by Tasha Stevens
A Guide for Women
Not all fat-loss tools are best for every person all the time. Fasted cardio is no exception. Here’s how and when women should do it if they must.
I remember the 4 AM alarm for early morning cardio. I wanted more sleep, but instead, I told myself that fasted cardio is best for fat loss. So I pushed through it. What happened? Absolutely nothing. In fact, I gained fat. I was frustrated and exhausted.
Does fasted cardio have value in specific cases? Yes. But if you’re a female, you need to know when to use it and when to avoid it. Here's the summary:
The Facts
- For women, fasted cardio isn’t all it’s cracked up to be. It can be effective if you keep your hormones at the center of your programming.
- Fasted cardio at the wrong time increases stress response and cortisol production, leading to increased fat stores.
- Fasted cardio disrupts thyroid, estrogen, and progesterone production, which means serious PMS.
- The best time to do fasted cardio is during your follicular and ovulatory phases. Post-recovery fuel is critical to avoid a spiked stress response.
- In the luteal and menstrual phase, opt for fed-state cardio to regulate blood sugar levels.
The Fasted Cardio Flaw
Fasted cardio does not deliver for women because it severely disrupts hormones. Hormones control every aspect of metabolic function. When they’re disrupted, it triggers the release of cortisol, a preservation and conservation hormone.
The premise behind fasted cardio sounds appealing: if you do low-intensity cardio in a fasted state, you’ll improve your body’s ability to burn fat. It will have to pull from energy reserves to sustain the cardio demand (1). When your body has no other option, it’ll tap into fat reserves to provide energy. Cool idea, right?
But for women, it's not that simple due to our 28-day hormone cycle. Sex hormone fluctuations directly impact our body’s ability to tap into fat reserves, its metabolic rate, its ability to handle workout intensity, and its ability to handle and respond to stress.
Fasted cardio induces fatigue and stimulates the hypothalamus-pituitary-adrenal axis (HPA axis) due to the drop in blood sugar levels. Low blood sugar stimulates the release of cortisol and triggers your stress response (2). When blood sugar is low, your body sounds an alarm. It says you need energy fast, and cortisol works to quickly reallocate energy reserves to respond to this demand. But there are long-term implications.
Cortisol isn’t just a fat-mobilizing hormone; it’s also a fat-storage hormone. It works to help you survive the stressor by temporarily reallocating energy to survive the immediate need (think fighting or running away from a predator). It also increases fat stores to ensure that organs and systems have reserves to pull from if the stressor is prolonged (think surviving a famine).
When cortisol is high, it tells your body to “halt” all non-essential functions to ensure that it can survive the demand at hand. So, functions like fat burn, lean mass development, ovulation, and optimal hormone production are compromised.
Moreover, women have a greater sensitivity to fasted states and changes in energy demand. Perceived famine or prolonged fasted states can lead to increased release of kisspeptin, a neuropeptide responsible for sex hormones and endocrine and reproductive function. The increased production disrupts the production of estrogen and progesterone (3). So, not only do you not get the promised results, but you get an array of symptoms like PMS, bloating, fatigue, muscle weakness, and mood swings.
To add insult to injury, increased cortisol levels lead to increased storage of belly fat to serve as a protective barrier for your organs, and it can depress thyroid function. You need optimal thyroid hormone levels to stabilize metabolism, give you energy, and regulate luteinizing hormone and ovulation (4). Without ovulation, progesterone isn’t produced, increasing the risk of estrogen dominance and fat storage in the arms and hips (4).
Timing: The Missing Link
There’s still a metabolic benefit to fasted cardio, and women can benefit from controlled doses to improve metabolic efficiency. But the timing matters. And I don’t mean time of day. I mean the time or phase you’re in within your 28-day cycle.
Women have four distinct phases every month. Across these phases, there are distinct shifts in hormones that influence metabolic rate, your body's ability to burn fat, recovery capacity, and stress threshold. Using fasted cardio during the phases in which your body can handle the intensity of the fast and workload, and where your body is willing to burn fat, is critical.
Follicular & Ovulatory Phase: Fasted Cardio is Fine
Your follicular phase begins right when your period ends. This time is marked by increases in estrogen and follicle-stimulating hormone (FSH) to prepare a follicle to develop into an egg for ovulation. This phase lasts between 4-6 days. Then comes ovulation, marked by a peak in estrogen, a spike in testosterone, and a spike in LH to trigger ovulation.
