Sex differences of human cortical blood flow and energy metabolism

Abstract

Brain energy metabolism is held to reflect energy demanding processes in neuropil related to the density and activity of synapses. There is recent evidence that men have higher density of synapses in temporal cortex than women. One consequence of these differences would be different rates of cortical energy turnover and blood flow in men and women. To test the hypotheses that rates of oxygen consumption (CMRO₂) and cerebral blood flow are higher in men than in women in regions of cerebral cortex, and that the differences persist with aging, we used positron emission tomography to determine cerebral blood flow and cerebral metabolic rate of oxygen as functions of age in healthy volunteers of both sexes. Cerebral metabolic rate of oxygen did not change with age for either sex and there were no differences of mean values of cerebral metabolic rate of oxygen between men and women in cerebral cortex. Women had significant decreases of cerebral blood flow as function of age in frontal and parietal lobes. Young women had significantly higher cerebral blood flow than men in frontal and temporal lobes, but these differences had disappeared at age 65. The absent sex difference of cerebral energy turnover suggests that the known differences of synaptic density between the sexes are counteracted by opposite differences of individual synaptic activity.

Keywords: Aging; cerebral blood flow measurement; energy metabolism; gender; positron emission tomography.

Figure 1.

Average images of CBF (mL/100 g/min) for males (top) and females. Before calculating average images, the individual scans were adjusted for study and age effects, and pCO₂ and hemoglobin concentration values.

Figure 2.

Average images of CMRO₂ (µmol/100 g/min) for males (top) and females. Before calculating average images, the individual images were adjusted for study and age effects.

Figure 3.

Plots of cortical gray CBF (mL/100 g/min) and CMRO₂ (µmol/100 g/min) in men (black circles) and women (red circles), adjusted for the effects of studies and age, and in the case of CBF also adjusted for PCO₂ and hemoglobin concentration. Black lines are regression line for men and red lines for women. For more details, see Table 1.

Figure 4.

Top row shows areas where women under 50 years have higher CBF than men in the same age group in percentage (not tested for significant differences). Color bar refers to percentage differences. Bottom row illustrates areas where women had significantly higher CBF than men. CBF differences and statistically significant clusters are superimposed on an average MRI brain template in MNI space.