Characteristics of Lipids

  • Lipids are a group of biomolecules that are insoluble in water but soluble in organic solvents.
  • They are composed of carbon (C), hydrogen (H) and oxygen (O) atoms, but have a higher proportion of C and H compared to O, making them less polar than carbohydrates or proteins.
  • Lipids provide more than twice the energy per gram than carbohydrates or proteins when metabolised.
  • They are generally hydrophobic, meaning they do not dissolve in water.

Types of Lipids

  • Triglycerides are the main lipids in the diet and body, made up of glycerol and three fatty acids.
  • Phospholipids have a similar structure to triglycerides but one fatty acid is replaced by a phosphate group. They are crucial in forming cell membranes.
  • Sterols or steroids, such as cholesterol, play roles in cellular structure and certain hormones.
  • Waxes are long-chain fatty acids linked to long-chain alcohols, providing protection and waterproofing in plants and animals.

Role of Lipids in the Body

  • Lipids are a major store of energy in the body, with fat cells being specially adapted for this purpose.
  • They are important for insulation and protection. Subcutaneous fat helps to maintain body temperature and around internal organs, fatty tissue provides cushioning to protect against damage.
  • Lipids are key in making up cell membranes, contributing to membrane fluidity and facilitating the transport of certain substances.
  • Certain lipids play a crucial role in the production of hormones, which regulate many body functions like metabolism and immune response.
  • Lipids, such as cholesterol, are necessary for the synthesis of vitamin D and bile salts.

Lipid Metabolism

  • Digestion breaks down dietary lipids into fatty acids and monoglycerides that can be absorbed by the cells of the small intestine.
  • Once absorbed, these components are reassembled into triglycerides and packaged with proteins and phospholipids into chylomicrons for transport in the blood.
  • Excess dietary lipids not immediately used for energy are stored as body fat.
  • When energy is needed, stored lipids are broken down through lipolysis into usable forms.
  • Lipids can also be synthesised in the body from excess dietary carbohydrates or proteins through a process called lipogenesis.

Lipids in Diet and Health

  • The types and amounts of lipids in the diet can greatly influence health.
  • Saturated fats, found in animal products and tropical oils, can raise levels of LDL cholesterol and potentially contribute to heart disease when consumed in excess.
  • Unsaturated fats, found in plants and fish, can lower LDL cholesterol levels and are a necessary part of the diet.
  • Trans fats, present in some processed foods, raise LDL cholesterol and also lower beneficial HDL cholesterol, and are associated with an increased risk of heart disease.