A new randomised controlled trial has found that middle-aged men with an elevated BMI (body mass index) and high fasting blood sugar levels can’t process carbohydrates well in the evening.
This has to do with the link between our biological or circadian clocks and nutrient partitioning, i.e., how the body responds to the intake of carbs, fats and protein.

According to researchers from the German Institute of Human Nutrition (DIfE), someone who is already showing signs of insulin resistance and not metabolising carbs in an optimal way should avoid eating carb-rich meals later in the day.
We already knew that carbohydrate tolerance varies between individuals and that certain factors, like exercise increasing the uptake of glucose, can somewhat increase it. This means the body becomes more sensitive to insulin.
On the contrary, an eating pattern where carbs are consumed preferentially in the evening appears to significantly lower glucose tolerance.
In the study, 29 men – of whom 11 were deemed to have prediabetes – followed two diet plans for four weeks each where the time of day differed in which they consumed mostly carbs or fats.
The participants either consumed carb-rich foods from the morning until about 1:30 pm and high-fat foods from 4:30 pm to 10 pm (A), or high-fat foods in the morning and high-carbohydrate foods in the afternoons and evenings (B).
The results showed that men with prediabetes had a measurable increase in their blood sugar levels following plan B. The same time-dependent decline in carb tolerance, although not as pronounced, was found in healthy controls.
There are a few reasons for this. One theory researchers are exploring is that the secretion of hormones which are determinants of blood sugar regulatio, such as glucagon-like peptide-1 (GLP-1) and peptide YY (PYY), depends upon circadian clocks.
Another well accepted hypothesis is that insulin sensitivity is higher in all cells early in the day but decreases towards the afternoon and evening, thus we tend to partition carbs ingested at this time not as efficiently.
The high BMI of participants also contributes to decreased insulin sensitivity, which compounded with an unfavourable timing of eating, can further impact on blood sugars.
The researchers conclude that people with prediabetes should avoid large, carbohydrate-rich dinners to improve blood sugar control.

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