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Trans fats, Alzheimer disease and cognitive decline

The "Dietary Fats and the Risk of Incident Alzheimer Disease" study

A study published in Archives of Neurology in February 2003 showed that the intake of both trans fats and saturated fats promoted the development of Alzheimer disease - a truly terrible form of dementia, in which people of middle age and older are progressively stripped of their memories, identities, personalities and ultimately their lives.

The study, "Dietary Fats and the Risk of Incident Alzheimer Disease" by Martha Clare Morris, ScD et al., was based on a random sample of 815 people of 65 years or more in age, who were not affected by Alzheimers at the outset. It found a series of strong statistical correlations between dietary intake of fats and the risk of developing Alzheimers. Trans fats and saturated fat increased the risk of developing Alzheimers, while polyunsaturated and monounsaturated fats of vegetable origin reduced the risk.

The strongest effect was observed in the case of trans fats: the top 80 percent in trans fat consumption had, on average, four times the risk of developing Alzheimers (after adjusting for the effects of other fats), than the 20 percent with the lowest trans fat consumption (who typically ate only 1.8 grams of trans fat daily). Thus:

"We observed a strong increased risk of Alzheimer disease with consumption of trans-unsaturated fat."

In the case of saturated fats, there was a smilar but weaker effect: the top 80 percent in terms of saturated fat intake had a 2.5 times greater chance of developing Alzheimers than the 20 percent with the lowest intake (after adjusting for the effects of other fats).

Similar but inverse correlations were found for intake of omega-6 polyunsaturated fat and monounsaturated fats, of vegetable origin. The 20 percent with the lowest consumption of unsaturated vegetable fats had a five times greater chance of developing Alzheimers than the 20 percent with the greatest consumption (after adjusting for the effects of other fats).

The study also found a synergistic effect whereby the damaging effect of trans fat was increased if the diet was low in polyunsaturated fat, especially where polyunsaturated fat intake was less than 10 grams daily:

"The deleterious effect of increased trans-unsaturated fat intake on the risk of developing Alzheimer disease was substantially greater among persons with low polyunsaturated fat intake, whereas the effects were minimized among persons with high polyunsaturated fat intake"

Thus a person eating a high amount of trans fat and a low amount of polyunsaturated fat was found to have a nine times greater risk of developing Alzheimers than someone eating a low amount of trans fat and a high amount of polyunsaturated fat.

The study concluded:

"High intake of unsaturated, unhydrogenated fats may be protective against Alzheimer disease, whereas intake of saturated or trans-unsaturated (hydrogenated) fats may increase risk ... These data [...] provide promising evidence that diets high in unsaturated, unhydrogenated fats and low in saturated and trans-unsaturated fats may protect against dementing disease."

The "Dietary fat intake and 6-year cognitive change in an older biracial community population" study

A further study in Neurology published in 2004, also by Martha Clare Morris et al., found similar effects in a wider study of cognitive ability in people over 65. The study, Dietary fat intake and 6-year cognitive change in an older biracial community population followed 2,560 participants of the Chicago Health and Aging Project, ages 65 and older.

It found that higher intakes of trans-unsaturated and saturated fat

"were linearly associated with greater decline in cognitive score over 6 years. These associations became stronger in analyses that eliminated persons whose fat intake changed in recent years or whose baseline cognitive scores were in the lowest 15%. Inverse associations with cognitive decline were observed in these latter restricted analyses for high intake of monounsaturated fat and a high ratio of polyunsaturated to saturated fat intake. Intakes of total fat, vegetable and animal fats, and cholesterol were not associated with cognitive change."

Note also that the effect is weaker for people who had recently changed their diet: this indicates that they were still suffering, or enjoying, the effects of their diet in the period leading up to the study.

The study concluded:

"A diet high in saturated or trans-unsaturated fat or low in nonhydrogenated unsaturated fats may be associated with cognitive decline among older persons."

Other studies

A further study, "Dietary intake of fatty acids and fish in relation to cognitive performance at middle age" [Neurology Vol. 62, 2004, pp.275-280] found that omega-3 oils play a similar role to omage-6 and mono-unsaturates in the maintenance of cognitive function among middle aged and older people.

Niacin is likewise protective of mental function, as shown in Martha C. Morris et al., "Dietary niacin and the risk of incident Alzheimer's disease and of cognitive decline" [Journal of Neurology Neurosurgery and Psychiatry, Vol. 75, 2004, pp.1093-1099].

A further study, High monounsaturated fatty acids intake protects against age-related cognitive decline [Neurology Vol 52, May 1999, pp1563-1569, by Solfrizzi et al], shows the beneficial effect of dietary monounsaturates. It hypothesizes that as people age, their brain chemistry requires more monounsaturated fat to prevent degeneration.

"High MUFA intake per se could suggest preservation of cognitive functions in healthy elderly people. This effect could be related to the role of fatty acids in maintaining the structural integrity of neuronal membranes."

However it also concludes that polyunsaturates (PUFA) predispose towards Alzheimers - an opposite finding to Martha Clare Morris's research. One reason for this could be that the Solfrizzi report ignores trans fats, and trans fats will more often be associated with a high PUFA diet than a typical Italian diet rich in olive oil (which consists mainly of oleic acid, a monounsaturate). The negative findings on PUFA may thus result from trans fats in the diet, and not from PUFA at all.

Letting this aside, the statement of Solfrizzi et al regarding the role of fatty acids in "maintaining the structural integrity of neuronal membranes" is highly significant. Accepting that saturated and trans fats in the diet predispose to Alzheimers, it is reasonable to conclude that they do so by undermining the structural integrity of neuronal membranes.

The body will surely have a mechanism for limiting the amount of saturated fat that enters into the neuronal membranes - however trans fats, as unsaturates, may escape this mechanism. If confirmed, this would help to explain why trans fats are worse than saturated fats in predisposing to Alzheimer disease.

Policy implications

Given the huge costs, both personal and to the public purse, of Alzheimer disease and other forms of dementia and cognitive decline among middle aged and elderly people, there is a strong public policy case for ensuring that persons of middle age or older (arguably those 50 years or more in age), should pursue a diet that favours the maintenance of full mental function, with the potential of substantial and worthwhile reductions in the incidence of Alzheimer disease amd other dementias.

Based on the studies quoted above, that diet should be:

We would argue that the strong correlation between the dietary intake of different forms of fat and both Alzheimer disease and other forms of mental decline among elderly people is sufficient to justify:

As the age profile of populations of the UK and other developed countries advances, the benefits of such measures will only become more important. The potential reduction in incidence of Alzheimer disease and similar conditions is of far too great value to society as a whole, and to affected individuals and families, for policy makers and legislators to ignore.

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