Dani Shugart
Do Birthdays Determine Leanness?
Can your birthday make you fat? New research says yes. Common sense says no. Here's the crazy study.
Let's think about your parents having sex. Or more precisely, let's think about when you were conceived and how cold it was back then. Why? Because it could be an indication of your metabolism.
So, do the math. If your birthday falls in autumn (September, October, November), you were conceived during the colder months. Well, research out of Japan shows that if your mom got pregnant during those cold months, you're more likely to have a higher amount of brown adipose tissue (BAT) and therefore an easier time being lean.
Quick refresher: You want more BAT on your body. Brown fat is more metabolically active than white fat, so the more you have, the easier it'll be to stay lean. Granted, the BAT that you're born with is called "constitutive" BAT, and it diminishes with age, but doesn't completely go away.
The cool thing is, there are some strategies that'll brown up a portion of your gross adult white fat, making it "inducible beige fat." We'll get to that in a bit. But first, let's talk about how those who were conceived in the winter (and born in autumn) may have the metabolic advantage.
The Study
Researchers analyzed 356 young males. Using imaging techniques, they measured the men's brown adipose tissue and evaluated their overall metabolic health: visceral fat accumulation, BMI, and daily energy expenditure.
They also collected participants' birth dates to estimate when the men were conceived. From there, researchers used weather data to determine outdoor temperatures during the estimated conception times.
The researchers found that those conceived during colder months had more BAT activity, increased energy expenditure, lower BMI, and less visceral fat in adulthood. These findings suggest that temperatures at conception may program BAT activity and influence metabolic health later in life.
Lifestyle Trumps BAT
So, were you born in the wrong month? Nah. Because your choices matter the most.
We can ask a couple questions to get to the bottom of it. For instance, do autumn-born babies usually become lean adults? And are people born in other months more likely to be overweight?
Well, I'm not sure what you're seeing, but from where I stand neither seems to be a trend. If people with fall birthdays were typically lean and everyone else was typically overweight, we'd probably notice. And eventually, the horoscope people would shout that there's something cosmically special about Libras and leanness. (Barf.)
So, while this study tells us that the development of BAT – due to the season of conception – may be somewhat advantageous for leanness later on, common sense tells us that behavior matters more.
And just in case this study still has you worried, here's a list of bodybuilders who've gotten quite lean and don't have an autumn birthday:
- Dorian Yates – April
- Shawn Ray – May
- Sergio Oliva Jr. – May
- Ronnie Coleman – May
- Arnold Schwarzenegger – July
- Kai Greene – July
- Frank Zane – June
Obviously, these men had supplemental help along the way, but even if such substances had never been invented, these guys would've still had the capability to be fit, if not downright ripped, through diet and exercise.
Plus, tons of natural lifters born outside of the autumnal season have gotten conspicuously lean, and tons of people born in the autumn months carry extra body fat. (As a September baby, I've been one!)
Increasing BAT in Adulthood
Can you increase your BAT? You can activate what you were born with but developing new BAT is pretty hard as an adult. You can, however, make white adipose tissue (WAT) browner.
When you brown your white fat, scientists refer to it as beige or as "brite," which is a combination of the words, brown and white. And you'll feel like a 12-year-old when you say it. And yes, you want to turn your white fat beige to increase energy expenditure throughout the day.
So here are a few strategies for browning your white fat:
1. Cold Exposure
Think ice baths, cold showers, cryotherapy, brisk winter walks without a jacket... you get the picture. Just don't die. That'll get in the way of your metabolic goals.
Can I rant now? Awesome, here goes. Fitness pros sometimes warn against cold exposure by saying it doesn't increase muscle growth. But that would be like saying, "Don't wipe! It doesn't prevent colon cancer!" See how silly that is? We can use cold exposure as a strategy for things outside of building biceps.
Yes, cold exposure immediately after your workout might hinder it from having as much of a hypertrophy impact. So, do it prior to training, or hours after your workout, or during an off-day. This isn't complicated.
2. Exercise
Exercise makes us produce a hormone called irisin that promotes the conversion of white fat into beige fat. High-intensity interval training (HIIT) and endurance training are the most effective, so get yourself out of breath if you'd rather skip the cold stuff.
3. Get Enough Sleep
Sleeping enough and having a consistent circadian rhythm influences BAT activity. Even if it didn't, there are about five thousand other reasons your body and mind do better with an adequate amount.
4. Diet and Supplementation
Diet and supplements help. Certain compounds in foods and supplements can help you develop browner, more metabolically active fat.
- Resveratrol: Research done on humans, mice, and cell cultures suggests resveratrol (Buy at Amazon) helps convert white fat into brown. It activates the protein SIRT1, which increases brown fat and reduces white fat, which suggests it promotes fat browning and energy expenditure.
- Cyanadin 3-glucoside: C3G (Buy at Amazon) helps by increasing the mitochondrial content of white fat and turning it into a more metabolically active beige fat.
- Omega-3 Fatty Acid: Activates brown fat cells by interacting with a receptor called GPR120, which triggers the release of a hormone (FGF21) that boosts energy burning. Like resveratrol, omega-3s also increase the expression of genes like UCP1, which helps cells burn fat for heat instead of storing it. Plus, chronic inflammation suppresses the browning of white fat, so take plenty of high-dose, inflammation-squashing fish oil (Buy at Amazon).
- Capsaicin: The hot stuff in peppers helps increase thermogenesis and may activate BAT. For some, this may be just as uncomfortable as jumping in an ice bath.
- Catechins: The compound in green tea can up your energy expenditure and help brown white fat.
References
- Yoneshiro, T., Matsushita, M., Fuse-Hamaoka, S. et al. Pre-fertilization-origin preservation of brown fat-mediated energy expenditure in humans. Nat Metab (2025).
- Andrade, João M. O., et al. "Resveratrol Increases Brown Adipose Tissue Thermogenesis Markers by Increasing SIRT1 and Energy Expenditure and Decreasing Fat Accumulation in Adipose Tissue of Mice Fed a Standard Diet." European Journal of Nutrition, vol. 53, no. 7, Oct. 2014, pp. 1503-1510, doi:10.1007/s00394-014-0655-6.
- Kim, Sangsu, et al. "Resveratrol Exerts Anti-Obesity Effects via Mechanisms Involving Down-Regulation of Adipogenic and Inflammatory Processes in Mice." Biochemical Pharmacology, vol. 81, no. 11, June 2011, pp. 1343-1351, doi:10.1016/j.bcp.2011.03.012.
- Vatner, Dorothy E., et al. "Brown Adipose Tissue Enhances Exercise Performance and Healthful Longevity." Aging, vol. 16, no. 1, 2024, doi:10.18632/aging.206179.
- Kim, Minji, et al. "Fish Oil Intake Induces UCP1 Upregulation in Brown and White Adipose Tissue via the Sympathetic Nervous System." Scientific Reports, vol. 5, 17 Dec. 2015, article 18013, doi:10.1038/srep18013.
- Kim, Jinkuk, et al. "EPA and DHA Differentially Modulate Adipose Tissue Formation and Beige Adipogenesis in Diet-Induced Obesity in Mice." The Journal of Nutritional Biochemistry, vol. 38, Dec. 2016, pp. 1-12, doi:10.1016/j.jnutbio.2016.08.009.
- Quesada-López, Tania, et al. "The Lipid Sensor GPR120 Promotes Brown Fat Activation and FGF21 Release from Adipocytes." Nature Communications, vol. 7, 18 Nov. 2016, article 13479, doi:10.1038/ncomms13479.
