Benefits
Lean mass with resistance training in young men
In a maker-sponsored 12 week trial of resistance-trained young men, lean mass rose 1.7 kg on 6.6 g a day and 1.68 kg on 19.8 g a day, while the 0.6 kg gain on a macronutrient-matched placebo was not significant; the group by time effect was significant. Strength rose equally in all groups. Only 37 of 45 men finished, and no independent trial has repeated this.
Muscle protein synthesis in older adults
In 20 healthy adults with an average age of 66, 19.8 g a day for 21 days gave higher average synthesis rates than a cheese powder placebo for most measured muscle proteins, about 18 percent overall. No single protein differed significantly on its own. Muscle mass and strength were not tracked, and the maker funded the study.
Strength gains with resistance training
In the 12 week trial of resistance-trained young men, bench press and leg press strength rose equally in the two Fortetropin groups and the placebo group. In young men with one leg immobilized, knee extensor strength fell about 19 percent and recovered with normal activity, with no difference between Fortetropin and placebo.
Muscle size during two weeks of leg immobilization
In 20 analyzed young men, 19.8 g a day did not protect the immobilized leg. Thigh muscle cross-sectional area fell about 8 percent and leg lean mass by DXA about 1.6 percent in both groups, and neither had recovered 2 weeks after the brace came off. Muscle fiber size also did not differ between groups. The maker funded the trial.
Blood myostatin levels, a marker
Over a 6 week trial that included 2 weeks of leg immobilization, plasma myostatin rose 75 percent on placebo and 4 percent on 19.8 g a day, a significant difference, yet muscle loss was the same. In older adults, 21 days of use did not change myostatin, and higher myostatin went with higher protein synthesis, the opposite of the proposed link. Blood levels may not reflect muscle effects.
Thigh muscle and mobility in dogs (animal studies)
In 100 dogs after knee surgery, thigh circumference shrank on placebo but did not change significantly on daily Fortetropin over 8 weeks, a within-group comparison, and weight bearing did not differ between groups. In senior dogs, owner mobility scores improved only within the Fortetropin group, with no difference between groups. Single doses did not lower myostatin in healthy dogs. These are animal results.
Mechanism of action
Myostatin lowering, as described by the maker
MYOS Corp presents Fortetropin as a natural myostatin reducer. Human data on this are uneven: blood myostatin did not rise with Fortetropin over a 6 week trial that included leg immobilization, but it did not change in older adults, and lower myostatin did not prevent muscle loss. Which component of the yolk would act on myostatin has not been identified.
Muscle growth and breakdown signaling in rats
In rats given Fortetropin by tube for 8 days before one bout of exercise, the exercised muscle showed greater phosphorylation of mTOR pathway proteins (4EBP1, rp6 and p70S6K) and no rise in ubiquitin markers of protein breakdown, unlike control rats. These are animal tissue signals, not measures of muscle growth in people.
Egg yolk protein and fat
Fortetropin is mostly fat (about 59 percent) with about 34 percent protein, so a 19.8 g dose adds roughly 6.7 g of protein and about 133 kcal. Because the human trials used placebos matched for macronutrients and energy, the results reported are not explained by the extra protein or calories alone.
Clinical trials
Randomized, placebo-controlled parallel trial with three groups (described as double-blind by a 2026 systematic review; blinding is not stated in the abstract): macronutrient-matched placebo, 6.6 g or 19.8 g of Fortetropin a day during 12 weeks of resistance training, plus a separate rat experiment; the abstract names no primary outcome and reports the two doses as separate groups; a 2026 systematic review lists the trial as MYOS-sponsored and not prospectively registered, and no registry number is given in the abstract (Sharp et al. 2016, J Am Coll Nutr)
45 resistance-trained college-aged men enrolled, 37 completed.
Lean mass showed a group by time effect (p of 0.05 or less): 6.6 g gained 1.7 kg and 19.8 g gained 1.68 kg, while the 0.6 kg gain on placebo was not significant, and muscle thickness rose only in the Fortetropin groups. Bench press and leg press strength rose equally in all groups. Later papers report that serum myostatin fell 18 to 22 percent in this trial; that figure is not in the abstract.
Randomized, double-blind, placebo-controlled trial; 19.8 g of Fortetropin or 19.8 g of macronutrient-matched cheese powder a day for 21 days, with heavy water labeling and a thigh muscle biopsy on day 21; fully funded by a grant from MYOS RENS Technology, which supplied both products; the authors declare no conflicts and none advised the company; no registry number reported (Evans et al. 2021, J Gerontol A Biol Sci Med Sci)
20 healthy older adults (10 men, 10 women), average age 66; all completed.
Mean synthesis rates were higher on Fortetropin for 32 of 38 myofibrillar, 33 of 44 sarcoplasmic and 12 of 17 mitochondrial proteins, significant by a binomial test, with an overall difference of about 18 percent. No individual protein differed significantly. Circulating myostatin did not change and did not differ between groups.
Randomized, double-blind, placebo-controlled parallel trial; 19.8 g of Fortetropin or macronutrient-matched cheese powder daily with breakfast for 6 weeks (2 weeks run-in, 2 weeks single-leg immobilization, 2 weeks recovery); primary aims were muscle cross-sectional area, leg lean mass by DXA and isometric strength; funded by Myos Corporation to the senior author; registered NCT05369026 after the study ended (Lim et al. 2023, PLoS One)
24 healthy young men randomized, average age 22; 2 dropped out because of the Covid-19 outbreak and 2 placebo participants were excluded for poor brace compliance, leaving 20 analyzed (11 Fortetropin, 9 placebo).
No primary outcome differed between groups: thigh muscle cross-sectional area fell 7.9 percent, leg lean mass 1.6 percent and peak torque 18.7 percent during immobilization in both groups (group by time p = 0.763, 0.870 and 0.579). Plasma myostatin rose 75 percent on placebo versus 4 percent on Fortetropin (p = 0.005). Routine blood chemistry showed no group differences.
Randomized, double-blind, placebo-controlled veterinary trial; Fortetropin at 300 mg per kg a day or a cheese powder placebo added to food for 12 weeks after tibial plateau leveling osteotomy; funded by the manufacturer, MYOS RENS Technology (White et al. 2020, PLoS One)
100 client-owned dogs; dogs with owner-reported egg allergy were excluded, and 14 did not return at 8 weeks, 3 of them for diarrhea.
From week 0 to 8, thigh circumference of the operated leg fell on placebo (minus 1.21 cm, p = 0.03) but not significantly on Fortetropin (minus 0.54 cm, p = 0.31), comparisons made within each group. Serum myostatin rose on placebo only. Weight bearing on the operated leg rose in both groups and did not differ between them (p = 0.10).
Randomized, double-blind, placebo-controlled crossover study; single 6 g and 12 g doses with blood sampling up to 72 hours; Myos Corp provided the products and funding for laboratory testing (Nugent Britt et al. 2021, Front Vet Sci)
10 healthy adult dogs aged 2 to 8 years owned by hospital employees.
Mean serum myostatin was not significantly lower after either dose than after placebo at any time point over 72 hours.
Randomized, double-blind, placebo-controlled veterinary study using an owner-completed mobility questionnaire (LOAD) over 12 weeks; funding is not stated in the abstract (Hetrick et al. 2022, Can Vet J)
Geriatric and senior dogs with reduced mobility; numbers are not given in the abstract.
Mobility scores improved from baseline in the Fortetropin group at week 6 (p = 0.03) and week 12 (p = 0.006), with no improvement on placebo, but no difference between the two groups was found at any time.