Plus, a new reason to try a keto diet and my favorite at-home HIIT workouts.
It’s summer and it feels like everyone is installing a cold plunge in their backyard or basement spa. As someone who is obsessed with my in-home sauna but despises being cold, I usually feel a mild sense of personal failure when I can’t bring myself to do the plunge. When we go to Bathhouse in NYC and everyone dutifully wades into the icy waters, I just keep walking.
Rather than continue to feel badly about this, I thought it was time to dive into the science behind the hype. Are cold plunges going to go the way of the NordicTrak and wind up as very expensive coat racks in five years? Or is it time to jump in with everyone else?
The fixation on cold therapy is based on emerging science (some of it really compelling, some not so much) and anecdotal reports.
The TL;DR: we need more information to recommend effective cold therapy protocols—especially for women. But the research is promising.
A high-quality meta-analysis of cold plunge studies published earlier this year in the journal PLOS One summed up the situation well: cold plunge research is “constrained by few RCTs, small sample sizes, and a lack of diversity in study populations.”
Here’s where we see the most well-supported benefits:
💪 Muscle soreness: A meta-analysis of 20 years of cold plunge studies found that cold water immersion (CWI) after exercise decreases delayed-onset muscle soreness, perceived exertion, and lactate in the blood.
🧠 Enhanced mood and mental clarity: CWI can promote enhanced connectivity in brain regions associated with emotion regulation (as shown by fMRI) and boost serotonin, dopamine, and endorphins. A small study with 33 participants showed that after 5 minutes of cold plunge, participants felt more active and alert and less nervous and distressed.
⛑️ Hormesis and resilience: A low-dose stressor like a cold plunge can improve your body’s resilience in the face of stress, which is key for longevity. The PLOS One meta-analysis found a brief cold plunge can lower reported stress for up to 12 hours afterwards. There’s also some evidence that brief cold exposure stimulates the vagus nerve, which regulates our relaxation response.
🏃Increased metabolic rate: Cold exposure activates brown fat via non-shivering thermogenesis (which burns energy to generate heat) and can “beige” white fat—i.e. make stored fat more metabolically active. If you let yourself get to the point of shivering thermogenesis, the benefits increase; the rapid involuntary muscle contractions burn glucose and increase your caloric expenditure.
😴 Better sleep: A cold plunge may help increase parasympathetic tone via the vagus nerve, leading to increased slow-wave sleep, which is key for physical and mental recovery.
What about cold plunge for women?
The science gets murkier when it comes to whether or not cold plunges are beneficial for women. Most of the research on cold therapy has been done in men, but women have different physiologies.
For example: women experience greater vasodilation after exercise. So a cold plunge, which helps send all that cooled blood back centrally, may have greater recovery benefits for women than for men.
But there are also some potential red flags for women to consider. Here’s what we know:
- Cold plunge may arrest muscle protein synthesis.
There’s a body of research showing post-workout cold plunge can blunt muscle growth. But the vast majority of that research was conducted using only men. So while a landmark study found that post-workout cold water immersion led to smaller gains in muscle mass and strength vs. active recovery and a more recent study found it may inhibit the mTOR pathway (critical for building muscle) in men, the evidence is insufficient for women. Given how important building lean muscle mass is to women’s longevity, understanding these effects for women is key. - It may cause hormonal fluctuations.
Anecdotally, some women report changes in menstrual timing or cramps. No clinical studies have assessed the impact of cold plunge on fertility but theoretically, excessive stress could impact the hypothalamic–pituitary–ovarian (HPO) axis, potentially throwing off the menstrual cycle. On the flip side, if cold plunge improves metabolic health, that could improve fertility, especially in someone with PCOS. - Women don’t need to be as cold as men.
Women don’t require (or tolerate) the same level of cold as men to experience vasoconstriction and metabolic and recovery benefits. When women go too cold (below ~15 degrees celsius) our bodies skip shivering and kick into a sympathetic stress response where the body increases cortisol instead of activating thermogenesis.
To be clear, there’s no science that says doing a cold plunge once or twice a week is going to hurt you. (Exceptions are: pregnant women, people with cardiovascular conditions, or poor circulation or neuropathy. If that’s you, stay warm.)
Based on what we do know—and what we don’t—I’m intrigued by the cold plunge. Specifically, I’m up for trying 1-2 minutes in a 55 degree plunge as a way to make my fat more metabolically active, increase dopamine and endorphins, improve sleep, and to support my regimen for improving resilience and recovery. I still love my sauna—if hot and cold contrast therapy can help me recover from stress faster, great.
