Here is evidence that too much protein in bees' diet can be harmful, calling into question the use of pollen supplements. That is to say, the amount of protein should be in the correct proportion, compared to a natural diet. 

Excerpts:

This study was conducted to determine the optimal dietary protein level for winter bees for rebuilding colony populations during early spring.

In this study, the highest dietary protein level resulted in lower survival and reduced population growth. Excess protein ingested by honey bees is responsible for the accumulation of undigested material in the gut. Herbert et al. (1977) reported that pollen substitutes containing 50% protein depressed brood rearing.

In the natural environment, it is well known that the protein content of bee-collected pollen varies considerably among species. However, in response to increasing demands for protein, bee colonies regulate the quantity of pollen collected rather than select pollen with a higher protein content. In addition, there is no evidence that bees prefer to collect pollen containing high levels of protein.

The overall survival of bees from the different groups in the study averaged 70.0% after feeding for 30 d. The survivorship curve indicates that bees that were fed with the diet containing 38.5% CP [crude protein] had the lowest survival rate, followed by the groups fed the 25.0, 29.5, and 34.0% CP diets. The highest survival rate was obtained with the control diet.

Zheng, B., Wu, Z., & Xu, B. (2014). The Effects of Dietary Protein Levels on the Population Growth, Performance, and Physiology of Honey Bee Workers During Early Spring. Journal of Insect Science, 14(1), 191.

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