Photoperiod, i.e., the relative day-length per 24h, may modulate the metabolic responses to high-fat diet (HFD) and sucrose consumption.
To test this hypothesis, hormonal changes, fat accretion and sucrose intake were measured in rats exposed to short- or long-day for 4 weeks and fed with a standard high-carbohydrate low-fat pelleted diet (high-carbohydrate diet (HCD)) or a high-fat, medium-carbohydrate pelleted diet (HFD), with or without free access to 10% sucrose solution in addition to water available ad libitum.
Plasma leptin and adiposity index, defined as epididymal white fat expressed as percentage of body mass, were markedly increased only in HFD-fed animals drinking sucrose under short, but not long, photoperiods. Voluntary ingestion of sucrose under short days was greater in HFD rats compared with HCD animals over the experiment, while a trend for the opposite effect was visible under long days. Total energy intake was not changed overall, as rats proportionally decreased chow intake when they drank sucrose. A noteworthy exception was the HFD group with sucrose access under short days that significantly increased their total calorie intake. Fasting blood glucose was generally unaltered, except for an increase in HFD-fed animals drinking sucrose under long days compared to control animals, suggesting a decrease in glucose tolerance. Insulin resistance was not yet affected by nutritional or photoperiodic conditions after 4 experimental weeks.
Even if photoperiod cannot be considered as an obesogenic environmental factor per se, the metabolic effects resulting from the combination of high-fat feeding and voluntary intake of sucrose were dependent on day-length. Exposure to short days triggers a larger increase of sucrose ingestion and hyperleptinemia in rats fed with HFD compared to the control diet. Considering that the cardinal symptoms of winter depression include carbohydrate craving and increased adiposity, the present data provide an experimental basis for developing new animal models of seasonal affective disorder.