The human brain produces fructose from glucose.
Level 4 - case-series / case-control
Single-arm experimental human physiological study without a separate control group
PubMed 28239653 · doi:10.1172/jci.insight.90508
What was done
Eight healthy adults (4 women, 4 men; mean age 28.8 ± 6.2 years; BMI 23.4 ± 2.6; HbA1C 4.9% ± 0.2%) underwent a 4-hour hyperglycemic clamp protocol targeting a plasma glucose level of 220 mg/dl. Intracerebral glucose and fructose concentrations were monitored throughout the clamp using 1H magnetic resonance spectroscopy (MRS) and analyzed using mixed-effects regression models.
What was found
Intracerebral glucose concentrations rose significantly above baseline from 20 to 230 minutes of the clamp. Intracerebral fructose concentrations also rose significantly above baseline from 30 to 230 minutes. Changes in intracerebral fructose were correlated with intracerebral glucose levels but showed no relationship with circulating plasma fructose levels.
Why it matters
This demonstrates that the human brain can endogenously convert glucose to fructose via the polyol pathway during hyperglycemia, indicating that CNS exposure to fructose does not depend entirely on blood-brain barrier transport of dietary fructose.
Limits
The sample size was very small (n = 8) and restricted to young, healthy, normal-weight individuals. The experimental hyperglycemic clamp (220 mg/dl for 4 hours) represents an artificial condition rather than typical daily glycemic excursions, there was no normoglycemic control cohort, and behavioral or downstream clinical endpoints were not assessed.
Cited by
- supports Only a small amount of dietary fructose enters the brain, but consumption of sugar, salt, and high-glycemic carbohydrates stimulates endogenous fructose production in the brain.