Mitochondria - the tiny power centers inside cells that burn nutrients like sugar to make energy - are tightly controlled by the body's biological or circadian clock. Consequently, there is an optimum time when sugar-burning is most efficient.
This was the main conclusion of a study by researchers from the Weizmann Institute of Science in Rehovot, Israel, and the Max Planck Institute of Biochemistry in Martinsried, Germany.
The study was published in the Proceedings of the National Academy of Sciences.
The researchers say their findings may explain why people who sleep and eat out of phase with their body clocks are more likely to become overweight and obese and develop chronic diseases, such as diabetes and metabolic syndrome.
Study leader Dr. Gad Asher, who heads a lab at the Weizmann Institute that specializes in circadian clocks and metabolism, says that like all living things, we each have a circadian clock that controls our sleep, activity, eating and metabolism. He explains:
"In a sense it's like a daily calendar, telling the body what to expect, so it can prepare for the future and operate optimally."
In the study, he and his colleagues identified hundreds of proteins in the mitochondria of mice and measured their levels at different times of day and night. They found that 40% of the mitochondrial proteins peak once a day - although not necessarily at the same time.
The team also identified the proteins that form the circadian clock of the mitochondria and control fluctuations in the other proteins.
Circadian proteins in mitochondria have peak times
The researchers found that most of the circadian proteins in the mice's mitochondria peaked 4 hours into the daylight part of their cycle. (Note that mice are active at night).
Of these circadian proteins, there was one - an enzyme - that appeared to be particularly important for controlling the rate of burning sugar for energy. This enzyme peaked 4 hours into daylight, suggesting the mitochondria's optimum time for sugar use was also around this time.
The researchers checked this by giving mitochondria sugar at various times of day and found, indeed, that 4 hours into daylight was the time when respiration - the intake of oxygen needed for burning the sugar - and glucose use were at their highest.
The team also investigated the fat cycle in mitochondria. They found that the protein that lets fatty acids into the power centers peaks at a different time to the protein that controls the rate of sugar-burning. And again, they found fat-burning was most efficient at this time.
Finally, Dr. Asher and colleagues ran tests on mice genetically engineered so their biological clocks did not work properly. They found the mitochondrial proteins in these mice did not show a rise and fall pattern, and they processed sugar and fat at a steady rate throughout the day and night.
Dr. Asher says the results support previous findings where they showed if mice only eat at night, when they are active, they eat the same amount of calories as mice that also eat during the day, but their lipid levels (e.g. blood cholesterol) are 50% lower. He concludes:
"In other words, the outcome depends not only on what you eat but also on when you eat it. If we could be more aware of the timing of our cellular activities, we might be able to take advantage of various nutrients in a healthier way."
In April 2015, from another study of circadian rhythms, Medical News Today learned why metabolic problems may affect late-nighters more than early risers. It found that people who stayed up late, regardless of other lifestyle factors, had a higher risk of diabetes or reduced muscle mass, compared with similar people who got up early.