Rancid fish oils, whether nutrient rich or not, can have a pro-oxidant effect within the body which is the opposite of an antioxidant effect. This can result in cellular damage. Such oils should strictly be avoided by infants, children, pregnant mothers and those with impaired immune systems.

A while back, a Norwegian Aquaculture Research Institute which specialises in marine raw materials and products conducted an initial series of in vitro studies on the extra virgin oils produced at ECO-Marine to determine their effects (versus the popular oils on the market) on oxidative stress parameters. Preliminary results suggest that the popular oils result in oxidative stress in the human cells tested, leading to cellular damage. The only oils which did not were the extra-virgin oils. The results are still early, but nevertheless very interesting.

Research indicates that rancid oils may also inhibit the cells’ own antioxidant system and reduces the inflammation-inhibiting effect of omega-3 fatty acids in cell models. Even more worrying is the fact that rancid fats have been implicated in increased rates of heart disease, atherosclerosis and are possibly carcinogenic. .

Analytical testing
Various methods can be used for screening and monitoring lipid peroxidation (oxidative rancidity) including measuring levels of:

  • Peroxide
  • Anisidine
  • Thiobarbituric Acid Reactive Substances’ (TBARS)
  • Free fatty acids
  • Acid values

At the initial stages of oxidative rancidity primary oxidation products are produced and the oil will have elevated levels of peroxide and anisidine. However, once the oil has been in a state of rancidity for a prolonged period of time the peroxide and anisidine values drop to normal or very low levels because the primary oxidation products (i.e. hydroperoxides) have been converted to secondary oxidation products. Hence, if you test an oil that is in an “advanced stage of rancidity” (or putrid) you will see low levels of peroxide and anisidine and assume that the oil is still fresh. Not so! The low levels are simply because the primary oxidation products have now been converted to secondary oxidation products. These secondary oxidation products can be detected by measuring TBARS. Oils which are in an advanced stage of rancidity will also typically have raised TBARS, a dark color and a strong taste and smell. The old time Norse fishermen and Vikings actually favored the clear/pale golden colored oil. This oil was fresh and healthy.

As a final note, another form of rancidity is “hydrolytic rancidity” which is evident in oils which have raised free-fatty acid levels and elevated acid values.

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