I guess you are implying that we don't all have the "smart" version of the gene for that reason. But the brain accounts for about 25% of glucose consumption in

I guess you are implying that we don't all have the "smart" version of the gene for that reason. But the brain accounts for about 25% of glucose consumption in your body. So for genes which enlarge the brain, evolution would have sought the optimal trade-off between increased intelligence vs increased metabolic cost. There may well be no "deficiency" other than that increased cost.