Here’s what I plan to do:
- Stick to 2-3 minutes. Stay in just long enough to induce shivering and then get out.
- Never workout cold. Working out cold can increase your risk of injury and reduce the effectiveness of your workout; cold decreases blood flow to muscles, which are less flexible and produce less power. Do a sauna, cold plunge, and then get back to normal temp before working out.
- Don’t use a cold plunge immediately after strength training. Save it for endurance exercise recovery or rest days.
- Keep it mild. Slightly milder temps (55-56 degrees) are enough to trigger metabolic pathways. Anything colder can trigger too great of a stress response for women.
What I’m Reading This Week
#1 Time to go keto?
For a while there, the keto diet was everywhere, but recently it feels like it’s fallen out of focus, no? Given the recent quiet on the keto front, I was interested to read this new study, which found material brain health benefits from a 3-week ketogenic diet.
Eleven participants with no neurological conditions or other chronic health issues followed two diets for three weeks each:
- Keto (5% carbohydrates, 20% protein, and 75% fat)
- Standard (45–60% carbs, 10–20% protein, and 25–40% fat)
After 3 weeks on a ketogenic diet, brain scans and blood tests revealed an impressive 47% rise in levels of brain-derived neurotrophic factor (BDNF)—a neurotrophin that promotes the growth of new brain cells, supports memory and learning, and regulates mood. They also showed a 22% increase in cerebral blood flow (which, in addition to helping your brain function optimally in the short-term, also lowers your risk of cognitive decline in the long-term).
There’s evidence that a keto diet may help you lose weight, clean up dead and damaged cells, and improve insulin sensitivity and blood glucose response. This study adds to the body of research suggesting keto isn’t just a fad or crash diet but can be a longevity tool when used correctly.
🏃 I don’t recommend going keto long-term because of the lack of evidence supporting its value when used this way. However, I sometimes prescribe a 1-week “keto sprint” for my patients who need a metabolic reset or are dealing with persistent symptoms of inflammation like brain fog. I share my protocol in detail in my book if you want to learn more!
❤️ Studies show being in a ketogenic state can improve cholesterol—for some. For others, a keto diet can send LDL cholesterol running wildly in the wrong direction; I always pair keto sprints with proactive lab testing to make sure we’re keeping heart health metrics in check.
#2 A proven way to build bone mass
One of the things I see so many of my patients in their 40s, 50s, and 60s struggle with asking is how can I actually build bone?
Nothing builds bone mass like high-impact exercise. So I was super excited to see new research showing HIIT workouts can be made twice as effective at building bone mass when paired with a vitamin D supplement.
Sedentary women ages 30-50 enrolled in a 16-week program including HIIT workouts and 800IU of daily vitamin D showed with a 3.2% improvement in hip bone mineral density. That’s huge considering the 1-year mortality rate for women who break a hip is 19.7%.
Bone loss is one of the most significant factors contributing to reduced healthspan as we age—especially for women. We lose 2-3% of our bone mass each year starting in menopause. HRT helps protect against this loss, which is a huge step towards staying healthy into your 70s. But to be healthier than ever, we need to be building bone throughout our lives, ideally from our 20s on, and especially in our 40s and 50s.
I know I personally need to be doing more HIIT workouts but honestly I struggle with finding the time. If you’re like me and getting to a regular Barry’s class is just not happening, I’m a big fan of at-home HIIT classes:
- Rocamoon is a mom of 3 who looks amazing and has fun positive vibes. (Personally, I’m not into being yelled at when I exercise.)
- Caroline Girvan inspires me to incorporate more heavy weights.
- Kirsty Godso can always be counted on to leave me completely exhausted when I want a challenge.
- Patrick Frost is great at helping me shake myself out of a workout rut.
How to be Healthier Than Ever This Week
🥶Take the plunge: If you’re ready to dip a toe into cold plunging, Ice Pod makes a super barebones plunge starting at $89. If you want to go straight to the high end, the Morozko ice bath actually makes its own ice. Plus, it self-sanitizes with microfiltration and ozone disinfection.
🍄 Be a Hiro: My household recently reached a huge milestone in that everyone is potty-trained! It’s hard to believe! But I am so impressed by Hiro Diapers, which are the first truly eco-friendly but high-performance disposable diapers, that I almost wish I still had a little one who needed them. Diapers are the #3 contributors to landfills and can take up to 500 years to decompose. Hiro diapers come with a plastic-eating fungi packet that starts digesting them after just a few weeks. Use code mycelia-parsley for 40% off!
⏳Test time: I shared on Instagram why I’m so excited about testing my epigenetic age (and will post my results when I get them). Get a TallyAge kit and test with me!