Fasting and muscle
From our Jason Fung profile
Fung uses long fasts; Attia moved away from them after losing muscle, and Lyon and Sims caution against fasting for active people in midlife.
Compare experts › Stacy Sims vs Jason Fung
Stacy Sims vs Jason Fung
Stacy Sims (exercise physiologist) and Jason Fung (physician, kidney specialist) side by side: how they train, eat, supplement and sleep, how strong the evidence is, and how to build a plan from either or both.
Words explained (9)
- Zone 2
- Steady cardio at a pace where you can still talk in full sentences, but not sing.
- Intervals
- Short hard efforts with easy recovery in between, such as 30 seconds fast, 90 seconds slow.
- 4 × 4
- Four rounds of 4 minutes hard with 3 minutes easy in between. Also called Norwegian intervals.
- Mobility
- Moving your joints freely through their full range.
- Creatine
- A well-studied supplement that supports strength and muscle.
- Omega-3 (EPA, DHA)
- Healthy fats found mostly in oily fish such as salmon, herring and sardines.
- Time-restricted eating
- Eating all your meals within a set window of the day, for example 8am to 6pm.
- Perimenopause
- The years of changing hormones before periods stop for good.
- Hormone therapy (HRT, MHT)
- Medicine that replaces hormones that fall at menopause. Only a doctor can prescribe it.
At a glance
| Measure | Stacy Sims | Jason Fung |
|---|---|---|
| Background | Exercise physiologist | Physician, kidney specialist |
| Training split | 60% strength · 10% aerobic · 25% intervals · 5% mobility | 45% strength · 35% aerobic · 12% intervals · 8% mobility |
| Signature habit | Heavy lifts + sprints | Fasting, less sugar |
| Protein | ~2.2 g/kg | No fixed target |
| Training style | Women 40+ | Fasting |
| Overall evidence | Strong | Mostly weak |
| Following | Instagram 1.4M · YouTube 372K | YouTube 1.5M |
Training splits are our estimates from each expert’s published routine. Following checked October 2026.
The key differences
- Sims puts more of the week into strength: about 60% against 45%.
- Fung puts more of the week into aerobic base (zone 2): about 35% against 10%.
- Sims puts more of the week into intervals: about 25% against 12%.
- In plans in their style, Sims' hard sessions are short sprints; Fung's are 4-minute intervals (Norwegian 4 × 4).
- We rate the evidence behind Sims' protocol higher: strong, against mostly weak for Fung.
- Supplements that go into a plan: Sims, creatine monohydrate; Fung, none (food first). Prescription items and personal stacks stay out of every plan.
- Neither has a separate mobility routine; plans add 10 minutes a day of basics.
Where they disagree
Protocols side by side
Each line is rated for the evidence behind it. “Doctor only” items never go into a plan.
Training
Stacy Sims
- Strength training 3–4 times a week, 45–60 minutes, built on compound liftsStrong evidence
- From perimenopause, lift heavy: about 3–5 sets of 6 reps or fewer, leaving 2–3 reps in reserve, with 2+ minutes restSome evidence
- Sprint intervals of 30 seconds or less, all-out, with full recovery; 1–2 sessions a weekSome evidence
- Jump and impact work (e.g. squat jumps, box jumps) for bone; one studied protocol is 10–20 jumps twice a daySome evidence
- Keep easy cardio (walking, easy rides) as recovery; she plays down long moderate-intensity sessions and zone 2 for midlife body-composition goalsPersonal habit
- 'Ten-minute rule': if you still feel flat after a 10-minute warm-up, scale the session downPersonal habit
Jason Fung
No specific guidance in what we reviewed.
