Blogs – by Iliriana Zeneli
Leptin 101: The Hormone That Could Be Behind Your Fatigue, Weight Gain, and Hormonal Chaos
Let’s talk about a hormone that almost no one is talking about but everyone should be: leptin.
You’ve probably heard about insulin, cortisol, maybe even ghrelin. But leptin? It's the quiet commander running the show behind your appetite, metabolism, fertility, and fat-burning power. And if you’re stuck in a cycle of low energy, stubborn weight (especially around your arms, belly, or hips), and irregular hormones… leptin might be the missing piece.
Leptin is often referred to as the "satiety hormone" or the "fat thermostat." It’s secreted primarily by your fat cells (adipocytes), and its job is to travel through the bloodstream to the brain, specifically the hypothalamus (which is also part of the downstream to your adrenals) to deliver one main message: how much energy you have stored.
When energy (body fat) is sufficient, leptin tells the brain:
You’re fed
You can stop eating
You can burn stored fat
You have the resources to maintain hormonal balance and reproduce
This is part of a tightly regulated feedback loop between fat cells and the brain called the leptin-melanocortin pathway. But here’s where things get tricky: if leptin levels are too low or the brain stops responding to them (aka leptin resistance commonly seen with PCOS), the body interprets it as starvation, even if you're eating enough. As a result, it shifts into fat conservation mode: slowing your metabolism, increasing hunger, and shutting down non-essential functions like reproduction.
In other words, when leptin is dysregulated, your body becomes biologically wired to not let go of fat, no matter how hard you're trying.
What Is Leptin?
Leptin is a hormone made by your fat cells that signals to your brain, specifically your hypothalamus, about your energy status.
Think of leptin as your body’s fuel gauge. When leptin is working properly, it tells your brain:
“I’m full.”
“I have enough energy stored.”
“It’s safe to burn fat.”
“It’s safe to reproduce.” !!!
But when leptin is low or your body stops listening to it, things start to spiral: slow metabolism, constant hunger, poor satiety, fertility struggles, and inflammation.
Leptin’s Role in Fertility
Leptin isn’t just about weight and hunger, it plays a critical role in reproductive health. It acts as a messenger to the hypothalamus, signaling whether your body has enough energy and nutritional reserves to support pregnancy. If leptin is too low (often due to chronic under-eating, overexercising, high stress, or being underweight) your brain may shut down the reproductive system to conserve energy. This can lead to irregular or missing periods, anovulation (not releasing an egg), and poor progesterone production (aka me wondering why my progesterone was so chronically low even though my testosterone and estrogen were seemingly fine).
This led to an “unexplained infertility” explanation from the IVF clinic because my labs WERE normal but leptin was NEVER checked. This means I would have been given the option of IUI or IVF.
Even in cases where ovulation is medically induced, like during IUI or IVF,leptin levels still matter. Research has shown that low leptin can impair endometrial receptivity, meaning the uterine lining may not be optimal for implantation. In IVF, this could reduce the likelihood of a fertilized embryo successfully implanting, even if the egg retrieval and fertilization went smoothly. For IUI, low leptin could disrupt hormone balance and hinder the timing of ovulation and implantation.
Functional fertility practitioners often want to see leptin levels at 8–10 ng/mL minimum before trying to conceive to ensure the body is hormonally “ready” to support conception and pregnancy.
AND this is how I found out my low leptin was contributing to my difficulty getting pregnant. I was completely shocked when I found out how it worked like a fertility signaling hormone. Meanwhile, the IVF clinic I went to didn't even bother checking it. I further went on to learn that if I did pursue IUI or IVF (which was given to me as my only option since they said whatever I had going on what “unexplained”) there would have been a very high chance both procedures would have failed over and over again because my body is making it very clear it does not have the “energy” for a baby right now. So it makes me think how many women out there are being poked and prodded over and over again without a single doctor asking them about their leptin.
How Leptin Becomes Low (or Resistant)
Even though leptin is made by fat cells, more fat doesn’t always mean more functional leptin. Here’s how things go wrong:
Low Leptin Happens When:
You’re under-eating or skipping meals regularly (like breakfast!)
