In mice fed a high-fat diet, shorter light and dark cycles delayed obesity by about eight weeks

In mice fed a high-fat diet, shorter light and dark cycles delayed obesity by about eight weeks

On September 1, researchers reported in The FASEB Journal that they had compared three light-dark schedules in female mice from a single laboratory strain fed a high-fat diet. Each group contained eight animals. The schedules differed in the duration of each light-dark cycle.

Mice have an internal clock. In constant darkness, their activity follows an intrinsic period called tau. Early in the experiment, tau was longer in mice fed a high-fat diet than in mice fed a low-fat diet, even before obesity developed. This observation led the researchers to test whether an external light schedule aligned with the internal rhythm would alter the time required for obesity to develop.

To test this possibility, mice on the high-fat diet were exposed to a standard 24-hour day, a cycle of about 23.64 hours that was close to tau, or a cycle of 23.28 hours. Light and darkness each occupied half of every cycle. Under the 24-hour schedule, obesity was recorded after 4.0 ± 1.1 weeks. Under the two shorter cycles, it was recorded after 12.1 ± 1.6 and 11.3 ± 1.6 weeks. Obesity developed in all three groups, but the shorter days delayed its onset by about eight weeks.

Cumulative energy intake was similar across the light schedules. From the second week onward, mice exposed to the shorter cycles maintained higher levels of locomotor activity. Differences in body weight appeared later. The authors interpret this sequence as evidence of higher energy expenditure because the difference in the timing of obesity developed under the same high-fat diet and without a difference in cumulative energy intake.

After exposure to the shortest schedule, 23.28 hours, the internal period measured in constant darkness also became shorter. After exposure to the cycle close to tau, the internal period remained consistent with both the cycle period and the period measured in constant darkness. Both shorter schedules delayed obesity, so the aftereffect observed with one schedule cannot explain the entire result. In this model, the time required for obesity to develop depended on the relationship between the external light schedule and the mice's internal rhythm.