Eating
Stacy Sims
- Do not train fasted: about 15 g protein before lifting, plus about 30 g carbohydrate before cardio of up to an hourSome evidence
- After training: 40–60 g protein (perimenopause on) within about 45 minutes, and carbohydrate within 2 hoursSome evidence
- Daily protein around 2.2 g/kg body weight, the top of the 1.4–2.2 g/kg range in her ISSN position standSome evidence
- Protein at meals 30–40 g, snacks 15–20 gSome evidence
- Avoid long fasts, keto and big calorie cuts if active; a small deficit (about 150–200 kcal) in the evening, away from training, if aiming to lose fatPersonal habit
Jason Fung
- Cut added sugar and refined grains; eat whole foods with natural fats.Some evidence
- Stop snacking: three meals a day, then skip one meal a day.Personal habit
- Eat within a shorter window of the day (time-restricted eating).Some evidence
- Fasts of 24 hours (skipping two meals in a row) two or three times a week, as used in his clinic's programme.Personal habit
- Longer fasts of several days; in 2016 he said fasts of up to a month were fine.Doctor only
- For type 2 diabetes: no sugary drinks, fructose or processed food, and more natural fats.Doctor only
Supplements
Stacy Sims
- Creatine monohydrate 3–5 g a day (start with 1–2 g if unsure), paired with strength trainingStrong evidence
- Choose third-party tested products (NSF or Informed Sport)Note
- Vitamin D3, omega-3, collagen and turmeric mentioned as options; no doses givenPersonal habit
- Iron only if a blood test shows low ferritinDoctor only
Jason Fung
No specific guidance in what we reviewed.
Sleep & recovery
Stacy Sims
- Traditional (Finnish-style) sauna at about 60–80 °C after training; she prefers heat to cold for womenSome evidence
- If using cold water, choose cool (about 16 °C) rather than ice-coldPersonal habit
- Treat easy days as true recovery between hard sessionsPersonal habit
- Sees hormone therapy as a valid option for hot flushes and bone risk, but not required for every womanDoctor only
Jason Fung
No specific guidance in what we reviewed.
Stress & mind
Stacy Sims
- Keep hard intervals short; she says longer efforts and very long sessions raise cortisol, already a concern in menopauseSome evidence
- In perimenopause, have caffeine with food before training if it makes you shaky or light-headedPersonal habit
- Listen to your own body and track responses rather than following a fixed planPersonal habit
Jason Fung
No specific guidance in what we reviewed.
Brain health
Stacy Sims
- Creatine for brain fog and mood; she cites a trial where 5 g a day added to an antidepressant helped women with depressionSome evidence
- Favours truly hard intervals over moderate or zone 2 cardio for midlife cognitive and metabolic benefitsSome evidence
Jason Fung
No specific guidance in what we reviewed.
Weak spots to know
Stacy Sims
- Some researchers say her sex-specific advice goes beyond the evidence. A 2023 umbrella review co-authored by Stuart Phillips found no clear effect of menstrual-cycle phase on strength or training gains, and Sims publicly debated Lauren Colenso-Semple in 2025 on fasted training and protein timing, framed as mechanisms versus outcomes.
- Her case against fasted training leans heavily on short-term hormone and appetite responses; critics note that longer studies measuring actual fat loss and muscle gain show little difference when daily calories are adequate.
- Many of her menopause-specific prescriptions (exact sprint lengths, playing down zone 2, tight post-exercise protein windows) come from extrapolation and her own coaching experience rather than trials in midlife women.
Jason Fung
- Red Pen Reviews scored The Obesity Code at 31% for scientific accuracy, and The Diabetes Code at 33%.
- His central claim that calories do not matter is contradicted by controlled studies.
- Much of his support comes from case reports, small older studies and his own clinic.
Build a plan from Sims, Fung or both
Your free plan shows a week of gym sessions, cardio, a day of eating, supplements and mobility, fitted to your age, time and health.
Questions
What is the main difference between Stacy Sims and Jason Fung?
Their training split differs most in aerobic base (zone 2): Sims about 10%, Fung about 35% (our estimate from their published routines). Sims' signature habit is Heavy lifts + sprints; Fung's is Fasting, less sugar.
How much protein do Sims and Fung recommend?
Sims: ~2.2 g/kg. Fung: No fixed target.
Whose advice has stronger evidence?
We rate the evidence behind Sims' protocol higher: strong, against mostly weak for Fung.
Do Sims and Fung disagree?
Yes, on fasting and muscle. See "Where they disagree" on this page.
Can I combine Sims and Fung?
Yes. In Build my mix you choose one expert per area, for example one for strength and another for eating. Your free plan shows the result straight away.
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Full sources are on each profile: Stacy Sims · Jason Fung. Reviewed October 2026.