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Nutrition · Chapter 4

Saturated, Unsaturated, Trans

A fatty acid is a chain of carbons with an acid group on one end. Almost everything a fat does — whether it is solid or liquid, how it stacks, how it behaves in the body — traces back to two questions about that chain: how many double bonds does it have, and which way do they bend it. Change the bonds below and watch the shape and the melting point move together.

Stearic acid

single bondcis double bondtrans double bond

The same chain, five copies side by side

Temperaturecool it down or warm it up
solid
Each faint dot is a point of contact — a weak van der Waals attraction — between neighbouring chains. More contacts means more heat is needed to pull the chains apart, so the fat melts at a higher temperature.
Stearic acid18:0 · saturated

Start from a familiar fat

Chain lengthcarbons, counted from the acid end
18 C
Double bondshow many C=C sit in the chain
0
Melting pointmeasured
69.6 °C
Fridge 4°C—
Room 20°C—
Body 37°C—
Reference intakes. There is no RDA for total fat in adults. The National Academies set an Acceptable Macronutrient Distribution Range of 20–35% of total energy from fat for adults 19 years and older. Adequate Intakes are set for the two essential fatty acids: linoleic acid 17 g/day (men 19–50 y) and 12 g/day (women 19–50 y); α-linolenic acid 1.6 g/day (men 19 y and older) and 1.1 g/day (women 19 y and older). The Dietary Guidelines for Americans advise less than 10% of daily calories from saturated fat for ages 2 years and older, and keeping trans fat intake as low as possible. No Tolerable Upper Intake Level has been set for any of these.
Energy checkevery fatty acid, whatever its shape
g
Melting points are laboratory values for the pure fatty acid. Real foods are mixtures of many fatty acids, so a whole fat softens over a range rather than melting at one temperature. Nothing here is individual medical or dietary advice.