Bears lose 30% to 40% of their body weight while hibernating. It’s important that they regain this weight before returning to the den in late fall. Mountain grizzlies and black bears rely on plants for a large part of their diet and it is during berry season that much of this weight gain is accomplished.
During spring and summer bears replenish their levels of protein, but it’s during late summer and fall that large doses of carbohydrates supply the weight gain necessary for winter survival.
The US Dept of Agriculture has analyzed the nutritional value of many foods. You can view this data at their site the Nutrient Database for Standard Reference. A quick look tells why berries are an excellent source of carbohydrates and why a good berry crop during the fall is so important to the bears. The following data for a few select foods is based on 100 grams of raw material.
• Spinach, a representative green leafy food, contains 23 kCal of energy, 2.86 grams of protein, .39 grams of fat and 3.63 grams of carbohydrates.
• Blueberries, more than twice the energy value and almost four times the carbohydrate level, contains 57 kCal of energy, .74 grams of protein, .33 grams of fat and 14.49 grams of carbohydrates.
• Insects, represent a significant protein source in the summer, contain 20.6 grams of protein, 6.1 grams of fat and 3.9 grams of carbohydrates.
• Salmon makes up a large portion of the Alaskan Brown Bear’s diet. Winter killed deer and elk are important protein sources when bears first emerge from the den. Salmon and venison, almost equal in nutritional value, contains 157 kCal of energy, 21.8 grams of protein, 7.13 grams of fat and 0 grams of carbohydrates. Just about what you get from an insect!
Notice that the food available in the wild does not contain high levels of fat. Fat is generated by the bears from consuming large amounts of carbohydrates.
Additionally, blueberries are rich in Vitamins A, C, E and beta-carotene as well as rich in the minerals potassium, manganese and magnesium. They are very high in fiber and low in saturated fat, cholesterol and sodium. But this is just the tip of the nutritional iceberg, for recent studies tell us that of all fresh fruits and vegetables, berries provide the most health-protecting antioxidants, those valuable elements which prevent cancer-causing cell damage and may limit the changes wrought by age related diseases.