The graphs at left show how composition of the mother's milk varies over the course of a cub's first year of life. (Parenthetically, it is often a challenge to know where to place information given that so many topics necessarily intergrade; these graphs could just as well go under Growth).
As you can see, composition of the milk changes substantially after the mother leaves the den and transitions from hibernation metabolism to active metabolism--a natural consequence of shifting from subsistence almost wholly on fat stores to subsistence on ingested food (see Physiology). Protein content of the milk increases substantially, as does energy, at least for a while. This shift naturally promotes growth of lean body mass in cubs. Energy more-or-less tracks lipid content of the milk, which reflects the high metabolizable energy concentration of fats.
The above graphs show the growth of cubs during their first year of life. These results derive almost wholly from the work of Charlie Robbins and his students at Washington State University, most notably here, Shawn Farley. (You will see me reference Dr. Robbins' work virtually anywhere there is something to say about nutrition, growth, or physiology). Panels A and B at left above show weight growth, highlighting the period that cubs are in dens--which is roughly the first 90 days of life. Panel B also differentiates the growth of individual cubs that are part of a litter of twins versus a litter of triplets. Panel C translates growth in absolute size into a rate, which is contrasted with the amount of energy that cubs are obtaining from their mother's milk (in Panel D).
Finally, in the two graphs to the upper right the fatness of mothers upon entering hibernation is related to (Panel A) the size of cubs after 90 days of growth (at emergence from the den, differentiating male cubs from female cubs) and (Panel B) their birth date (essentially the day in January).
Cubs grow quite rapidly and reach a peak growth rate as late as 7-8 months of age. Their most rapid growth occurs after they leave the den, just prior to the onset of a major decline in intake of energy from nursing. This pattern makes sense given that this period closely coincides with when cubs are obtaining energy not only from milk, but also from sampling food that their mother is finding. Aside from these dominant themes, cubs that are part of litters of two grow faster than cubs that are part of threesomes. This is not surprising given that a cub that is one of triplets is sharing a limited supply of milk with two siblings rather than one, which implicates the trade-offs for a mother of producing triplets versus twins. The mother may increase the odds of replacing herself by having triplets, but only as long as the overall odds engendered by an extra cub offset the reduction in survival of individual cubs--which is a predictable result of being smaller. The other key theme is that cubs born to fatter mothers enjoy several advantages, including having a longer period of growth within the protection of a den, and being able to reach a larger size prior to leaving, regardless of the time advantage. Again, this result is not surprising. Fatter mothers provide their cubs with more milk and are able to endure the rigors of denning for a longer period of time. Interestingly, the benefits of a fatter mother apparently do not differ for male versus female cubs or for cubs that are one of twins versus triplets.
The graph farthest to the right places the milk produced by bears (the large brown dots) in context of other mammals (the beige dot and bar summaries). Bears produce milk that is amongst the richest in fats and also the least rich in carbohydrates. The dominance of fats in bear milk is a natural consequence of the critical role that body fat (adipose reserves) plays in hibernation, which is when cubs are born and first nursed (for more details on this see Physiology).