I found this:
Growth, Kidney Disease, and Longevity of Syrian Hamsters (Mesocricetus auratus) Fed Varying Levels of Protein
Donald B Feldman, Ernest E McConnell, and Joseph J. Knapka
Laboratory Animal Science © 1982 Academic Press
This was an excellent and extremely interesting paper to read. Its beauty is in its simplicity:
Four groups of hamsters that were fed foods with 4 levels of protein throughout their lives: 6%, 12%, 18%, 24%. The scientists monitored the death rates and nephritis (swelling of the kidneys) rates in these groups over time.
1) Hamsters fed on a diet of 6% protein were found to have significantly shorter lifespans than those fed on higher protein diets.
This shouldn't come as much of a surprise to anyone--6% protein is inadequate at any age. It was assumed that this was because of malnutrition--several of these hamsters died of diseases that they were presumably too weak to fight off. Many of the younger ones died of enteritis (wet tail or something similar to it).
2) Statistically speaking, there was no difference in lifespan between the hamsters fed 12%, 18%, and 24% protein.
As for the other levels, there simply wasn't enough difference in the lifespans of the hamsters to be sure that it wasn't just random chance. They all had similar lifespans regardless of the amount of protein they were fed, even at levels as high as 24%.
Hamsters fed on diets with high levels of protein did not die sooner than those fed lower levels of protein. And actually, of the group of hamsters that lived to be the oldest, half of them (7 out of 13) were from the group fed 24% protein.
3) Hamsters fed on higher protein diets had higher levels of nephritis (kidney swelling). However, this did not contribute to higher death rates (as stated above).
High-protein diets did appear to contribute to kidney swelling. However, this paper did delve into what that actually means for the hamsters:
Quote
"Paradoxically, seven of the 13 hamsters that survived longer than 20 months were those fed the highest level of protein. Most of these animals had moderate to severe nephritis, but renal function apparently was adequate for survival." (pg 617)
Quote
"Furthermore, the blood urea nitrogen was markedly elevated in only a few animals with advanced nephritis, whereas others with comparable lesions had values that fell within the normal range. Consistently higher blood urea nitrogen values in hamsters fed 18% and 24% protein compared to those consuming lower levels of protein at all time points probably reflected the degree of nitrogen metabolism from available protein rather than the inability of the kidney to remove waste products, at least prior to 20 months." (pg 61
Basically, what this is saying is that while these hamsters had kidney swelling, it didn't seem to actually have any effect on kidney function in most cases. Their kidneys were still functioning normally. Furthermore, when blood urea nitrogen was high, it could have been from the simple fact that these animals were eating more protein, rather than their kidneys failing (basically, more goes in, more comes out).
I also thought this point was very interesting:
Quote
"Females fed either 18% or 24% protein had heavier kidneys (p<0.05), than those females fed either 6 or 12% protein (Table 3). The kidneys of females fed 12% protein were in turn heavier (p<0.05) than those females fed 6% protein." (pg 615)
Yes, females with 12% protein had more swollen kidneys. But they still had a much better survival rate than the hamsters fed 6% protein. So kidney swelling does not necessarily equate to an earlier death.
4) Significant differences in kidney size were not noted until 20 months of age or more
Quote
"After adjusting for differences in body weight, significant differences in kidney weights were not evident until the hamsters reached 20 months of age." (615)
This makes me question the recommendation of dropping protein as early as 1 year. If hamsters fed 24% protein (which is way more than we ever recommended for any age) didn't show any significant kidney swelling until 20 months...well, I'm not sure that not reducing protein from a mere 17% - 18% at only 12 months is absolutely essential.
Assorted Recommendations
Various Sources
"Commercial rodent feed is generally used as the basic diet for omnivorous hamsters, sometimes in combination with alfalfa cubes, to provide 16% - 24% protein, 60 - 65% carbohydrates, and 5 - 7% fat."
Biology and Diseases of Hamsters
F. Claire Hankenson and Gerald L. Van Hoosier Jr.
LABORATORY ANIMAL MEDICINE 2nd Edition
"The results of this study indicate that when the protein source is composed of common feed ingredients, the requirement is over 15% of the diet."
Protein Nutrition of the Golden Hamster
Charles A. Banta, Richard G. Warner and James B. Robertson
The Journal of Nutrition 105: 38-45, 1975
"Although the nutritional requirements have not been determined specifically, a pelleted rodent diet that contains approximately 16% protein and 4 - 5% fat is typically provided and appears to proved a nutritionally adequate diet."
Biology and Medicine of Rabbits and Rodents 5th Ed.
John E. Harkness, Patricia V. Turner, Susan VandeWoude & Colette L. Wheler
© 2010 Blackwell Publishing
Clinical studies, scientific reports?
. Thank you for the links I’ll read them 
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