The human brain is, pro rata bigger and is far more complex in structure than in any other species. It tends to be a very busy organ and consumes about 25% of the daily caloric intake of an average person. This increases to about 50% of caloric intake in the children aged 1 to 6 years and reaches 55% in 4-6 month olds and a staggering 75% in newborn babies. Around about the age of 30, the human brain begins to shrink at the rate of 1 milligram per year and if that seems a small rate of decline (a lifetime reduction of 8% volume), the evolution of the human brain to its present size was also 1 milligram per year. Without question, the biggest fear people have in entering old age is a loss of cognitive function with Alzheimer’s disease the worst-case scenario. The issue is so important that it attracts all forms of snake-oil merchants promising this or that diet to stave off any decline in cognitive function.
The best place to start the task of ensuring a healthy brain throughout adult life is during pregnancy. During the third trimester of pregnancy, there is a growth spurt in brain development, which continues for 24 months. The human brain is about 60% fat and so when it comes to any discussion of nutrition and brain function, fat is bound to dominate. Specifically, the brain is rich in long chain highly polyunsaturated fatty acids that are abbreviated to EPA (Eicospentaenoic acid) and DHA (Docosahexaenoic acid). These fats cannot be synthesised by the human body and therefore have to be obtained from our diet. Fatty fish are by far the best source of these fatty acids and this raises an interesting question: Why, if the human brain was so important in our evolution, did we not develop the capacity to synthesise these fatty acids ourselves? Stephen Cunnane, author of “Survival of the Fattest” makes the case that man migrated from the Savannah to the shoreline of lakes, rivers and deltas where an abundance of fish, shellfish, eggs, bird and wild life existed. Such a food chain is rich in the brain type fatty acids, EPA and DHA, and there would have been no evolutionary advantage in having the energy demanding metabolic pathways to manufacture these fatty acids ourselves.
The growing foetus is totally dependent on a maternal supply of these fatty acids for brain development and when these enter the mother’s blood supply after a meal, they are preferentially transferred to the foetus. Other types of fats might be shared with the mother’s own fat reserves but not these precious fats. After birth, breast milk should contain adequate levels of these fatty acids and so too should infant formula. The problem begins to arise when post-natal nutrition is inadequate. The first 24 months of life sees a tremendous growth in brain complexity, especially in the frontal cortex through which the new baby acquires the social norms and language of its environment. Inadequate nutrition in this period will greatly diminish intellectual capital for the rest of such an individual’s life.
It goes without saying that an adequate intake of these fatty acids is required throughout adulthood and there are ample studies showing that inadequate intakes of fish oil type fatty acids are associated with a higher risk of loss of cognition in later adulthood. However, when the putative link between these fatty acids and cognitive decline are tested in dietary intervention studies, the evidence just evaporates. One possible reason for this is in the genetic predisposition to Alzheimer’s disease. There is a protein that is strongly involved in fat transport and distribution, abbreviated to Apo E and it can exist in three different forms of (Apo E2, E3 and E4) and we can inherit any two varieties from either parent. Thus 60% of the population has E3/E3 and they account for 65% of all Alzheimer’s cases. A smaller number (23%) of the population has the E4 variety either alone (E4/E4) or in combination (E4/E3) and this quarter of the population account for nearly half of all cases of Alzheimer’s disease. Thus with such a strong genetic dimension, intervention studies will eventually have to be conducted in which the individuals genetic make-up is taken into account. Moreover, the duration of the studies will have to become much longer if we are to identify a truly protective effect should such an effect actually exist. In addition to fish oil type fatty acids, there has been similar data for some of the B vitamins, most notably, folic acid and vitamin B12. Again, the association data seems very strong but again, when dietary intervention studies are conducted, little supporting evidence emerges. A higher body mass index in middle age is also a risk factor for Alzheimer’s disease and my guess is that this may arise because the more adipose tissue you have the more you will move EPA and DHA to that tissue away from blood which would normally be the route to the brain.
In Celtic mythology, the salmon was referred to as the fish of knowledge and maybe, even as these mythologies evolved, there was anecdotal evidence that fish was good for the brain. In modern Celtic Ireland we have a low intake of EPA and DHA and remarkably, 75% of our intakes of these vital nutrients come from fish oil capsules rather than fish. Some achievement for an island race!