Sexual dimorphism in the rate of weight gain upon dietary manipulation.
To assess sex differences in response to DIO, we challenged both male and female C57BL/6J mice with three different diets that are commonly used in DIO studies for a total of 22 weeks: 1) 10% calories from fat (low fat diet, LFD), 2) 45% calories from fat (Western Diet, WD), and 3) 60% calories from fat (high fat diet, HFD). The LFD was sucrose matched to the HFD and the primary source of fat was lard for each diet (Table 1). To determine changes in body composition over time, mice were weighed weekly and body composition was measured by quantitative magnetic resonance imaging at 4, 12 and 20 weeks of dietary intervention. As expected, both male and female mice gained significantly more body weight when fed a WD or HFD compared to LFD-fed mice. However, male mice gained significantly more body weight after only 2 weeks of HFD feeding compared to mice fed a LFD (21.98g ± 0.444 vs 19.04 ± 0.306 respectively, p=0.0001), whereas male mice fed a WD showed a significant increase in body weight after 6 weeks of WD feeding compared to mice fed a LFD (27.59g ± 0.825 vs 24.24 ± 0.466, respectively, p=0.0033) (Fig 1A). Female mice, by contrast, did not show a significant weight increase until 11 weeks of HFD feeding compared to LFD-fed mice (25.79g ± 1.175 vs 21.20 ± 0.346 respectively, p=0.0038), and 13 weeks of WD feeding compared to LFD fed mice (24.40g ± 0.829 vs 20.83 ± 0.375, respectively p=0.0017) (Fig 1C). Similar to other published data (23), female mice did not gain as much weight as male mice early during feeding of either WD or HFD. However, at the end of the 22-week challenge, both male and female mice gained the same proportion of weight (252% increase in total body weight in males compared to 236% increase in females when fed a WD, p=0.899, n=6-8, and 267% increase in body weight in males compared to 228% body weight increase in females fed a HFD, p=0.166, n=6-8 (Fig. 1 B, D). Fat mass was significantly increased in male mice fed either WD or HFD at 12 and 20 weeks of feeding (Fig 1E), while lean mass did not significantly change in males until 20 weeks of HFD feeding compared to LFD-fed controls and WD fed mice (Fig 1F). Female mice showed increased fat mass only at 20 weeks with either HFD and WD feeding (Fig 1G), and exhibited significantly more lean mass at 20 weeks on either a HFD or WD compared to LFD-fed mice (Fig 1H).
Sexual dimorphism in food intake and activity upon dietary manipulation.
Next, mice were placed in metabolic cages after 8 and again after 16 weeks of diet to monitor food intake, activity levels, and respiratory exchange. Measurements were obtained over a continuous 48 hour period at 8 and 16 weeks of diet. Dietary intervention with WD or HFD in both male and female mice led to sexual dimorphism in the quantity of food consumed during light and dark cycles. In particular, male mice fed either a WD or HFD had reduced food consumption during both light and dark cycles compared to LFD-fed controls at 8 weeks (Fig 2A). In terms of locomotor activity, male mice exhibited reduced activity levels after 8 weeks of WD or HFD compared to LFD-fed controls (Fig 2B, C). Female mice did not exhibit any differences in total grams of food consumption on either diet at 8 weeks (Fig 2D), and they reduced their activity levels upon WD feeding during the light cycle at 8 weeks, but not during the dark (active) cycle with WD or HFD feeding (Fig. 2E, F). At 16 weeks of dietary intervention, male mice on WD or HFD continued to eat significantly less than LFD-fed mice during the light cycle and significantly less on the HFD during the dark cycle (Fig 2G). At 16 weeks of DIO, locomotor activity levels in male mice was also reduced during the light cycle in mice fed both diets, and in mice fed the WD during the dark cycle (Fig 2H, I). Female mice, by striking contrast, did not change their food intake or activity levels during the light or dark cycles at 16 weeks of diet (Fig 2J-L). These results are in agreement with the body of literature demonstrating that consumption of a HFD results in reduced locomotor activity in C57/BL6J male mice compared to female mice (23,24),
