[sushi_78]

I don't think rationing dry food is the best way to achieve weight loss. I've often had to slim down gerbils (as they get very prone to weight gain as they get older) and it's more effective to increase their fresh food. Increasing their ratio of fresh vegetables to dry food while keeping their overall food quantity the same reduces their calorie intake while not leaving them hungry or thinking there's a famine coming. Try to feed a variety of vegetables and especially highly nutritious ones such as spinach, kale, dandelion leaves, herbs, collards, rocket blackberry/strawberry/raspberry leaves and so on.

I have a dwarf hamster who was obese when I brought her home, and with both her and the gerbils, the key has always been to identify why they are overweight and reverse that. With obesity there always has to be a cause. So sometimes with the gerbils it's been because their main diet just wasn't right for their needs, and changing to a different diet solved it. With my hamster it was because her cage was too small and she was too big for her wheel, so she didn't have enough opportunity to exercise. It was fixed by correcting her environment. Sometimes it's just from too many treats and extra foods and cutting those out is enough. They have a kind of inbuilt normal weight which their body for the most part prefers to remain at, and will naturally remain at unless something internal (i.e illness, aging) or external (diet or environment) knocks them out of equilibrium.

I would recommend thinking about whether the food he was on previously might not have been right for him. That doesn't necessarily mean it's a bad food. The food that caused my gerbils to gain weight was Getzoo, which is a fine food, it just didn't suit them. It's always worth trying a different food if the previous one didn't seem to be producing good results.

Hamsters don't overeat in terms of volume (i.e they will eat until they are full and then stop, not keeping stuffing themselves as humans might), but they may pick out the nicest (and usually fattiest) bits from a mix and leave the rest. You shouldn't give them unlimited quantities of food as that encourages them to be picky, but only as much as they need to eat each day plus a little extra for hoarding. So another thing to look at would be whether the quantity of food he was getting was right for him.

You've probably already considered this one, but the other thing to think about would be all the foods he gets on top of his usual mix, which can add up. Fatty seeds are very healthy, they tend to be among the most nutritionally dense foods, but they're still a dense source of energy so could cause weight gain if fed in excess.

Lastly, and you've probably already got this covered as well, but look at his environment. Make sure his cage is big enough so he has enough space to move around, that's it's enriching so he has a reason to move around, and most importantly that is wheel is big enough for him and spins smoothly. If he's very big he might even need a larger wheel than a dwarf hamster would typically need. Even if his back is straight on his current wheel, he might still find a larger one more comfortable which would mean he uses it more. You can also try giving him more time outside his cage in a playpen or free-roaming, if he enjoys it (as some hamsters are stressed by it).