An unsaturated fat is a fat or fatty acid in which there are one or more double bonds in the fatty acid chain. A fat molecule is
if it contains one double bond, and
if it contains more than one double bond.
Where double bonds are formed, hydrogen atoms are eliminated. Thus, a
is "saturated" with hydrogen atoms. In cellular metabolism
hydrogen-carbon bonds are broken down - or oxidized - to produce energy, thus an unsaturated fat molecule contains somewhat less energy
than a comparable sized
The greater the degree of unsaturation in a fatty acid (ie, the more double bonds in the fatty acid), the more vulnerable it is to lipid peroxidation
(rancidity). Antioxidants can protect unsaturated fat from lipid peroxidation. Unsaturated fats also have a more enlarged shape than
Double bonds may be in either a cis or trans isomer, depending on the geometry of the double bond. In the cis conformation hydrogens are on the same
side of the double bond, whereas in the trans conformation they are on opposite sides. Saturated fats are popular with manufacturers of processed
foods because they are less vulnerable to rancidity and are generally more solid at room temperature than unsaturated fats. Unsaturated chains have
a lower melting point, hence increasing fluidity of the cell membranes.
fats can replace
in the diet, though trans unsaturated fats should be avoided.
Replacing saturated fats with unsaturated fats helps to lower levels of total cholesterol and LDL cholesterol in the blood. Trans unsaturated fats are
particularly bad because the double bond stereochemistry allows the fat molecules to assume a linear conformation which leads to efficient
packing (i.e., plaque formation). The geometry of the cis double bond introduces a bend in the molecule precluding stable formations. Natural
sources of fatty acids are rich in the cis isomer (olive oil
Saturated fats (like butter or lard) and fatty acids with trans double-bonds (like margarine) tend to be solids at room temperature, whereas natural
fatty acids with cis double-bonds (like vegetable oils) tend to be liquids. By artificially hydrogenating vegetable oils, the food processing industry
reduces the number of double bonds and causes the formation of trans fatty acids. Hydrogenation results in margarines that are more solid and less
vulnerable to rancidity. Hydrogenation results in peanut butter with a trans-fat-containing oil that does not separate from the peanut paste. But
when trans fatty acids are incorporated into cell membranes, the membrane fluidity is reduced and the cells do not function as well. Not all trans
fatty acids in the diet are due to food processing. For example, natural butter is 5% trans fat.
Although unsaturated fats are healthier than saturated fats
, the old
Food and Drug Administration (FDA)
recommendation stated that
the amount of unsaturated fat consumed should not exceed 30% of one's daily caloric intake (or 67 grams given a 2000 calorie diet). The new dietary
guidelines have eliminated this recommendation.
Most food contain both unsaturated and saturated fats. Marketers only advertise one or the other,
depending on which makes up the majority.
Thus, various unsaturated fat vegetable oils, such as
, also contain