by Dewey M. Caron 
Honey bee dietary needs are carbohydrate (sugar) for energy and protein, amino acids, lipids, vitamins, minerals, phytochemicals and cholesterol for growth and gland development. Beekeepers know the major bee food of nectar and pollen must first be processed with conversion of nectar into honey and pollen into bee bread.  
Pollen is especially critical for bee larvae, the growth stage of metamorphosis, and for new adults which need the proteins, lipids, minerals, amino acids, and cholesterol to kick-start their glands, enabling them to produce brood food, pheromones, beeswax, etc. Proper nutrition is especially critical for colonies expanding rapidly in  spring, newly started colonies and for fall reared adults that need fat body vitellogenin to suspend the aging process and successfully overwinter. 
Studies show bees with access to high quality pollen have lower pathogen levels, have more brood for overwintering success and are less susceptible to Nosema the gut parasite.  Plant phytochemicals (including sterols), supplied by pollen, although in minute quantities, are responsible for molting hormone (the chemical that guides bees through metamorphosis), cell membrane stability and undoubtedly other vital physiological processes in bees. Intact cell membranes are vital to keeping things corralled in cells that need be there and for keeping other things that shouldn’t be there (like pesticides and pathogens) from entering cells.  
Studies are underway at OSU, directed by post doc Priyadarshini Chakrabarti, to better determine both the quality and quantity parameters to define nutritional adequacy of the bee diet, especially as regards plant phytochemicals and cholesterol. Natural nectar and pollen sources vary greatly. Not all flower pollens necessarily supply the essentials of a bee diet. Diversity in the diet helps make up for source deficiency. Likewise, our managed bees are heavily supplemented with various protein sources, a variety of vegetable oils and sugars, such as high fructose corn syrup. How do these supplements factor into the nutritional needs of bees? 
A recent publication by Priya from OSU studies reviewed a spectrum of bee diets, both natural and supplemental, toward seeking a better understanding as to what amino acids, phytochemicals and cholesterol are critical. Title: The omics approach to bee nutritional landscape published in a September issue of Journal  Metabolomics. doi: 10.1007/s11306-0191590-6. Free access here: https://www.ncbi.nlm.nih.gov/pubmed/31538263. It is an “easier read” than title suggests. Poor nutrition can be a factor in weakened colonies or colonies not developing seasonally. Loss of weeds in agricultural fields and homeowner lawns due to increasingly heavy use of herbicides, monocultures of only single flowering sources available in the bee diet (as when the bees are involved in commercial pollination), landscape alterations that remove natives and substitute plants of little use to bees, effects on the bees’ intestinal microflora (perhaps due to heavy reliance on artificial feeding of colonies and antibiotic/pesticide exposure), and decreasing natural forage (principally due to human disturbance of the habitat where bees forage) all have cumulative negative impact. Bees that are weakened because of poor or inadequate nutrition become more susceptible to other factors. Instead of the diet aiding the bees to get through challenges like mites/miticides, a poor diet may in fact be one of the underappreciated factors in heavy colony losses. How are your bees doing so far this winter?  
Above is a photo of how bees were utilizing a candy board (photo from J. Schifler, Chester Co,)