Determining the Minimum Dietary Carbohydrate Requirement in Apparently Healthy Adults: Findings from a 16-Day Controlled Feeding Trial.

BACKGROUND

Defining the minimum carbohydrate requirement is crucial for health and disease prevention. However, the estimated average requirement (EAR) for carbohydrates in adults was based on limited data that might not reflect physiological adaptations or individual differences.

OBJECTIVES

We aimed to assess 1) the minimum dietary carbohydrate requirement in apparently healthy Chinese adults by identifying the highest carbohydrate intake that avoids nutritional ketosis, 2) the effect modification by sex and age, and 3) validate a novel ex vivo method to monitor the 24-h dynamics of β-hydroxybutyrate (BHB).

METHODS

In this controlled feeding trial, 22 healthy adults first completed a 3-d very low-carbohydrate diet (20 g/d) to induce nutritional ketosis (fasting venous BHB ≥0.5 mmol/L). Carbohydrate intake was then progressively increased (40, 50, 70, 90, and 110 g/d) while maintaining constant protein and fat intakes. We measured circulating BHB levels via the reference-standard venous blood biochemistry and a novel ex vivo method for continuous ketone monitoring (CKM). Linear mixed-effects models were used to estimate the carbohydrate intake threshold corresponding to a BHB concentration of 0.5 mmol/L.

RESULTS

During the study period, participants' resting metabolic rate remained unchanged, although a shift from carbohydrate to fat oxidation was confirmed during ketosis. A significant negative nonlinear relationship was observed between carbohydrate intake and BHB levels. The minimum carbohydrate requirements were ∼105 g/d for males and 93 g/d for females, as indicated by venous blood measures, and 88 g/d for males and 78 g/d for females via CKM. There was a strong agreement between the CKM and venous measurements (intraclass correlation coefficient = 0.93).

CONCLUSIONS

Findings from this controlled feeding study established the minimum carbohydrate requirement to avoid significant fat mobilization in healthy Chinese adults, revealing a marked sex difference, and validated the clinical utility of a novel ex vivo method for dynamic metabolic monitoring. This trial was registered at clinicaltrials.gov as ChiCTR2500102659. (https://www.chictr.org.cn/showproj.html?proj=272503).