Fasting for Health: Competing Perspectives

Written By Nicki Choudhary

In the ever-evolving landscape of health and nutrition, fasting has emerged as a compelling approach to wellness that goes far beyond simple weight loss. When evaluating approaches, always ask yourself why you are seeking something out–is it to help with specific symptoms, is it a preventative measure, is there something in your health toolkit that you are trying to replace, etc? 

Defining Fasting: A Spectrum of Approaches

Unlike many dietary interventions, fasting doesn't have a single, universally accepted definition. Researchers and health practitioners present varying interpretations:

  1. Time-Restricted Eating: Popularized by Dr. Satchin Panda from the Salk Institute, this approach limits food intake to a specific window of 8-12 hours per day. In implementing this approach, the broadest benefits come from aligning nutrition with the body's circadian rhythms, which includes eating in the morning and wrapping up any food intake several hours before bedtime.

  2. Intermittent Fasting: This broader category includes time-restricted eating methods above including more restrictive eating windows (6 or less) as well as consecutive or alternate days of fasting like the 5:2 method, where individuals dramatically reduce calorie intake for two days per week.

  3. Prolonged Fasting: Longer periods of significant calorie restriction while ensuring necessary nutrient levels for multiple consecutive days and all under medical supervision. Fasting for multiple days is cycled with a person’s routine eating patterns. Both water fasting and fasting-mimicking diets are in this category. Dr. Valter Longo, a longevity researcher at the University of Southern California, has been a prominent advocate of this approach, especially fasting-mimicking diets. This fasting approach is typically high in unsaturated fats and low in overall calories, protein, and carbohydrates and is designed to mimic the effects of a water-only fast while still providing necessary nutrients.

The Biological Magic: Why Fasting Impacts Health

Interventional studies comparing different fasting approaches and comparing fasting approaches to caloric restriction are helping us start to compare the benefits of these approaches for different demographics and health conditions. At its core, fasting appears to have positive benefits on health due to 1) Caloric Restriction 2) Caloric Quality Improvement 3) Circadian Rhythm Synchronization  

Caloric Restriction

Most of the clinical trials that compare fasting regimens to caloric restriction diets show that there is a reduction in calories consumed as a result of eating windows, an average of 20% reduction. This reduction in calories is generally accepted as driving the positive changes in health. Fasting is perceived as a stressor by the body and as a result can activate stress response pathways and may help cells become more resilient. When we fast, our cells initiate a housekeeping process called autophagy. Dr. Yoshinori Ohsumi, who won the Nobel Prize in 2016 for his work on autophagy, demonstrated how cells break down and recycle damaged proteins and mitochondria during periods of nutrient scarcity. Please note that a majority of the work done on autophagy has been in mice and a few limited trials in humans.

Caloric Quality Improvement

Although quality of calories is not a prerequisite for different fasting approaches, what we see in real-world studies by ZOE and Dr. Panda from the Salk Institute is that quality does improve. Notably, after a longer period of fasting at night, frequency of breakfast intake and quality of breakfast can improve. In addition, consumption of alcohol tends to go down–when the kitchen closes in the evening so does the bar.  

Circadian Rhythm Synchronization

Our bodies aren't designed for constant eating. Dr. Panda's research reveals that aligning food intake with our natural circadian rhythms can improve metabolic health, enhance sleep quality, and potentially reduce inflammation. In doing so, each cell in our body can better anticipate what’s coming next, prepare for it, and support it fully. Hormones like melatonin, TSH, cortisol, ghrelin, leptin, and insulin that help orchestrate a variety of biological processes need to be in sync with our 24-hr cellular clocks to perform their best. In addition, giving our bodies ample and consistent time in a sleep state allows it to switch over to critical activities like clean up and repair that would not be possible in wakeful hours.

Not a One-Size-Fits-All Solution

While the science is promising, fasting isn't universally appropriate. Different populations may require nuanced approaches to fasting.

Fasting for Women: Competing Perspectives

We’ll look at two perspectives in the scientific community regarding fasting for women that focus on hormonal health and performance. There are many others that range from weight loss to longevity to psychological health. 

Different experts may promote different approaches as a result of the studies they are looking at. Always make sure the experts you are listening to are citing human studies and that the populations in the studies are representative of you. For example, you wouldn’t want to follow fasting recommendations from clinical research done on obese men if you are not an obese man. Sometimes there just isn’t enough clinical evidence on an intervention for specific demographics or conditions, so what do you do then? In that situation, you could compare studies that are representative of different aspects

  • Effect of early time-restricted eating on cardiometabolic health for females

  • Effect of early time-restricted eating on cardiometabolic health for mid-life adults

  • Effect of early time-restricted eating on cardiometabolic health for south asians

In the absence of high-quality clinical evidence that is representative of you, you’ll have to think through the risk of just trying something and seeing how well it works for you. If an intervention has been tested on human populations not representative of you, there may already be some basic safety data that can give you the confidence to try something even though it may not be optimal for you. Take your questions and any information you’ve collected to your doctor to see if something might be a good fit for you. 

And for fasting in particular, anyone with unique energy requirements (pregnant or breastfeeding) or those taking any type of medications should always talk to their doctor first. 

