Deer Feeding by Humans
Worse than a belly-ache.
It takes two to four weeks of feeding on a new food source for deer to establish the microorganisms necessary to obtain nutrients from that food. The time and energy it takes to convert to new microorganisms uses precious fat reserves that could have been spared if the deer had fed continually on natural winter browse. Studies, including some in Pennsylvania, have documented the death of wild ruminants from feeding on highly digestible, high energy, low fiber feed such as corn in winter. This rapid exposure to a concentrated grain diet can cause a fatal disruption of the animal's acid-base balance. Those that survive the immediate effects often die in the days or weeks that follow, due to secondary complications of the disease.
Feeding by humans causes the concentration of a large herd which attracts predators such as bears, mountain lions, foxes, etc. When the human, especially a child, is feeding the deer and there are predators around, as the human cannot run as fast as the deer, the predator, not being fussy about what it eats, will eat the human. There are several recorded instances where the human child was killed and eaten by the predator. Bears don't kill their prey before they start to eat it . . . human or otherwise.
It seems to the human, especially the human child, that they are helping the deer by giving them a treat and it's a fun thing to do, but they are actually killing the deer. Irrespective of the predators, the deer themselves can become aggressive and harm the human.
If one feels that animals are in distress, such as injured or due to severe weather conditions and snowfall, he should call the proper agency for advice and help.
The Dead of Winter
Winter mortality will never be eliminated, it's nature's way of ensuring that only the strongest of the species survive to reproduce. Winter survival is determined by the availability of high quality fall food (to ensure fat accumulation) and winter thermal cover (to conserve energy). By late fall, deer (even captive deer) instinctively reduce their food intake and continue to do so through most of the winter. During that time, deer rely heavily on fat reserves and their ability to conserve energy, thereby making those reserves last longer. They travel less and seek protection in cover where snow is less deep, wind is less severe and tempertures are warmer. Winter energy conservation, especially important to fawns which use a good portion of their fall food to grow bone and muscle, not build up fat reserves. If an animal's fat reserves are used up before the end of winter, it is much more likely to die.
That being said, any activity that causes increased energy demands can harm deer by compelling them to waste essential fat reserves. Supplemental feedig can cause deer to expend more energy by coercing them to travel farther and more often and can increase winter starvation by luring in more animals than the feed can support. In one study, feeding was found to increase the winter death rate from 25 to 42 percent. Supplemental feeding also lowers the quality of the herd by enabling less fit individuals to avoid selective, natural winter culling. High concentrations of wildlife at feeding sites also attract predators. Animals expending energy to avoid those predators burn fat reserves that would have otherwise enabled them to survive the winter.
Sources: Wildlife conservation websites.
http://www.ruidosonews.com/story/news/local/2017/01/12/feeding-wildlife-could-fatal-wild/96491254