PTH by stimulating ornithine decarboxylase in osteoblasts produces osteogenic polyamines such as putrescine, spermidine, and spermine (42)
It seems that the reason carnitine was unable to increase muscle carnitine concentration in these studies for 2 reasons: Poor bioavailability (only 20% for of a 2-6 g dose is absorbed) and Because the transport of carnitine into the muscle was limited
In animal studies, MOTS-c administration has been found to increase exercise capacity and improve metabolic parameters independent of activity levels
Furthermore, elevated CSF homocysteine level has been associated with migraine (95), migraine aura (95), PD (96), and post l-DOPA therapy (97) attributed to S-adenosylmethionine consumption and consequent S-adenosylhomocysteine elevation (88)
Jiang L, Yin M, Xu J, et al
Therefore, it is not a question of whether tirzepatide can activate the mGIPR, but rather what doses of tirzepatide are required to do so