You’ve been dieting on and off for years
You’re underweight or have low body fat
You have HPA-axis dysfunction (adrenal burnout)
You’re constantly stressed or inflamed
Leptin Resistance Happens When:
Your leptin is high, but your brain stops responding to it (similar to insulin resistance)
You have too much inflammation, oxidative stress, or poor sleep
You have excess body fat but still feel hungry all the time
You’re overeating ultra-processed foods or eating too often
How Leptin Interacts with Circadian Rhythm + Light
Leptin is a circadian hormone, which means it follows a daily rhythm based on your internal clock and exposure to light.
Morning Light = Better Leptin SensitivityWhen you get sunlight in the morning (especially within the first hour of waking), it helps synchronize your suprachiasmatic nucleus (SCN), the part of your brain that regulates your circadian rhythm. This same region also controls leptin signaling.
If your light exposure is off (too little sunlight, too much artificial light at night), it throws off the timing of your leptin secretion. This leads to:
Increased cravings at night
Impaired fat burning during sleep
Disrupted sleep + blood sugar crashes
Dysregulated hunger signals throughout the day
Fixing your light environment = a natural leptin reset.
Get sunlight within 30–60 minutes of waking (10+ minutes outdoors)
Dim lights after sunset
Limit screens after 8–9pm or use blue light blockers
Align meals to your body clock (bigger breakfast, lighter dinner)
If your metabolism feels backwards (no appetite in the morning, ravenous at night), poor circadian cues may be confusing your leptin clock.
Symptoms of Low vs. High Leptin
Low Leptin
High Leptin (Resistance)
Low appetite or nausea in the morning
Always hungry, never satisfied
Poor satiety even after eating
Strong cravings for sugar or snacks
Trouble building muscle or feeling “frail”
Difficulty losing fat despite effort
Infertility or irregular periods
PCOS-like symptoms or estrogen dominance
Cold hands/feet, slow thyroid
Signs of inflammation (puffiness, water retention)
Brain fog, fatigue, low motivation
Sleep issues, poor recovery after workouts
How to Test Your Leptin Levels
Functional testing is ideal, but here are the common options:
1. Blood Test (Fasting Leptin)
Ask your practitioner for a fasting leptin blood draw. Reference ranges vary, but here’s a rough idea:
Women: 4–25 ng/mL
Men: 1–9 ng/mL
⚠️ Optimal ranges vary based on context. For example, many FUNCTIONAL experts want to see leptin between 8–10 for healthy ovulation and hormonal resilience. So if yours is a 4 and you’re being told it’s normal, then we need to think of the functional range.
I personally did mine through Function Health - you can use IBALAJ10 at checkout!
2. Signs of Leptin Resistance (With Normal or High Levels)
Elevated fasting insulin
Elevated triglycerides
Inflammatory markers (like CRP)
Midsection weight gain despite “eating clean”
Myth: “Fasting Will Always Make Leptin Look Low”
One common misconception is that fasting before blood work automatically makes leptin appear falsely low, so the results can be ignored. But that’s not entirely true. While leptin does decrease slightly during fasting, this is part of its normal circadian rhythm, and fasting provides a consistent baseline to assess your body's overall leptin status.
If your leptin is significantly low in a fasted state, it’s clinically relevant because it reflects how your body is functioning in a rested, unstimulated state. This is when your brain gets its most honest “energy status report.” A low fasted leptin level still signals to your brain that there isn’t enough energy available, which can suppress metabolism, fertility, and recovery even if you’re eating well later in the day.
Fasting doesn’t invalidate the result it actually helps reveal the deeper issue so don't let conventional doctors ignore low leptin just because it can be slightly lower in a fasted state for blood work purposes.
How to Improve Low Leptin Naturally
The goal is to increase leptin production and restore sensitivity to it so your brain and body get back in sync.
✅ Step 1: Eat Enough (Especially in the Morning)
Low leptin = underfed. Focus on nourishing, not restricting.
Protein: Aim for 30g+ per meal (especially breakfast)
Carbs: Don’t fear them, leptin needs carbs (think root veggies, fruits, sprouted grains)
Fat: Support hormones with quality fats (pasture-raised eggs, avocado, grass-fed butter)
✅ Step 2: Strength Train 2–4x/Week
Muscle helps improve leptin sensitivity. Bonus: it helps regulate blood sugar and burn fat more efficiently.
