Hmm…Looking around a bit more, if they’re eating only sunflower seeds as you say, that can cause problems:
“Sunflower seeds contain a whopping 47% – 49% fat. While they can contribute to a balanced diet alone they are a deficient food much too high in fats. Sunflower seeds and peanuts are all high fat foods that need to be restricted to a small part of their diet. High fat diets can shorten the life of a hamster. Although nuts and seeds are packed with valuable and much needed nutrients they are also very high in fat as well as calories. But the oils / fats they contain are mono and polyunsaturated types, the good fats, and these are less likely to promote clogging or hardening of the arteries than animal fat does. Although they are considered the good fats, no fat in excessive consumption is good. Sunflower seeds as the main ingredient part of the diet invite disease. High fat diets contribute to the same health problems in hamsters as it does in humans. Hamsters’ can become obese and have high cholesterol levels, suffer strokes, heart attacks, and other heart problems from a high intake of fat.
Many hamsters just love sunflower seeds this is because they taste better than the other seeds in their diet. Eating too many and only just theses seeds he may experience a condition called ketosis. This is because he may be eating too many of his favorite sun flower seeds and not enough carbohydrates. Sunflower seeds are high in fats and low in carbohydrates, you may think that because he is eating fat he will get fatter. Not necessarily, it doesn’t work like that. If you eat no carbohydrates and eat just fat you will not only stop getting fat but will actually get thinner. Sunflower seeds as the main ingredient part of his diet is unbalanced and may cause deficiency problems. If he is eating more fat (seeds) and less carbohydrates he may induce ketosis. Ketosis merely means that the body is using fat for energy. Although this does not mean he is actually losing weight it means the body is becoming a more efficient fat burner. So a once bonny hamster can use his body fat store very quickly appearing to lose weight”
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