During these phases, your body’s metabolic rate is lower. However, insulin function and blood sugar regulation are optimal. When insulin is functioning optimally, your body is extremely effective at using this energy and ensuring that blood sugar levels remain stable. Stable blood sugar also regulates cortisol (5).
When blood sugar is stabilized, your cortisol levels remain stable, and energy production pathways and fat oxidation pathways function optimally. This means you have the potential to burn fat. During these phases, your body responds well and can handle the stress associated with fasted cardio. So, in moderation, you can reap benefits.
To avoid any potential risks of increased stress using fasted cardio, eat a meal with protein and a complex carbohydrate within 30 minutes of finishing the cardio workout. Additionally, continue to eat at regular intervals (every 3-4 hours) for the remainder of the day to stabilize blood sugar and avoid a spike in cortisol.
Luteal and Menstrual Phase: Avoid Fasted Cardio
After ovulation, your estrogen and testosterone drop as progesterone rises, marking the start of your luteal phase. Progesterone is your “pro-gestation” hormone. It prepares your body for potential conception by slowing down digestion, reducing energy output, and focusing on preservation to ensure successful egg implantation. While your base metabolic rate has increased (you need more calories) your insulin function greatly diminishes (6).
Because of this, your body has an increased production rate of fat cells. Your body is no longer effectively reallocating blood sugar for immediate use or glycogen stores. So, any excess blood sugar remaining in the bloodstream for too long gets allocated to fat cells. So instead of your body effectively reallocating blood sugar to immediate energy use or glycogen stores, the body creates fat cells as additional storage space (7).
This is only heightened as you enter your menstrual phase, marked by the start of your bleed. That’s when your hormones are at their lowest. Your body is focused on shedding excess endothelial tissue and this results in severe nutrient depletion. This process is energy-expensive and taxing. Any additional stress or caloric deprivation can trigger increased secretion of cortisol and increased fat production.
During these phases, do fed, low-intensity cardio. Progesterone and estrogen increase responsiveness to steady-state cardio. Additionally, the incorporation of cycle-synced strength training during this phase improves insulin sensitivity and reactivity to improve fat-burning outcomes in your follicular and ovulatory phase.
Also, your body needs rest during this phase. It’s better to focus on a quality night’s sleep than wake up early for cardio.
References
- Schoenfeld BJ, Aragon AA, Wilborn CD, Krieger JW, Sonmez GT. Body composition changes associated with fasted versus non-fasted aerobic exercise. J Int Soc Sports Nutr. 2014 Nov 18;11(1):54. doi: 10.1186/s12970-014-0054-7. PMID: 25429252; PMCID: PMC4242477.
- Vilaça-Alves J, Muller F, Rosa C, Payan-Carreira R, Lund R, Matos F, Garrido N, Saavedra FJ, Machado Reis V. Cardiorespiratory, enzymatic and hormonal responses during and after walking while fasting. PLoS One. 2018 Mar 1;13(3):e0193702. doi: 10.1371/journal.pone.0193702. PMID: 29494664; PMCID: PMC5833199.
- Kumar S, Kaur G (2013) Intermittent Fasting Dietary Restriction Regimen Negatively Influences Reproduction in Young Rats: A Study of Hypothalamo-Hypophysial- Gonadal Axis. PLOS ONE 8(1): e52416.
- Lizcano F. Roles of estrogens, estrogen-like compounds, and endocrine disruptors in adipocytes. Front Endocrinol (Lausanne). 2022 Sep 21;13:921504. doi: 10.3389/fendo.2022.921504. PMID: 36213285; PMCID: PMC9533025.
- Walter KN, Corwin EJ, Ulbrecht J, Demers LM, Bennett JM, Whetzel CA, Klein LC. Elevated thyroid stimulating hormone is associated with elevated cortisol in healthy young men and women. Thyroid Res. 2012 Oct 30;5(1):13. doi: 10.1186/1756-6614-5-13. PMID: 23111240; PMCID: PMC3520819.
- Solomon SJ, Kurzer MS, Calloway DH. Menstrual cycle and basal metabolic rate in women. Am J Clin Nutr. 1982 Oct;36(4):611-6. doi: 10.1093/ajcn/36.4.611. PMID: 7124662.
- NIH study shows how insulin stimulates fat cells to take in glucose. (2010, September 7). National Institutes of Health (NIH). Retrieved January 16, 2024.