Intermittent Fasting for 16 Hours VS 12 Hours Daily: Muscle Loss, Testosterone & Metabolic Health Improvements

by Mike Mutzel


Long-term intermittent fasting strategies haven’t exactly been popular among the sports and conditioning community due to fears that prolonged daily fasting strategies will lead to muscle and strength losses. A new 12-month time-restricted feeding (TRF) study found that prolonged daily fasts by subjects who regularly resistance train is associated with visceral fat loss, various cardiometabolic improvements, and reductions in inflammation with minimal muscle or strength loss compared to subjects who eat three meals daily (no feeding window compression).




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Video Time Stamps:


0:00 Intro
0:18 New Study
0:08 Testing 16 Hour VS 12 Hour Feeding Windows
0:27 Markers of Inflammation decreased
1:17 Only four hour difference in feeding window
2:22 Study Title:
3:43 Testing your metabolic health
4:42 Berberine & metabolic health
5:50 Feeding Window comparisons
6:43 How Calories were distributed
7:36 Whey protein post workout
7:50 TRF group ate less calories by accident
8:33 Strength and muscle loss between two groups
11:08 Testosterone did decrease
11:53 Insulin, Leptin and Adiponectin
12:44 HDL, Triglycerides and glucose
13:52 Chronic Inflammation
15:11 Trade offs to consider


Longer Daily Fasts Associated with Better Cardiometabolic Profiles, Less Chronic Inflammation

Researchers randomized subjects with five or more years of consistent weight lifting experience into two different feeding groups for the duration of the 12-month study. The TRF group was instructed to eat three time daily at 1 p.m., 4 p.m. and 8 p.m., and a normal diet (ND) group instructed to eat three times daily at 8 a.m., 1 p.m. and 8 p.m. In essence, the study was designed to test the long-term effect of a 16-hour daily fast versus a 12-hour daily fast.

Dietitians worked with study subjects to ensure meals contained sufficient energy and macronutrients to meet daily energy requirements, and subjects had weekly calls with dietitians to ensure adherence to the study protocols, meal timing and the like. It’s worth noting the calorie distribution over the course of the study: subjects in the TRF arm had 40%, 25% and 35% of calories for breakfast, lunch and dinner, respectively, while the ND group had 25%, 40% and 35% of their calories at their respective breakfast, lunch and dinner.

Weight training wasn’t supervised over the course of the study, but subjects in both groups were advised to weight train between 4 p.m. and 6 p.m. during the study. Subjects in both groups were advised to consume a 20-gram whey protein shake after their afternoon training session.

Main Findings: Body Composition, Hormones, Insulin Sensitivity and Muscle Mass

Researchers found subjects in the TRF group had a spontaneous 6.1% decrease in energy intake over the course of the study compared to the ND group. This may be why the ND group gained 3.37% body mass over the course of the study while the TRF group actually lost 3.36% of body mass and 18.8% visceral fat (VAT). The ND group didn’t lose any VAT.

With regard to strength, both groups significantly increased their 1 rep max test for upper and lower body as assessed by bench press and leg press. (If we’re being honest, the TRF group had a small percentage strength increase in these tests over the ND group.) There were differences in the circumference of muscle between the two groups. The TRF group lost 4.31% and 2.9% muscle cross sectional area (CSA) in their arms and legs, respectively. The CSA of the arms and legs increased in the ND group by 11.87% and 6.95%, respectively. As a small anecdote, I’ll say I notice my arms appear smaller when I strictly adhere to a more compressed feeding window. But that trade-off, in my opinion, is worth it from a long-term metabolic health perspective, as you’ll soon see.

Subjects in the TRF group experienced a 16.8% reduction in testosterone over the course of the study, but there was no observed change in the ND group. However, the concentration of leptin decreased by 24.9% and adiponectin increased by 21.3% in the TRF group while no alterations were observed in these hormones in the ND group. The researchers speculated that the changes in appetite-related hormones may have impacted the testosterone levels.

Significant improvements in glucose and insulin signaling as well as in lipids were observed in the TRF group but not in the ND group. At the end of the study, HDL increased by 15.39%, and glucose, insulin and triglycerides dropped by 9.26%, 28.26% and 20.98%, respectively, in the TRF group compared to no changes in the ND group.

Markers of chronic inflammation were also significantly reduced in the TRF group, but not so in the ND group. Compared to baseline levels, IL-1 beta, IL-6 and TNF-alpha all significantly decreased by 22.97%, 25.22%, and 13.88% in the TRF group after the 12-month study.

In summary, there are trade-offs with everything in life, and following a compressed feeding window meal plan is no exception. If maximizing muscle size and testosterone is a primary goal, and you’re not interested in favorable shifts in cardiometabolic and inflammatory markers, consider a 12-hour fast and not more. If you’re seeking to balance long-term health with muscle and strength gains, consider a 16-hour compressed feeding window.


Related: Fasting is needed to for Calorie Restriction health benefits


It would be interesting to see how an earlier feeding window would have impacted the outcome of this study as prior earlier time-restricted feeding studies have found significant improvements in fat mass compared to no feeding window compression, which wasn’t necessarily discovered here.

As for me, I’ll stick to my 11 a.m. to 7 p.m. feeding window and regular resistance training. I’ll trade slightly smaller arms for better cardiometabolic and inflammatory health all day long!



Moro, T., Tinsley, G., Pacelli, et al. (2021). Twelve Months of Time-restricted Eating and Resistance Training Improves Inflammatory Markers and Cardiometabolic Risk Factors. Medicine and science in sports and exercise, 53(12), 2577–2585.

  1. You call the Normal eating a 12 hr protocol, but it looks like it has a 13 hr eating window ( or even longer if later evening snacking is allowed)
    Also, It’s not clear whether
    1. Snacking was allowed
    2. Was the 16 hr fast a water only fast?
    3. Were the normal diet people allowed to have snacks after 8 PM
    I think these are important to clarify,

  2. Muscle size is a product of hydration and Carbohydrate intake and timing. . . What is that ratio 1:4(carbs : water retention)

  3. Quick questions I may have missed was this research were the subjects men only and if so is it as effective for women as I know fasting for men is far more effective for men than women.

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