The effects of initiating a 24-hour fast with a low versus a high carbohydrate shake on glycemic control in older adults: a randomized crossover study.
Level 2 - randomized trial
Individual randomized crossover trial
PubMed 40234969 · doi:10.1186/s12986-025-00920-5
What was done
Twenty-four sedentary, overweight adults over age 50 (BMI ≥ 27, < 150 min weekly exercise) participated in a randomized crossover study to test how pre-fast macronutrient composition affects time-to-ketosis and glycemic markers during a 24-hour fast. Participants began each fast with either an isocaloric high-carbohydrate/low-fat/moderate-protein (HC/LF) or low-carbohydrate/high-fat/moderate-protein (LC/HF) shake. Outcomes included interstitial glucose every 15 min, capillary beta-hydroxybutyrate (BHB) at 0, 1, 4, 8, 12, and 24 h, and plasma insulin, glucagon, GLP-1, and GIP at 0, 1, 24, and 48 h.
What was found
Glucose levels were higher with HC/LF from 15 min to 2.25 h post-ingestion (p < 0.05). Nutritional ketosis (BHB ≥ 0.5 mmol/L) was reached on average by 12 h in the LC/HF condition but was not reached on average at any point in the HC/LF condition (condition-by-time interaction F = 3.84, p < 0.01). At 1 h post-meal, LC/HF resulted in 41.9% lower insulin (t = 6.13, p < 0.01), 23.6% higher glucagon (t = -4.72, p < 0.01), 26.8% higher GLP-1 (t = -5.16, p < 0.01), and 34.4% higher GIP (t = -3.41, p < 0.01) versus HC/LF.
Why it matters
Consuming a low-carbohydrate pre-fast meal accelerates the onset of nutritional ketosis and suppresses postprandial insulin surges during a 24-hour fast.
Limits
The sample was small (n = 24) and limited to older, sedentary, overweight individuals. The study evaluated acute responses over a single 24-hour fast rather than sustained health outcomes.
Cited by
- supports Individuals on high-carbohydrate diets almost never have blood beta-hydroxybutyrate levels above 0.3 millimolar.