From this paper's conclusions:
The totality of the evidence supports a summary conclusion. Enhanced
nutrition has made us taller people and poor nutrition has made us more obese. But
our diet today probably does not make us very different people from our grandparents as far as cognitive competence is concerned. Our brains have altered since 1900, and they are better brains for solving the problems of our time. But they have altered rather like a muscle, that is, they have altered because we use them differently than our parents and grandparents did. The causes of this are many and the effects of nutrition, at least since privation has been banished, are too weak to stand out from the crowd.
Economics & Human Biology doi:http://dx.doi.org/10.1016/j.ehb.2009.01.009
Requiem for nutrition as the cause of IQ gains Raven's gains in Britain 1938 to 2008
James R. Flynn
The hypothesis that enhanced nutrition is mainly responsible for massive IQ gains over time borrows plausibility from the height gains of the 20th century However, evidence shows that the two trends are largely independent. A detailed analysis of IQ trends on the Raven's Progressive Matrices tests in Britain dramatizes the poverty of the nutrition hypothesis. A multiple factor hypothesis that operates on three levels is offered as an alternative instrument of causal explanation.
The Raven's data show that over the 65 years from circa 1942 to the present, taking ages 5 to 15 together, British school children have gained 14 IQ points for a rate of 0.216 points per year. However, since 1979, gains have declined with age and between the ages of 12 to 13 and 14 to 15, small gains turn into small losses. This is confirmed by Piagetian data and poses the possibility that the cognitive demands of teenage subculture have been stagnant over perhaps the last 30 years.