belief, we didn’t see any benefit of omega- 3 supplements for
stopping cognitive decline,” says lead author Emily Chew of the
National Eye Institute, part of the National Institutes of Health
(Bethesda, Maryland, USA). Nor did the researchers see benefits
of any supplement that they tested for either cognitive decline
A recent meta-analysis of six cohort studies with a total
of 22,402 participants likewise failed to detect an association
between fish oil intake and the risk of dementia or Alzheimer’s disease (Wu, S., et al., http://dx.doi.org/10.1016/j.neubio-
rev.2014.11.008, 2015). However, a higher intake of fish was
associated with a 36% lower risk of Alzheimer’s disease. For each
100 g of fish eaten per week, there was an 11% reduced risk of
Ismail suspects that negative results for neurocognitive
studies may be explained by insufficient dosages of DHA. A 2015
meta-analysis indicated that greater than 1.0 g of DHA per day
was required to improve memory in adults (Yurko-Mauro, K., et
al., http://dx.doi.org/10.1371/journal.pone.0120391). “There is a
cutoff where every single study conducted with over one gram of
DHA has found a benefit on cognitive function,” says Ismail. “And
almost every single study that had less than one gram found no
benefit.” (Fig. 1)
“Obviously, when there’s not an effect, you are always con-
cerned that you didn’t have the right dose of EPA or DHA, or it
was in the wrong ratio, but we point out these limitations in our
paper,” says Chew. “Also, our patients are much older, with an
average age of 73, so it may be that we are getting in the game
too late.” Perhaps starting omega- 3 supplementation at an
earlier age could have an effect on cognitive decline, she says.
MORE HARM tHAN gOOD?
Some researchers have hypothesized that instead of being
beneficial, omega- 3 supplements may actually be harmful to
human health because the polyunsaturated fatty acids are
highly susceptible to oxidation (Albert, B. B., et al., http://dx.doi.
org/10.1155/2013/464921, 2013). Omega-3s contain multiple
double bonds and bisallylic carbons (carbon atoms between
two double-bonded carbon atoms), which make them prone to
hydrogen loss and free radical formation. In a chain reaction, the
lipid radical can generate lipid peroxides and more radicals from
unoxidized polyunsaturated fatty acids. Lipid peroxides then
degrade into secondary oxidation products—aldehydes such as
4-hydroxyhexanol and malondialdehyde. These primary and
secondary oxidation products can damage cellular membranes,
proteins, and DNA.
During the manufacture of fish oil, the deodorization
process that removes the fishy odor often involves high
temperatures, which may accelerate secondary oxidation.
Antioxidants, most commonly vitamin E, added to fish oil can
reduce but not prevent oxidation. As a result, fish oil supple-
FIG. 1. Omega- 3 cognitive function studies and their outcomes sorted by DHA dosage.
Credit: Adam Ismail, GOED