Sugar - The Brainfood That's Making You Dumb | Surprise: Fructose and Artificial Sweeteners are Not a Problem!

None of these qualifies as "brain booster".

A few years ago the results of a recent study from New Zealand would probably have made the mainstream news: A double-blind, placebo-controlled, cross-over experimental design was used. "Results revealed that ingestion of glucose and sucrose led to poorer performances on the assessed
tasks as opposed to fructose and the placebo" (Ginieis 2017). These days, however, sugar is everyone's boogieman, anyway. It is thus unlikely that you will read about Rachel Ginieis paper anywhere else, but here - and that despite the fact that its results could have a profound influence on your cognitive abilities.
As the authors point out, in recent years, there has been increasing interest in studying cognitive effects associated with sugar consumption. Interestingly enough, most research has hitherto focused on the confirmation of glucose, as a main energy substrate for the brain - one that can momentarily benefit cognitive performances, particularly for memory functioning.

Glucose ingestion has been found to have a facilitating effect on memory tasks known to depend on the hippocampal region 8 in both young and elderly groups, including individuals with neurological diseases or metabolism conditions. In contrast, studies that assessed glucose effects on other cognitive domains have produced somewhat mixed results. Specifically, previous studies have examined glucose effects on reaction time, attention, face recognition, working memory , and related types of tasks. Of these, a few studies observed small facilitative effects on information processing speed and attention, when measured using the trail-making test, letter symbol digit test, or the Stroop test. By contrast, other studies found either no difference or deteriorative effects " (Genieis 2017).

While scientists have long been speculating that the differences could be a result of differential effects of sugar on different tasks with the test results depending on the type of test that was used, it is or rather has not been clear if the effects differ between different forms of "sugar" (e.g., fructose and sucrose), ... until now:

"The present study tested in 49 people the effects of three common dietary sugars against a placebo sweetener (i.e., sucralose), on performance of three well-studied cognitive tasks - simple response time, arithmetic, and Stroop interference, all of which are suggested to rely on the prefrontal lobe. A double-blind, placebo-controlled, cross-over experimental design was used" (Ginieis 2017).

A total of 49 individuals participated. They were randomly assigned into an overnight 10-hour fasting group (N=26; 15 females; 22.6 ± 4.2 years of age) or a non-fasting group (N=23; 13 females; 24.3 ± 4.9 years of age). Participants fell within a BMI range of 17.6 – 32.7 kg/m2 (fasting: 23.2± 3.56; non
fasting: 24.1±3.87). A set of univariate analyses confirmed that neither age [t(47)=0.89, p=.38]
nor BMI [t(47)=0.984, p=.17] differed significantly across the fasting and non-fasting groups.

Figure 1: Performance measures in each of four experimental conditions on the three cognitive tasks (Ginieis 2017).

Data collection took place over 16 consecutive weeks. Each participant was assigned to a testing session and asked to attend the same session every week for 4 weeks. In order to minimise biases due to differences in testing time, effort was made to counterbalance testing times between the two experimental groups.

• For the fasting group, the 26 participants were randomly assigned to the 7.00h and 10.00h sessions. 
• For the non-fasting group, eight participants were assigned to the 7.00h, ten participants were in the 10.00h, and the remaining five participants attended the session at 15.00h.

As previously pointed out, the study's "[r]esults revealed that ingestion of glucose and sucrose led to poorer performances on the assessed tasks as opposed to fructose and the placebo (p<0.05)" (Ginieis 2017) - with the effects being particularly pronounced under the fasting condition in comparison to the non-fasting condition (p<0.001).
References:

• Ginieis, Rachel,  Elizabeth A. Franz, Indrawati Oey, Mei Peng. "The 'sweet' effect: Comparative assessments of dietary sugars on cognitive performance." Phb (2017) - doi:10.1016/j.physbeh.2017.12.010