Nineteen healthy individuals took part in a crossover study in which, for three days in a clinical setting, they adhered to diets compliant with the World Health Organization’s healthy eating guidelines (for example, rich in fruit, vegetables and fibre and low in saturated fat, sugar and salt) and unhealthy, typically ‘Westernized’ diets. Fasting, after breakfast, after lunch and after dinner blood samples (each postprandial sample taken at 2 hours) and 24-hour urine samples were taken on day 3 for analysis. A panel of five SCFAs, three methyl-branched SCFAs and two hydroxylated carboxylic acids were analysed by gas chromatography–mass spectrometry.
The concentrations of serum SCFA depended more on the time of day and relative timing of meals than on dietary quality. For example, there was no effect of having eaten breakfast on SCFA concentration or composition — with the authors suggesting that an overnight fast equilibrates SCFA concentrations and, with typical colonic transfer time of 6 hours and fermentation taking several hours, the 2-hour sampling timepoint might be insufficient to capture effects. After lunch and after dinner, differences emerge between healthy and unhealthy diets. For example, 2-methylbutyric acid (derived from leucine, an amino acid associated with animal source food; and associated with metabolic syndrome) and 2-hydroxybutyric acid (associated with oxidative stress) were higher in the healthy diet groups than the unhealthy diet groups. Lactic acid, on the other hand, was higher after consumption of the unhealthy diet. Butyric acid peaked during the overnight fast; acetic and propionic acids peaked after lunch.
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