It's important to note that the tortoises carried on the Zond flight were probably not given any food or water, and I suspect they were selected because they could survive that for a long period. So weight loss would be inevitable. (We know that Apollo 17 carried pocket mice in the Command Module to study radiation exposure, and that the species was chosen for their ability to survive on very small amounts of water.)
For people it's completely different as of course they are well supplied.
Muscle mass through atrophy is common if exercise is not taken. There is a good summary on the Wikipedia page Effect of spaceflight on the human body, and the headline claim there is:
without regular exercise astronauts can lose up to 20% of their muscle mass in just 5 to 11 days
Also see a NASA factsheet on muscle atrophy which is the source of this figure.
We should remember the "up to 20%" figure is for muscle loss, so the overall body mass loss will be much less and anyway will depend on the calorie intake...