By Dr. Jenn Simmons MD.
I stumbled into the world of fasting back in college—completely by accident. The girl who lived next door to me came back sophomore year looking like a new person. When I asked how she did it, she shrugged and said, “I stopped eating after 6 PM.” Naturally, I copied her immediately. I lost the freshman 15—and then some—that semester.
Fast forward to now, and fasting isn’t just a trend for biohackers wearing glucose monitors like jewelry. It’s one of the most underutilized (and free!) tools in the prevention and healing of breast cancer. And let me be clear: fasting is the quickest way to healing. Once I dove into the science, I knew I was never going back.
But let's be honest: fasting sounds intimidating. No food? For how long? Will I survive without my almond milk latte? (Spoiler: yes.)
If you’re wondering what role fasting could play in your breast cancer journey—or in your mission to never have one—this post is for you.
And don’t worry, I’m not tossing you into a 72-hour fast without a plan. (Unless you want to join me for our next 72-Hour Water Fast Challenge—more on that later.)
First, What Actually Happens When You Fast?
Fasting isn’t just “not eating.” It’s a carefully timed physiological reset. Here’s what your body is doing when you step away from the fridge:
⏱️ 12–14 Hours:
Blood sugar and insulin drop
Fat burning begins
Digestive processes take a break
⏱️ 16 Hours:
Autophagy begins (hello, cellular cleanup crew)
Inflammation decreases
Growth hormone rises
⏱️ 24 Hours:
Gut lining repair initiates
Stem cell production begins
More autophagy
⏱️ 36–48 Hours:
Ketosis deepens (fat becomes your fuel)
Deeper immune regeneration begins
Cancer-suppressing pathways activate
⏱️ 72 Hours:
Full-body immune reboot
Senescent cells (old, dysfunctional ones) cleared
Stem cell surge
Now imagine what that means for a woman healing from breast cancer.
Why Fasting is a Game-Changer in Breast Cancer
🔬 Reduced Insulin and IGF-1 Breast cancer cells thrive in high-insulin environments. Fasting lowers insulin and insulin-like growth factor 1 (IGF-1), which can slow tumor growth.
📉 Estrogen Modulation Intermittent fasting and extended fasts support estrogen metabolism by improving liver detoxification and increasing SHBG (sex hormone binding globulin), which helps regulate circulating estrogen.
🧫 Autophagy and Cancer Cell Cleanup Fasting triggers autophagy, where the body recycles damaged or precancerous cells before they become a problem.
💥 Cancer Cell Vulnerability Dr. Valter Longo (yes, the Dr. Longo, who spoke at my Breast Cancer Breakthroughs Summit) showed that fasting makes cancer cells more sensitive to chemotherapy while protecting normal cells—a phenomenon called differential stress resistance.
See the research here
Types of Fasting: Which One is Right for You?
Let’s break down the different ways to fast, because no, it doesn’t always mean days without food.
1. Time-Restricted Eating (TRE)
Eat within a 10-, 8-, or 6-hour window (e.g., 12–8 PM). Fast the rest.
Best for: Beginners, hormone balancing, daily detox
Bonus: Align with your circadian rhythm for added metabolic benefits
2. Intermittent Fasting (IF)
Fasting for 16+ hours several times per week. Popularized by women like
Mindy Pelz, DC and
Cynthia Thurlow, NP.
Best for: Fat loss, metabolic health, hormonal support
Pro tip: Avoid daily long fasts if you’re in perimenopause or under chronic stress.
3. Fasting Mimicking Diet (FMD)
A 5-day low-calorie, high-fat, plant-based protocol designed by Dr. Longo. You’re technically eating, but your body thinks it’s fasting.
Best for: Supporting chemo, enhancing autophagy without full food restriction
Note: ProLon is the commercial version, but a DIY version can work too.
4. Extended Water Fasts (24–72+ Hours)
No food. Just water, electrolytes, maybe some herbal tea.
Best for: Deep autophagy, immune reset, stem cell renewal
Warning: Don’t attempt extended fasts without guidance if you’re on meds or underweight.
Fasting Around Chemotherapy: Yes or No?
Short answer: yes—but smartly.
Dr. Longo’s work shows fasting before and after chemo can:
-Increase efficacy of chemo
-Decrease side effects (nausea, fatigue, brain fog)
-Protect healthy cells
-Typical protocol: fast for 48 hours before chemo and 24 hours after. You can use the fasting mimicking diet instead of a full water fast if needed.
More on chemo-fasting synergy here
Seasonal Fasting: Did You Know Your Fast Should Shift With the Seasons?
In summer (longer days), you might do better with a 14:10 eating window. In winter (shorter days), 16:8 or longer fasts feel more natural.
Why? Our circadian biology evolved with seasonal rhythms. Fasting in harmony with light exposure can amplify metabolic benefits.
When NOT to Fast
Let’s be clear—
fasting isn’t for everyone, and
timing is everything. Avoid or pause fasting if you:
-Are pregnant or breastfeeding
-Are underweight or have an eating disorder
-Are in active cancer treatment without guidance
-Have adrenal fatigue or extreme burnout
-Have poorly controlled diabetes (work with your provider)
How Often Should You Fast?
Here’s a general guide for my breast cancer patients:
Time-restricted eating: 5–6 days/week
Intermittent fasting (16+ hours): 2–3 times/week
Fasting mimicking diet: Once every 1–3 months
Extended fasts (48–72 hours): Every 3–6 months, ideally guided
How to Break a Fast (Don’t Blow It!)
After all that effort, don’t break your fast with a bagel and a latte.
✨ Best Fast-Breaking Foods:
-Steamed or sautéed veggies
-Bone broth or miso soup
-Avocado or soft-boiled egg
-Gentle protein: wild-caught fish or lentils
-Hydration: herbal tea, mineral water
*Pro tip: chew slowly and savor it. Your digestion needs a gentle re-entry.*
Real Talk: The Fasting Mindset
Fasting isn’t punishment—it’s a gift. A time to give your body a break, to listen, to heal, to reset.
Yes, there may be hunger. Yes, your brain may try to tell you that you’re dying (you’re not). But what’s on the other side? More energy. Sharper focus. Improved labs. Better hormone balance. A stronger body. A healthier terrain.
It’s one of the most powerful things you can do for free to help your body resist, repair, and recover from breast cancer.
Ready to Fast with Support?
Join our 72-Hour Water Fast Challenge!
Get guidance, recipes for re-entry, and a supportive community of women just like you walking the path to better health.
Stay tuned for the next one or
book a discovery call to join our Breast Health Blueprint community, where fasting is just one of the tools we use to shift the terrain from sickness to health and reclaim our power.
With Gratitude,
Dr. Jenn Simmons
Founder, Real Health MD
Integrative Oncologist | Functional Medicine Physician
www.realhealthmd.com
