and growth of the brain during the first two years of life coincide with peak accretion of brain complex polar lipids, including sphingolipids 1

5. Signaling and Regulatory Functions

• Fatty acids influence inflammatory and immune processes through effects on cell membrane structure and function, modification of inflammatory mediator profile, and alterations in gene expression 1
• Omega-3 fatty acids generally act in a less inflammatory or anti-inflammatory manner, while omega-6 fatty acids may promote inflammatory processes 1
• Sphingolipids like sphingomyelins and ceramides act as both structural and signaling lipids 1

6. Cholesterol Metabolism and Transport

• After absorption as free cholesterol in the intestine via the NPC1L1 protein transporter, cholesterol is re-esterified within enterocytes by ACAT2 2
• Cholesterol is then packaged into chylomicrons, secreted into the lymphatic system, and eventually enters the bloodstream 2
• Formation of cholesterol esters is catalyzed by lecithin-cholesterol acyltransferase (LCAT) in plasma and by acyl-CoA:cholesterol acyltransferase (ACAT) intracellularly 2

Clinical Implications

Cardiovascular Health

• Dyslipidemia is highly prevalent in patients with overweight and obesity, often following an atherogenic pattern characterized by:
  • Hypertriglyceridemia
  • Lower concentrations of HDL-cholesterol
  • Formation of small dense LDL and oxidized LDL 1
• Abnormal cholesterol ester metabolism is associated with increased risk of atherosclerosis and metabolic syndrome 2

Metabolic Health

• Lipid metabolism is a key regulator in storage and provision of energy 1
• Disruptions in lipid metabolism can lead to metabolic disorders and contribute to conditions like gestational diabetes mellitus and hypertensive disorders of pregnancy 1

Brain Development

• Essential fatty acids that cannot be synthesized by the human body (α-linolenic acid and linoleic acid) must be obtained through diet 1
• These essential fatty acids serve as precursors for signaling lipids and are critical for proper brain development and function 1

Practical Considerations

• Essential fatty acids: Ensure adequate intake of α-linolenic acid and linoleic acid through diet, as these cannot be synthesized by the human body 1
• Balanced fatty acid intake: Consider the balance between omega-3 and omega-6 fatty acids, as omega-3 fatty acids can counter the potentially pro-inflammatory actions of omega-6 fatty acids 1
• Cholesterol management: Maintain healthy cholesterol levels through diet, exercise, and if necessary, medication to reduce cardiovascular risk 2, 3

Understanding the complex roles of lipids in the body is essential for maintaining optimal health and preventing diseases related to lipid metabolism disorders.