Macronutrient-preference is modulated by biological sex and estrous cycle in mice

This study demonstrates that adult female mice exhibit a stronger preference for fat and greater overall food intake compared to males, who favor fat-carbohydrate combinations, with female feeding behavior further modulated by the estrous cycle.

The Gist

Imagine a giant, high-tech cafeteria where the only things on the menu are three specific items: a bowl of pure butter (fat), a bowl of chocolate sugar cubes (carbs), and a bowl of butter mixed with sugar (the combo).

Now, imagine filling this cafeteria with mice. But here's the twist: we aren't just watching them eat; we are watching who eats what, how much, and how long they hang out near the food. This study did exactly that to figure out if male and female mice have different "foodie" personalities and if a female mouse's monthly hormonal cycle changes her cravings.

Here is the breakdown of what they found, using some everyday analogies:

1. The Great Gender Divide: The "Fat Fanatic" vs. The "Combo Lover"

The researchers found that male and female mice are basically different species when it comes to food choices.

• The Female Mice (The Fat Fanatics):
  Think of the female mice as people who walk into a buffet and immediately ignore the salad and the bread, heading straight for the richest, creamiest dessert. They ate significantly more food overall than the males, but it wasn't because they were just "hungrier." They were specifically hunting for fat.

  • The Metaphor: If the food choices were a car, the females were driving a sports car that only runs on premium diesel (fat). They spent the most time hovering near the fat bowl, sniffing it out, and eating it. Even when they were near the other foods, they just didn't care as much.

• The Male Mice (The Combo Connoisseurs):
  The males were a bit more indecisive, but they had a clear favorite: the combination of fat and sugar. They didn't care much for the plain fat or the plain sugar on their own.

  • The Metaphor: The males were like someone who says, "I don't want a plain cookie, and I don't want a plain glass of milk, but give me a cookie dipped in milk!" They liked the "combo" dish the best. Interestingly, they didn't seem to have a strong preference for the plain fat until they had been eating for a while (after two weeks), suggesting they had to "learn" to like it.

2. The "Monthly Mood" Effect: The Estrous Cycle

Female mice have a cycle similar to the human menstrual cycle, where their hormone levels (specifically estrogen) go up and down. The researchers tracked this to see if it changed their food choices.

• The "Party Mode" (Estrus):
  When the female mice were in their high-estrogen phase (let's call it "Party Mode"), they ate more food overall. But here is the kicker: they didn't eat more fat. Instead, they ate more of the Combo (fat + sugar).
  • The Metaphor: Imagine you are in a great mood and suddenly crave a "cheat meal" that has everything in it—salty, sweet, and creamy. That's what happened during this phase. Their bodies wanted the complex mix, not just the pure fat.
• The "Chill Mode" (Diestrus):
  When their hormones were lower, they went back to their normal eating habits, sticking mostly to their usual amounts.

3. Time Spent vs. Speed of Eating

The study also looked at behavior, not just stomachs.

• The "Hanging Out" Factor: The females didn't just eat the fat; they lingered near the fat bowl. It was like they were in a deep conversation with the food. The males, however, didn't hang out near any specific food as much; they were more like "grab-and-go" diners.
• The "First Bite" Speed: Interestingly, no matter if they were male or female, or what their hormones were doing, they all approached the Combo food the fastest. It was the most tempting "magnet" for everyone. But once they got there, the females stayed for the fat, while the males stayed for the mix.

Why Does This Matter?

Think of our bodies as complex machines. This study suggests that male and female machines are built with different fuel gauges.

• For Women (and female mice): The body seems wired to crave and process fat differently, perhaps due to how their brains and guts talk to each other. This might explain why, in the human world, women often report different food cravings than men and have different risks for certain metabolic diseases.
• For Men (and male mice): They might need a bit more time to develop a taste for pure fat, preferring the "flavor explosion" of mixed foods first.

The Bottom Line:
You can't just study "mice" or "humans" as one big group when it comes to diet. Biology matters. A female mouse is a different eater than a male mouse, and even a female mouse changes her menu depending on her hormonal "season." If we want to understand obesity, diabetes, or healthy eating, we have to stop treating everyone the same and start respecting these biological differences.

Technical Summary

1. Problem Statement

Dietary choices are critical determinants of metabolic and neurological health, yet the biological mechanisms driving macronutrient preference remain poorly understood. Existing literature presents conflicting findings regarding sex differences in food preference:

• Human Data: Often relies on self-reporting, which is prone to bias. While some studies suggest women consume more fat, others highlight higher obesity rates in females potentially linked to fat intake, despite self-reported diet-consciousness.
• Rodent Data: Often utilizes "high-fat" diets that are actually combinations of fat and carbohydrates. This confounds the ability to isolate preferences for specific macronutrients (pure fat vs. pure carbohydrate vs. mixed).
• Gap: There is a lack of controlled experiments using distinct, pure macronutrient sources to determine if biological sex and hormonal states (specifically the estrous cycle) drive specific nutrient preferences rather than general caloric intake.

2. Methodology

The study employed a controlled Three-Food Choice (3FC) paradigm over a 14-day period using adult C57BL/6J mice.

