What is the TOTOX value and why does it matter for fish oil supplements?

The TOTOX value is a number that indicates how rancid a fish oil supplement has become. It is calculated as two times the so-called peroxide value plus the anisidine value. The first measure reflects recent oxidation, the second reflects breakdown products that have been accumulating for longer. Together they give a more complete picture than either number alone. The international GOED standard (a quality benchmark for fish oil) sets the upper limit at 26.

In practice, that standard is regularly exceeded. In a measurement of 72 American omega-3 supplements, 68% of the flavoured products had a TOTOX value that was too high. For unflavoured products, that figure was 13%. In New Zealand, only 8% of 32 tested products simultaneously met all three oxidation standards. In South Africa, more than 80% of the supplements measured in both 2009 and 2012 had a peroxide value that was too high. Price, expiry date and country of origin turned out to be unreliable signals of quality.

In addition to excessive oxidation values, most products also contain fewer active fatty acids (EPA and DHA) than the label promises. In New Zealand, 69% of products contained less than two thirds of the stated amount. In South Africa, the same was true for more than half of the products. So sometimes you get less of what you want and more of what you do not want.

Whether rancid fish oil is actually harmful to humans is difficult to establish conclusively. Animal studies show damage from oxidised fats, but clinical studies in humans have rarely reported the oxidation status of the oils tested to date. The potential health risks are therefore plausible but not yet clinically proven. What oxidation products such as aldehydes do cause is the familiar fishy smell and taste, one of the reasons people avoid supplements.