I have always preferred the warmer months to the colder, darker ones. Since I was a teen, I noticed I had less energy in the cooler, darker months and was sleepier. A few decades later, I noticed that I became less lean in the cooler months. No one would notice my condition because my energy levels are adequate and the extra weight is minimal, but I can see the seasonal difference in my body composition. My energy increases, and my leanness returns when the days lengthen, and it gets warmer. My transition to my more energetic, leaner self took longer this year. As always, I looked for a reasonable explanation for why I tend to gain some winter season weight, and why it stayed longer this year. This post will cover a few reasons for seasonal body compositional transitions and factors that can alter the normal process.

Warmth and Physical Activity

A paper published in 2020 discussed the seasonal variation in one individual over twenty years. During the cold winter months, the subject has a higher body mass due to decreased physical activity. Conversely, the body mass decreased when physical activity peaked in mid-summer. The changes in body mass were mostly due to variations in body fat. However, in the last eight years of the study, the subject's lean tissue decreased while their fat increased. 1 Sarcopenia, a normal condition of muscle loss with age, is the probable cause. I have a post on sarcopenia here if you want more info. Speaking for myself, I don't hike with my dogs in the winter because they are not bred for the cold. I spend much less time outside due to the cold, and as a consequence, I gain weight.

Thermogenesis of Eating

It makes sense that the more active we are, the more energy we expend, and the lower our mass will be. There is an additional factor; how hot one feels is not simply a matter of the environment; one's own activity may generate heat, so being active may be equivalent to raising the environmental temperature. When we eat, our bodies produce heat, which combines with the heat in our environment. This can lead to stronger appetite suppression after a meal in a hot environment. Brobeck stated in 1948 that "everyone knows ... that appetite fails in hot weather."

Since the body needs calories to generate heat when it is cold, excessive cold should result in a greater intake of calories. Johnson and Kark found this to be true when studying soldiers stationed in warm versus cold areas. 2

Circadian Rhythms and Cortisol

Our biological clocks use variations in light to establish circadian rhythms, which affect sleep, hunger, weight, mood, and other processes. Several hormones are involved, including cortisol. Cortisol has many functions and is sometimes called the stress hormone because it remains continuously high with chronic stress. This may be unfair because cortisol has many beneficial functions. For instance, it has similar properties to adrenalin. It rises slowly during sleep, and spikes in the morning (the cortisol awakening response), increasing our blood sugar and blood pressure to get us going for the day. It falls back down quickly and is at its minimum when we are ready for bed, facilitating our ability to sleep. 

Cortisol and Weight Gain

Researchers have found that cortisol has different effects depending on when and how it pulses. With stress, cortisol can remain higher than normal and for longer periods. One of its other effects is fat cell differentiation and growth. In a 2018 paper published in Cell Metabolism, researchers determined that the loss of normal patterns of daily hormone secretion is linked to obesity in humans. They found that certain patterns did not influence fat cell development, while others had large influences. Interestingly, high pulses had no effect, meaning that it is not the amount of cortisol; otherwise, we would all build fat cells in the morning when we have our normal spikes. Instead, as far back as 2011, Campbell et al. noted that more continuous cortisol signals found with stress were linked to increased fat mass. 3

This is why it took me longer than usual to lean up this year: My father was terminally ill for six months and passed away on March first. As much as I thought I was "coping," the situation was a source of stress. I am sure my cortisol was running higher than normal, causing me to hold on to some of the extra weight I normally gain in the colder months. But now that the nice weather is here, I am much more active. I can ride my bike, hike, take walks, etc. Additionally, I am coming to grips with my father's passing, and the stress over his suffering is gone. 

Putting it All Together

Losing weight seemingly effortlessly in the warmer months is no sure thing. I eat a whole-food carnivore diet. I eat almost no carbohydrates. I have avoided processed food like the plague it is for decades, so it comes naturally for me without any additional effort. 

If you have an ice cream addiction in the summer, you will probably gain weight, not lose it. Having said that, as long as you are more active and keep your stress down, and eat less, your body composition should change. As far as cortisol goes, proper sleep is a must. As I mentioned, cortisol is a circadian hormone; it aids in sleep when pulsing properly. As we have seen, poor sleep elevates it; elevated cortisol will create new fat cells and weight gain. Other tricks to keep your cortisol down are walking, especially in forests, breathing exercises, meditation, yoga, and spending time with trusted friends.