Intermittent fasting is among the most popular dieting protocols for weight loss and other health benefits, including decreasing fasting blood glucose and insulin resistance, leptin levels, and increasing adiponectin levels [1]. While time-restricted feeding (TRF) diets don’t prescribe a set schedule, the most optimal fasting window is a highly debated topic.
In a video shared on Dec. 21, 2023, natural bodybuilder-turned-powerlifter and nutritional scientist Dr. Layne Norton delved into a scientific review published in Nov. 2023 in Reviews in Endocrine and Metabolic Disorders that examined whether fasting during the morning or evening leads to different outcomes in metabolic markers like HbA1c (hemoglobin A1C (blood test)), HOMA-IR (homeostatic model assessment for insulin resistance), fasting blood insulin, and glucose. [2]
Check out the video below, courtesy of Norton’s YouTube channel:
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Understanding Time-Restricted Feeding (TRF)
Time-restricted feeding (TRF), a form of intermittent fasting, involves consuming food within a specific time window each day. The study in question categorized TRFs into two types:
- Early time-restricted feeding (eTRF) — 8:00 a.m. to 4:00 p.m.
- Late time-restricted eating (lTRF) — 12 noon to 8:00 p.m.
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Study Findings and Analysis
The meta-analysis revealed that, overall, TRF led to a reduction in HbA1c levels. However, it did not significantly affect fasting glucose or insulin levels. Norton was not surprised by these findings as calorie restriction, a common outcome of TRF, typically leads to HbA1c improvements.
However, the meta-analysis resulted in a notable finding: the eTRF and lTRF subgroup analysis showed that eTRF was more effective in lowering fasting blood glucose and appeared slightly more beneficial in reducing fasting blood insulin levels.
Norton expressed skepticism about these findings as well, arguing that if eTRF had a long-term effect, it would be reflected in the HbA1c data, which was not the case.
HbA1c was not better for early time-restricted feeding versus late time-restricted feeding.
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The Importance of HbA1c
HbA1c is a site on hemoglobin, the molecule that can become glycosylated. Since red blood cells take about 120 days to turn over, the amount of glycosylated hemoglobin is a good indicator of long-term blood glucose levels; the higher the overall blood glucose, the more glycosylated hemoglobin will be.
Norton opined that the researchers didn’t find a difference in glycosylated hemoglobin because they took the fasting blood glucose measurements at the same time in the morning for both TRF groups.
“If you are early time-restricted feeding and your final meal is at 4 p.m. versus late time-restricted feeding where your last meal is at 8:00 p.m., you’ve had 4 hours extra to fast,” Norton said.
To validate that eTRF is more effective in lowering fasting blood glucose and insulin levels, Norton suggested staggering the blood glucose measurement times in future studies to account for the difference in fasting duration.
Dr. Layne Norton’s Takeaway
Despite the study’s findings, Norton emphasized the importance of adhering to a feeding window that aligns with an individual’s lifestyle and preferences, whether eTRF or lTRF.
The slight fasting blood glucose differences observed in the study, if legitimate, should not override the importance of following a fasting pattern that fits an individual’s lifestyle and can be maintained over the long term.
References
- Albosta M, Bakke J. Intermittent fasting: is there a role in the treatment of diabetes? A review of the literature and guide for primary care physicians. Clin Diabetes Endocrinol. 2021;7(1):3. Published 2021 Feb 3. doi:10.1186/s40842-020-00116-1
- Rovira-Llopis S, Luna-Marco C, Perea-Galera L, Bañuls C, Morillas C, Victor VM. Circadian alignment of food intake and glycaemic control by time-restricted eating: A systematic review and meta-analysis. Rev Endocr Metab Disord. 2023 Nov 22. doi: 10.1007/s11154-023-09853-x. Epub ahead of print. PMID: 37993559.
Featured image via Shutterstock/vetre
