Intermittent Fasting Preserves Muscle Gains During Resistance Training: What the Research Shows

Medical disclaimer: This article summarises published research for informational purposes only. It is not medical advice and is not a substitute for guidance from a qualified health professional. Always consult your doctor before starting any fasting protocol, especially if you have an existing health condition or take medication.

Study at a Glance

What This Study Looked At

One of the most persistent concerns about intermittent fasting is whether it interferes with muscle building. The worry makes intuitive sense: if you're eating in a compressed window, can you consume enough protein at the right times to support muscle protein synthesis and recovery from resistance training?

Tinsley and colleagues at Texas Tech University designed this trial to answer that question directly — specifically in resistance-trained women, a population underrepresented in fasting research. They wanted to know whether an 8-hour time-restricted feeding window, placed in the afternoon and evening, would blunt the muscle-building and strength adaptations from 8 weeks of supervised resistance training.

For related background, see does intermittent fasting burn muscle? and can you build muscle while intermittent fasting?.

Who Was Studied

Participant profile: Active, resistance-trained females. Mean age early-to-mid twenties. All participants had existing resistance training experience, ensuring the study was examining muscle maintenance and hypertrophy in trained individuals rather than beginner gains.

How time-restricted feeding worked in this study: The TRF groups consumed all of their daily calories between noon and 8:00pm — an 8-hour eating window with a 16-hour fast. There was no caloric restriction prescribed. Participants were simply told not to eat outside this window. All groups consumed supplemental whey protein as part of the study protocol.

The resistance training protocol: All three groups completed 8 weeks of supervised resistance training, 3 sessions per week, using a structured periodised programme covering upper and lower body muscle groups.

What the Researchers Found

Body Composition

Key findings:

• Fat-free mass (muscle and lean tissue) increased similarly across all three groups — approximately 2–3% relative to baseline. The TRF groups did not lose or fail to gain lean mass compared to the control group.
• Fat mass decreased in both TRF groups while increasing slightly in the control group. This differential fat loss occurred despite protein and calorie intake being similar across groups.
• The TRF + HMB group showed the greatest fat mass reduction (-4% to -7%), suggesting the supplement may have additive effects on body composition beyond TRF alone.

Muscular Strength and Performance

Muscular strength (measured by bench press and leg press performance) improved in all three groups across the 8 weeks of training. There were no statistically significant differences between groups in strength gains — meaning TRF did not impair the development of maximal strength.

Muscle Hypertrophy

Ultrasound imaging of muscle cross-sectional area showed skeletal muscle hypertrophy occurred in all groups, again with no significant difference between the TRF and control groups. Muscle grew at similar rates regardless of feeding window.

Energy Intake

Despite TRF groups consuming all their calories in an 8-hour window versus the control group eating throughout the day, total daily energy and protein intake (~1.6 g/kg body weight) did not significantly differ between groups. This is important context — the study controlled for intake, meaning the body composition differences were attributable to the timing of eating, not a simple calorie restriction effect.

What Did Not Change

• Maximal strength outcomes (no between-group differences)
• Muscle hypertrophy rates (similar across groups)
• Total daily caloric intake (matched across groups)

What the Researchers Concluded

Tinsley and colleagues concluded that time-restricted feeding does not attenuate resistance training adaptations in active women. Similar fat-free mass gains, skeletal muscle hypertrophy, and muscular strength improvements were achievable across dramatically different feeding schedules — provided energy and protein intake were matched. The TRF groups additionally showed favourable changes in fat mass that the control group did not.

What This Means If You Fast

• Muscle building is compatible with intermittent fasting. This study provides direct evidence that an 8-hour eating window does not prevent women from building muscle during resistance training — a concern that often keeps people from combining fasting with their gym work.
• Protein intake still matters. The TRF groups matched the control group's protein intake (~1.6 g/kg/day) within their 8-hour window. Meeting your protein target remains essential — fasting doesn't exempt you from this requirement.
• TRF may give you better body composition outcomes than eating all day. The TRF groups gained the same muscle while losing fat; the control group gained muscle but also gained fat. If body recomposition is your goal, timing your eating can help.
• You don't need to eat immediately post-workout. This study's TRF groups ate between noon and 8pm, which for some participants likely meant training in a fasted state or training before the eating window opened. Results were still equivalent to the group eating throughout the day.
• Women specifically benefit from this data. Most resistance training + fasting research has been conducted on men. This study addressed that gap and showed that the combination works for trained women as well. See intermittent fasting and muscle preservation for women for more.
• Adding HMB may provide extra fat loss. If fat loss is a priority alongside muscle building, the TRF + HMB group's results suggest this combination is worth exploring — though HMB is an optional addition, not a requirement.

Study Limitations

• Small sample size. With 24 per-protocol completers split across three groups, each group had approximately 8 participants — too small to detect modest effects with confidence.
• Women only. Results may not generalise to men, though existing male-focused studies (such as Moro et al. 2016 in Journal of Translational Medicine) also show muscle preservation with TRF and resistance training.
• Short duration. Eight weeks is sufficient to detect body composition changes but may not capture longer-term effects or hormonal adaptations in women, particularly across different menstrual cycle phases.
• Specific population. Participants were already resistance-trained. Results may differ in beginners, older adults, or those with lower baseline fitness.
• HMB manufacturer funding. The involvement of Natural Alternatives International (which manufactures HMB) in the supplementation arm is a potential conflict of interest for the HMB-specific findings.

Source

Tinsley GM, Moore ML, Graybeal AJ, Paoli A, Kim Y, Gonzales JU, Harry JR, VanDusseldorp TA, Kennedy DN, Cruz MR. Time-restricted feeding plus resistance training in active females: a randomized trial. American Journal of Clinical Nutrition. 2019;110(3):628–640. PMID: 31268131

Frequently Asked Questions

Does intermittent fasting prevent muscle growth?

This study found the opposite — time-restricted feeding did not prevent muscle growth during resistance training. Fat-free mass increased at similar rates in both TRF and control groups, suggesting fasting-compatible muscle building is achievable when protein intake is adequate.

How much protein should I eat while fasting and training?

The TRF groups in this study consumed approximately 1.6 g of protein per kilogram of body weight per day — all within their 8-hour eating window. For a 65 kg woman, that's roughly 104g of protein per day consumed in two or three meals. This is consistent with general resistance training recommendations.

Can I train in a fasted state and still build muscle?

The study's TRF groups consumed food between noon and 8pm. Depending on each participant's training schedule, some may have trained fasted. Yet muscle hypertrophy was equivalent across groups. This suggests that fasted training does not necessarily impair muscle-building outcomes when total daily protein intake is maintained.

Should women combine intermittent fasting with resistance training for fat loss?

This study suggests yes — the TRF groups lost fat mass while gaining similar muscle compared to the control group. If fat loss alongside muscle building (body recomposition) is the goal, TRF combined with structured resistance training appears to be a more effective approach than resistance training with unrestricted eating patterns.

What was the eating window used in this study?

All food was consumed between 12:00pm and 8:00pm — an 8-hour eating window with a 16-hour daily fast. This is the standard 16:8 protocol that many people already use or find relatively easy to adapt to.

Related Research and Articles

• Does intermittent fasting burn muscle?
• Can you build muscle while intermittent fasting?
• Should you work out in a fasted state?
• Does fasted cardio burn more fat?
• Intermittent fasting and muscle preservation for women
• What should you eat after working out during intermittent fasting?
• Intermittent fasting and weight loss: what 50 studies show

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