In 2001 I was introduced into one to multi-day fasting through the Vineyard Christian Fellowship, which I occasionally do since then. In spiritual tradition, fasting helps to focus the mind on prayer. Physically, fasting changes the body. One statement I heard repeatedly over the years from multiple friends is that fasting reduces muscle tissue not fat , for which I found no scientific evidence yet. I rather found studies confirming fasting reduces fat long-term and saves muscle strength. Do you know more scientific studies?
Prolonged fasting produces a body-composition picture that is initially alarming but more nuanced on closer reading.
Prolonged Water‐Only Fasting Followed by a Whole‐Plant‐Food Diet Promotes Fat‐Free Mass Recovery and Continued Fat Mass Loss in Adults With Overweight or Obesity. Gabriel+ 2025 found in their study on 14-day water-fasting followed by 6+45 days of whole‐plant‐food diet:
- prior research suggested “fat‐free mass (FFM) accounts for approximately two‐thirds of weight loss” , which was replicated: “FFM accounted for 74% of total BW lost”. Of this, there is a “lack of clinically significant muscle wasting”, “a large portion of FFM lost during fasting likely reflects changes in hydration status rather than lean tissue catabolism”. Muscle protein is a minor portion.
- After 14 days, fat mass was reduced by 6%. After 65 days (at follow-up) reduced by 14%, “fat mass loss accounted for 67% of total body weight lost”.
Is muscle and protein loss relevant in long‐term fasting in healthy men? A prospective trial on physiological adaptations, Laurens+ 2021 found in their study on a 10-day Buchinger Wilhelmi (200–250 kcal/day) fast followed by a 4-day food reintroduction and 90 day follow-up:
- fat mass “FM loss accounted for 40 ± 2.1% of changes in total body weight”
- lean soft tissue “LST loss accounted for 60.7 ± 2.1% of body weight loss, explained for 44% (−1.6 ± 0.1 kg) by extracellular water loss (which includes loss of faeces by enemas), for 14% (0.50 ± 0.01 kg) by early glycogen loss and its associated water, and for 42% by metabolic active (liver, kidneys, heart, intestine, muscles, etc.) tissues (1.5 ± 0.1 kg representing 25% of weight loss)
- “Strength was maintained in non‐weight‐bearing muscles and increased in weight‐bearing muscles (+33%, P < 0.001)”
- “3‐Methyl‐histidine, a marker of skeletal muscle breakdown, transiently increased during the first 4 days of fasting (Figure 2G) and returned thereafter to baseline values.”
- Our results support the hypothesis: “muscle function to be preserved if fasting adaptations were selected during evolution”
- “In our study, we did not notice changes in muscle physical properties”
It seems that fasting (water-only or Buchinger Wilhelmi)
- does decrease fat
- does decrease muscle mass in a minor portion in the beginning, with no change in muscle strenght
- which is consistent in an evolutionary
I do eat supplements while fasting. I want to learn more about which are clever, based on scientific repeatable measured evidence.
Do you know more relevant papers about this?