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E147
July 14, 2026
Why Women Gain Weight on the Same Diet as Men: Fueling, Fasting, and Biology with Dr. Stacy Sims
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In this episode of unPAUSED, Dr. Mary Claire Haver sits down with exercise physiologist and nutrition scientist Dr. Stacy Sims to unpack why so much fitness and diet advice built on male physiology leaves women confused about why the scale will not move, and why weight loss can feel so much harder for women than for men on the exact same plan. Dr. Sims, author of Roar and Next Level, a former professional cyclist who has taught and coached at Stanford, breaks down why women following the same diet and training program as their male partners often end up tired and stuck rather than leaner and fitter.

The conversation covers the biological reasons women conserve fat in a calorie deficit, how female circadian rhythm differs from male circadian rhythm, and why eating within an hour of waking helps regulate cortisol and appetite hormones like ghrelin and peptide YY. Dr. Sims explains why fasted workouts can backfire for women, why front loading calories earlier in the day supports better metabolic outcomes, and why time restricted eating works with a woman's biology in ways that intermittent fasting often does not.

Dr. Haver and Dr. Sims also dig into strength training for women in perimenopause and menopause, including why lifting heavier weights helps preserve fast twitch muscle fibers, power, and proprioception, all of which decline earlier in women than in men. They discuss the connection between high intensity exercise and lactate production, and why lactate is a preferred fuel for brain metabolism that may help protect against cognitive decline and dementia risk. The episode also touches on bone density and osteoporosis risk after menopause, the difference between calories in versus food quality and timing, and practical guidance on microdosing melatonin for better sleep.

Throughout the conversation, Dr. Sims makes the case that women's health depends on training and eating in ways that reflect female biology at every life stage, and that building muscle and bone during midlife matters more than chasing thinness.

Guest links:

Books:

Articles:

Chapters

00:00:00 – Why Calories Don't Tell the Whole Story
00:01:54 – Meet Dr. Stacy Sims
00:05:00 – Women Are Not Small Men
00:11:48 – Why Women Struggle to Lose Weight
00:17:17 – The Truth About Circadian Rhythm
00:27:17 – Should Women Exercise Fasted?
00:31:58 – How to Build Muscle After 40
00:38:04 – The Best Workout for Menopause
00:44:15 – Intermittent Fasting vs. Time-Restricted Eating
00:49:48 – Dr. Stacy Sims' Daily Nutrition Routine
00:53:53 – Why Exercise Beats Dieting
01:04:07 – Train for Healthspan, Not Weight Loss

About the guest

Dr. Stacy Sims

Dr. Stacy Sims is an internationally recognized exercise physiologist and nutrition scientist with an MSc and PhD. Her work is dedicated to transforming the way women approach exercise, nutrition, and performance by advancing research that reflects female physiology.

She has led research programs at Stanford University, Auckland University of Technology (AUT), and the University of Waikato, where her work focused on female athlete health and performance while challenging long-standing assumptions in sports science.

During her time at Stanford, Dr. Sims leveraged the innovation of Silicon Valley to translate her research into consumer products and her bestselling book, ROAR, which explores the differences between male and female physiology and how women can optimize training and nutrition throughout every stage of life. Her work has helped reshape conversations around women's health and performance, a message further amplified through her widely viewed TEDx talk, Women Are Not Small Men.

Dr. Sims has published more than 100 peer-reviewed research papers, authored several books, and is a sought-after speaker at professional and academic conferences around the world. Her contributions to sports nutrition and exercise science have established her as a leading authority on sex differences in training, nutrition, and health.

She currently serves as a Senior Research Associate with SPRINZ at Auckland University of Technology, holds an adjunct position with the Stanford Lifestyle Medicine Group, supervises PhD students, publishes academic research, and advises several innovative companies.

Through drstacysims.com, Dr. Sims provides science-backed education and online learning resources designed to help women train, fuel, and perform in alignment with their physiology throughout every stage of life.