The respiratory exchange ratio (RER) measures inherent composition and utilization of fats, carbohydrates, and proteins as they are converted to energy substrate. An RER of 1 indicates that only carbohydrates are being used as substrate, and an RER of 0.7 indicates that only fat is being used as substrate (25). Consistent with the utilization of fat as substrate, male mice fed WD or HFD exhibited significantly reduced RER (compared to LFD) in both light and dark cycles at 8 weeks of diet (Fig. 3A, C). Female mice also showed a significantly reduced RER in the dark cycle at 8 weeks of diet and showed no statistical difference in RER when fed a WD during the light cycle (Fig 3B, D). At 16 weeks of diet male mice continued to show reduced RER in both light and dark cycles under both WD and HFD feeding compared to LFD-fed mice (Fig 3E, G). Similarly, to 8 weeks of diet, females showed a reduction of RER in the dark cycle when fed a WD or HFD at 16 weeks and reduced RER when fed a WD only during the light cycle (Fig 3F, H). Collectively, these data demonstrate that consumption of diets high in dietary fat leads to changes in locomotor activity, food intake, and a change in the animal’s primary fuel source. Furthermore, while mice of either sex fed a LFD demonstrated a clear peak of nocturnal increased food intake with concomitant peaks in RER during the nocturnal cycle, HFD and WD feeding attenuated the diurnal peak pattern of feeding behavior (Fig 3A-B, E-F). This is consistent with reports that diets high in fat disrupt circadian rhythmicity in mice (26,27).
Effect of DIO on glucose tolerance and insulin resistance in male and female mice.
To assess changes in glucose homeostasis, glucose tolerance tests (GTTs) were performed at 4, 12, and 20 weeks of dietary intervention. After 4 weeks of diet, male mice on either a WD and HFD had worsening glucose tolerance compared to LFD-fed control mice (Fig. 4A, B), an observation that persisted at weeks 12 and 20 (Fig. 4E-F, I-J). Female mice also showed worsening glucose tolerance on either the WD or the HFD at 4 and 12 weeks of feeding (Fig. 4C-D, G-H). However, at 20 weeks of dietary intervention while female mice had worsening glucose tolerance on the HFD, the area under the curve (AUC) analysis was no different in WD-fed compared to LFD-fed female mice (Fig 4K, L). Furthermore, it is worth noting that area under the curve (AUC) values calculated from female GTTs on a WD or HFD intervention were lower compared to male AUC values on either the WD or HFD at all time points, suggesting an inherent resistance of females to glycemic deterioration caused by dietary intervention.
To determine if differences in insulin resistance contributed to the differences observed in glucose tolerance, insulin tolerance tests (ITTs) were performed at the end of dietary intervention at 22 weeks. Final ITTs performed on male mice fed either the WD or HFD showed reduced insulin sensitivity compared to LFD-fed mice (Fig 5A, B). Whereas insulin sensitivity was not as markedly affected. ITTs revealed that female mice on WD and HFD exhibited minimal to no impairment in insulin tolerance compared to females on LFD (Fig 5C, D).
Sexual dimorphism in adaptive β-cell hyperplasia
Increases in insulin resistance result in increases in islet β-cell mass/area as an adaptive mechanism to meet peripheral insulin demand (28). To assess for potential changes in islet morphometry, pancreata were removed after 22 weeks of dietary intervention and immunostained for insulin. Although male mice fed a WD did not show statistically significant changes in β-cell area compared to LFD-fed mice, male mice fed a HFD had significantly increased β-cell area (Fig. 6, top panel). Notably, HFD-fed female mice trended towards an increase in β-cell area (p=0.0706), but neither WD nor HFD resulted in significantly altered β-cell mass in females (Fig. 6, bottom panel), a finding consistent with the lack of changes in insulin resistance.