Fat (macronutrient and lipid compounds)

Structure and types

At a molecular level many dietary fats are triglycerides: a glycerol backbone esterified to three fatty acids. Fatty acids differ by chain length and by the presence and position of double bonds. Major categories include saturated fats (no double bonds), monounsaturated fats (one double bond), polyunsaturated fats (multiple double bonds, including omega-3 and omega-6 families) and trans fats (unsaturated fats with a trans configuration). Some lipids are sterols (e.g., cholesterol) or phospholipids that form cell membranes.

Functions in the body

Dietary and stored fats perform several essential roles: they provide concentrated energy reserves, insulate against heat loss, cushion internal organs, form critical components of cell membranes, and serve as precursors to signaling molecules and steroid hormones. Fats are also necessary for absorption and transport of vitamins A, D, E and K. Certain fatty acids, such as linoleic and alpha-linolenic acids, are essential because the human body cannot synthesize them.

Digestion, transport and health

During digestion bile salts emulsify fats and lipases break triglycerides into absorbable units. Intestinal cells reassemble lipids into chylomicrons that enter the lymphatic system and bloodstream. In circulation, fats are carried by various lipoproteins; their balance influences cardiovascular risk. Health effects depend on type and amount: high intakes of industrial trans fats and excessive saturated fat are linked to increased heart disease risk, while monounsaturated and polyunsaturated fats (including marine omega-3s) are associated with cardiovascular benefits when they replace less healthy fats or refined carbohydrates.

Dietary sources and uses

Common sources include animal fats (butter, lard), plant oils (olive, canola, sunflower), nuts, seeds, and oily fish. At room temperature many saturated fats are solid ("fats") while unsaturated fats are liquid ("oils"). Beyond nutrition, fats and oils are used in cooking, food processing, cosmetics, soapmaking (saponification), and industrial applications such as biodiesel.

Notable facts

• Fats supply about 9 kcal per gram — more than carbohydrates or protein.
• Essential fatty acids must be obtained from the diet.
• Food processing (hydrogenation) can create harmful trans fats; many regulations and recommendations now limit their use.