• Subjects:
  • 12 Male and 11 Female mice (3–4 months old).
  • Subjected to mild food deprivation to maintain body weight at 90–95% of free-feeding weight.
  • Behavioral testing occurred during the dark phase of a 12-hour reverse light/dark cycle.
• Stimuli (Three Food Types):
  1. Fat: Pure vegetable shortening (Crisco®).
  2. Carbohydrate: Chocolate-flavored sucrose pellets (20 mg).
  3. Combination: A 1:1 caloric ratio blend of ground sucrose pellets and shortening.
• Experimental Design:
  • Habituation: 6 days of arena exposure and single-food exposure in home cages.
  • Testing: Mice were placed in a 12x12 inch plexiglass arena with simultaneous access to all three food types for 45 minutes daily.
  • Duration: 14 consecutive days of testing.
• Data Collection:
  • Intake: Food weighed before and after sessions (normalized to body weight).
  • Behavior: Video tracking (Noldus Phenotyper/EthoVision) quantified:
    • Latency: Time to first approach a food zone.
    • Duration: Total time spent with the head in a food zone (≥2 seconds).
  • Estrous Cycle: Female vaginal cytology (Toluidine Blue stain) performed daily to classify stages (Proestrus, Estrus, Diestrus). Proestrus and Estrus were grouped as "Estrus" (high estrogen).
• Statistical Analysis:
  • Linear Mixed-Effects Models (LME) using lme4 in R.
  • Fixed effects: Sex, Food Type, Week, Cycle Stage, and interactions.
  • Random effect: Mouse ID (to account for repeated measures).
  • Post-hoc comparisons using Tukey adjustment.

3. Key Contributions

• Disentangling Macronutrients: Unlike previous studies using mixed "high-fat" diets, this study isolated pure fat, pure sucrose, and a controlled 1:1 fat-carb blend to identify specific nutrient drivers.
• Sex-Specific Behavioral Profiling: The study moves beyond simple intake volume to include behavioral metrics (latency and duration), distinguishing between anticipatory behavior (approach) and sustained engagement (time spent).
• Hormonal Modulation: Systematically tracked the estrous cycle to determine if cyclical hormonal fluctuations (specifically estrogen peaks) modulate nutrient-specific intake and behavior.

4. Key Results

A. Biological Sex Differences

• Total Intake: Females consumed nearly double the amount of food (normalized to body weight) compared to males.
• Nutrient Preference:
  • Females: Showed a robust, selective preference for pure fat. They consumed significantly more fat than both the combination diet and sucrose.
  • Males: Showed a preference for the Combination diet (fat + carb) over pure fat or sucrose. They did not show a strong preference for pure fat initially.
• Behavioral Engagement:
  • Females: Spent significantly more time in proximity to the fat zone compared to males and compared to other food zones. This suggests a sustained engagement with fat.
  • Males: Spent more time near the combination zone.
  • Latency: No sex differences were found in the latency to approach food, indicating that the differences lie in sustained motivation/valuation rather than initial exploration.
• Temporal Dynamics: Female preference for fat was stable across the 14 days. Males showed a delayed shift, increasing fat intake in Week 2, suggesting a learning component or slower acquisition of fat preference.

B. Estrous Cycle Effects (Females Only)

• Total Intake: Females consumed more total food during Estrus (high estrogen) compared to Diestrus.
• Nutrient Specificity:
  • The increase in intake during Estrus was driven specifically by the Combination diet.
  • Pure fat and pure sucrose intake did not differ significantly between cycle stages.
• Macronutrient Breakdown: When calculating total macronutrient intake (partitioning the combo diet), carbohydrate intake was significantly higher during Estrus, while fat intake remained stable.
• Behavior: Females spent more time near the Combination food zone during Estrus compared to Diestrus. Approach latency remained unchanged across cycle stages.

5. Significance and Implications

• Neurobiological Mechanisms: The findings support the hypothesis that sex differences in macronutrient preference stem from divergent peripheral-central metabolic pathways.
  • Females: The immediate and stable preference for fat suggests enhanced sensitivity to fat-mediated post-ingestive signals (e.g., PPAR-α mediated vagal afferents).
  • Males: The delayed preference for fat and preference for mixed diets may rely on reinforcement learning mechanisms or different reward circuitry.
• Hormonal Modulation: Estrogen fluctuations appear to selectively enhance the valuation of carbohydrate-containing or mixed macronutrient foods, rather than pure fat. This aligns with human reports of increased cravings for sweet/energy-dense foods during high-estrogen phases.
• Obesity and Metabolic Health: The study suggests that sex-specific vulnerabilities to diet-induced obesity may arise from distinct nutrient preferences. Females may be more susceptible to high-fat diets, while males may be driven by mixed macronutrient palatability.
• Future Directions: The authors highlight the need for future studies using ovariectomy/hormone replacement to isolate specific hormones and intragastric infusion to rule out orosensory (taste) confounds.

Conclusion: Biological sex is a primary driver of macronutrient preference in mice, with females exhibiting a stable, high-fat preference and males preferring mixed macronutrients. The estrous cycle further modulates this behavior, specifically increasing the consumption of carbohydrate-rich mixed diets during high-estrogen states. These results underscore the necessity of including sex and hormonal status as critical variables in nutritional neuroscience.