Addition of hydrogen at this site saturates the bond. However, binding to the catalyst is not stable, and if the fatty acid is released before saturation the double bond may be regenerated in either the cis or trans configuration. Since formation of the trans isomer is favored energetically, this structure tends to dominate. By varying the temperature, pressure, catalyst, processing time, and type of oil, fats with different characteristics can be produced. In recent years, these parameters have been manipulated to produce cooking fats with lower levels of trans fatty acids.
Soft margarines packaged in tubs have lower levels of trans fats than the harder, stick margarines.
The lower estimates were calculated from food-frequency questionnaires, which are relatively narrow in scope and do not reflect the wide variations in dietary habits. The higher values are estimated from market availability and disappearance data, which do not consider such factors as the disposal of hydrogenated frying fat after use. Some research has found a decrease in HDL cholesterol in response to trans fatty acids, but the data are inconsistent. The mechanisms by which these changes in serum lipids are induced are unknown, but the similar geometry of trans and saturated fatty acids may explain their similar physiological effects. It is notable that dietary changes to improve serum lipid profiles normally replace saturated, not unsaturated, fats with trans fats.