Hormonal Health Approach 

This approach is based on several clinical trials of women with PCOS and women with obesity and has been popularized by Dr. Mindy Pelz, a chiropractor, via her book Fast Like a Girl. Generally, this approach has limited scientific evidence outside of PCOS and Obesity. The studies are linked below. 

  • Fasting can disrupt delicate hormonal balances, especially during key points in the menstrual cycle like the luteal phase

  • Time-restricted eating (10-12 hour eating window) is preferable to more restrictive methods for hormone health

  • Eating windows earlier in the day can be more beneficial for hormone health

  • Improvements in weight and metabolic markers can be achieved and sometimes improvement in endocrine profiles  

Key studies:


The Performance Approach

This approach is based on a limited number of clinical trials and training work by Dr. Stacy Sims, an exercise physiologist and nutrition scientist. The studies are linked below.

  • Supports time-restricted eating like a 12-hr window which she refers to as just “normal eating” and eat within 1hr of waking up for active women and especially before any exercise

  • “Women are not small men.” Women have different brain and muscle morphology than men: 1) they have two areas in the hypothalamus that sense nutrients because of a more robust endocrine system vs one for men and 2) muscle morphology in women already supports higher metabolic flexibility than for men due to a higher density of slow-twitch muscle fibers.

  • For women, the order of accessing fuel sources during training is different from men. Women use blood sugar, free fatty acids and amino acids and then lean mass. Men use all stored glucose and then fatty acids.

  • Strategic nutrient timing should be just before training and within an hour after training

  • Longer fast periods and exercise are both seen as stressors by the body that can yield benefits around cellular repair and cleanup beyond what time-restricted eating already will provide. Exercise should be the preferred method to reap these additional benefits for women given the physiological differences between men and women. 

  • 12-hr eating window to align with circadian rhythm

  • Leverage exercise rather than longer fasting windows as stressor to reap benefits for metabolic health, cellular repair and general longevity

Key studies:

  • Low energy availability (LEA) for 10 days reduces myofibrillar and sarcoplasmic muscle protein synthesis in trained females, Journal of Physiology 2023, n=30

    • “The main finding of this study is that LEA during combined resistance and cardiovascular exercise training results in marked reductions in myofibrillar and sarcoplasmic muscle protein synthesis. The decline in muscle protein synthesis is accompanied by reduced lean mass, nitrogen balance androgen index, thyroid hormone concentrations, RMR and increased cortisol/insulin ratio. These findings suggest that LEA may have negative consequences for skeletal muscle adaptations in females performing exercise training.”

  • Time-restricted feeding (12-8p) plus resistance training (8w supervised) in active females: a randomized trial, American Journal of Clinical Nutrition 2019, n=40

    • “In summary, similar gains in FFM, skeletal muscle hypertrophy, and muscular performance can be achieved with dramatically different feeding schedules provided that energy and protein intake are similar during a progressive RT program. In a healthy, active female population, TRF did not produce changes in physiological variables including resting metabolic rate, substrate utilization, blood lipids, glucose and insulin, blood pressure, arterial stiffness, or cortisol responses. Although a possible benefit of HMB for fat loss was observed in the PP analysis, supplementation did not definitively improve outcomes during TRF. Due to their potential to favorably influence body composition without compromising physical performance, additional examination of various IF protocols in both sedentary and active populations is warranted.”

  • Effects of 3mo combined dietary intervention and physical-cognitive exercise on cognitive function and cardiometabolic health of postmenopausal women with obesity: a randomized controlled trial, Int Journal of Behavioral and Physical Activity 2024, n=92

    • “These findings suggest that combined physical-cognitive exercise and dietary intervention are promising interventions to improve cognition and obesity-related complications of postmenopausal women with obesity.At the end of the 3-month intervention, the exercise and combined group demonstrated significant memory improvement which was accompanied by significant improvements in plasma BDNF level, insulin levels, HOMA-IR, %body fat, and muscle strength when compared to controls (p < 0.05). Only the combined intervention group demonstrated a significant improvement in executive function and increased plasma adiponectin levels when compared to control (p < 0.05). Surprisingly, no cognitive improvement was observed in the diet group (p > 0.05). Significant reduction in cholesterol levels was shown in the diet and combined groups when compared to controls (p < 0.05). Among the three intervention groups, there were no significant differences in all cognitive outcomes and cardiometabolic outcomes (p > 0.05). However, all three intervention groups showed significant improvements in plasma BDNF levels, weight, BMI, WHR, fat mass, and predicted VO2 max, when compared to control (p < 0.05).”

  • Intermittent Fasting: Does It Affect Sports Performance? A Systematic Review, Nutrients 2024, 

There’s more than one way to better health.

For those who find fasting challenging, numerous other strategies can support metabolic health. Fasting represents a powerful tool in the wellness toolkit, but it's not a magic bullet. The key is understanding your body, consulting professionals, and finding an approach that feels sustainable and supportive of your overall health journey.

  • Balanced, nutrient-dense diets

  • Regular physical activity

  • Stress management techniques

  • Consistent sleep schedules

  • Mindful eating practices

Disclaimer: Always consult with a healthcare professional before making significant changes to your diet or lifestyle.

Nicki Choudhary
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