Winter Weight: Is It Your Hormones or Your Habits?
Every year around this time, someone asks me a version of the same question. “My weight creeps up in winter and drops off in summer, even when my training barely changes. Is something hormonal going on?”
It’s a fair question, and the honest answer is more interesting than a simple yes or no.
The insulin theory
There’s a real body of research looking at whether insulin sensitivity shifts with the seasons. Some of it is compelling. A Swedish study followed over 1,100 men and found insulin sensitivity was 11% lower in winter than summer, with the body’s insulin response to a glucose test running 16% higher in the colder months. A Japanese workplace study of 758 people found the same seasonal pattern, with insulin resistance and metabolic syndrome rates climbing in winter. In both cases, colder outdoor temperatures tracked with reduced insulin sensitivity.
Science flag: insulin sensitivity describes how effectively your cells respond to insulin and clear glucose from the blood. Lower sensitivity, or higher resistance, means your body needs to produce more insulin to do the same job, which is linked over time to fat storage and metabolic strain.
So winter insulin resistance is a genuine finding. But it’s not the whole story, and it’s not universal. A Danish study of healthy young men found no seasonal change in insulin sensitivity at all. A Taiwanese study found insulin resistance was actually higher in summer, the opposite direction. Researchers reviewing this field describe it as “controversial,” which in plain English means the results depend heavily on who’s being studied, their age, activity level, and location.
Where the stronger evidence points
If insulin isn’t a reliable seasonal switch, what is driving winter weight gain for most people? The research keeps coming back to the same two things: how much we move, and how much we eat.
A year long US study of nearly 600 adults found physical activity dropped to its lowest point in winter and peaked in spring, calorie intake was highest in autumn, and body weight peaked in winter, all consistent with each other even though the swings were modest, around half a kilogram. A UK study using pedometers found both normal weight and overweight adults took 1,000 to 1,700 fewer steps a day in winter compared to summer. And a remarkable 20 year single subject study found body mass tracked almost exactly with seasonal activity levels, rising when activity fell in winter and dropping when activity rose in summer, with nearly all the change showing up as body fat rather than muscle.
The pattern across the evidence is consistent. Winter doesn’t flip a hormonal switch that makes weight gain inevitable. It quietly reduces the incidental movement in our day, the walking, the standing, the general fidgeting around, while often nudging our food choices toward heavier, more carbohydrate dense meals. Add those together over a few months and the scale moves, insulin story or not.
What this means for your training
If you’re already training consistently through the colder months, you’re doing the single most protective thing the evidence points to. But structured training is only part of daily movement. Worth asking yourself:
- Has your incidental activity dropped even though your sessions haven’t, fewer walks, more time seated, less general movement around the house or workplace
- Has your eating shifted with the season, heavier meals, more starchy carbohydrate, smaller portions of vegetables
- Are you training somewhere cold enough that everything else around your session gets more sedentary
None of this requires blaming your metabolism. It means the usual levers, movement and food, are worth a closer look before assuming your body has changed how it works.
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