✅ Step 3: Morning Sunlight + Daily Movement
Circadian rhythms matter. Leptin communicates with your brain based on light cues and movement.
Walk within 30–60 minutes of waking
Get at least 7,000–10,000 steps/day
✅ Step 4: Support Sleep + Stress
Chronic stress and poor sleep blunts leptin. Simple changes:
Be in bed by 10pm
Use magnesium and blue light blockers
Try fascia work or breathwork before bed
Mock Leptin-Supporting Routine for a 9–5 Schedule
Here’s what a gentle but effective leptin-resetting day could look like:
🕕 6:30–7:30 AM – Wake Up + Light Exposure
Open your curtains, go for a 10–15 min walk (even just on the porch)
Sip a mineral rich drink to hydrate before any caffeine (if you drink caffeine)
🕗 8:00 AM – Breakfast
3 eggs, sweet potato, avocado + berries
1 cup coffee or matcha (after breakfast!)
AM supplements
🕙 10:30 AM – Short Walk Break
5–10 min walk outside or movement break between meetings
🕛 12:30 PM – Lunch
Salmon or chicken with rice, greens, olive oil dressing, and sauerkraut
Optional: digestive bitters or enzymes
🕔 5:30–6:00 PM – Strength Workout or Walk
30–45 min weights or Pilates + light cardio (walk or bike)
🕡 6:30 PM – Dinner
Grass-fed beef, roasted carrots, white rice or squash, cooked greens
🕘 9:00 PM – Wind Down
Magnesium mocktail, blue light blockers, foam rolling or fascia release
Be in bed by 10:00–10:30 PM
This is a very mock schedule based on a typical 9-5 but the key is really finding a routine, it doesn't need to be these exact times but it just needs to be the same consistent time everyday (give or take 30min). The other key is getting in short bursts of sunlight and movement whenever you can. I’ve worked in a corporate setting and go into an office even today and throughout the day many of our employees will step outside midday to get some steps in and move their bodies.
If you have a non-traditional schedule, young kids, or work before sunrise/after sunset, you can still support your leptin by mimicking natural light cues. Red light therapy in the morning can help stimulate your circadian clock when sunlight isn’t accessible and using blue light blockers at night helps protect your melatonin and leptin signaling even if you're exposed to screens or overhead lighting. It’s not about being perfect, it’s about protecting your brain from confusion and creating hormonal safety signals in whatever ways your routine allows.
While lifestyle is foundational, certain supplements can help support leptin production and sensitivity—especially during the early phases of healing:
Magnesium (Glycinate or Malate): Helps calm the nervous system, improve sleep, and support insulin sensitivity, all critical for leptin balance. The one linked has 4 types of magnesium but if you want just glycinate for PM then I would go for this one. Malate is found in Minerals & Chill.
Vitamin D3 + K2: Low vitamin D has been linked to leptin resistance and metabolic dysfunction.
Omega-3 Fatty Acids (Fish Oil): Supports inflammation reduction and improves cellular signaling, making your brain more responsive to leptin. (plant based Omega option)
Berberine: Can enhance leptin sensitivity and support blood sugar balance, but it’s best used under practitioner guidance.
Protein Powder: Can help get in extra protein for muscle growth, satiety and blood sugar control. It is always important to find one with clean ingredients.
Electrolytes/Minerals (like Minerals & Chill): Proper sodium, potassium, and magnesium balance supports adrenal health and energy metabolism, making the body feel “safe” to release stored fat.
*Always personalize supplements based on your lab work and symptoms, but these are generally safe starting points to help the body restore leptin signaling more effectively.
Final Thoughts
If you’ve felt like you’re “doing everything right” and still can’t shift your body composition, fix your cycle, or get your energy back, it’s not your fault. Your body may simply be stuck in energy conservation mode because it thinks it’s not safe.
Leptin is your body’s way of asking, “Am I safe to thrive?”
Support your leptin, and you’ll unlock metabolism, fertility, mood, and hormonal resilience in a way that dieting never could.
