There’s more to fat than you may think Stock Photo

A recent spell of very cold weather made me think about (among other things) bears hibernating in their dens, keeping warm by using the fat put on during the fall binge of eating. Because they are “burning” (oxidizing) fat, they don’t need to urinate. But if they used up all their stored fat, they would start metabolizing protein, in muscles, for example. Proteins contain nitrogen, and excess nitrogen from metabolism is excreted in urine. So a bear with depleted fat reserves would need to urinate.


Fats are a good way to store energy. They hold more than twice the calories of carbohydrates: about nine calories per gram versus four calories per gram. However, it takes a bit longer to break down fats to a usable form for metabolism. The relatively low caloric content of carbohydrates is why bears have to eat a whole lot of berries to pack in as many calories as they get from eating fish, especially the brains and roe. I’m told that ravens have figured this out — they regularly raid the tanks of used cooking oil at the haz-mat depot!

Mammals actually have two kinds of fat — the usual white fat (which hibernating bears depend on and which some of us humans have way too much of) and the so-called brown fat. Brown fat cells, unlike the white kind, contain mitochondria, those miniscule organelles that do all the work of metabolism. Brown fat (again, unlike white fat) is well supplied with capillaries, which carry the products of mitochondrial work to the rest of the body. One of those products is heat, and the primary function of brown fat is thermogenesis — generating heat. Brown fat “gears up” when the body is chilled, helping to prevent hypothermia.

Brown fat is stored chiefly around bones in the upper torso and around some internal organs (in humans, anyway, and presumably other mammals). Human babies have more brown fat than older people, perhaps because they can’t shiver to generate heat; the amount of brown fat decreases with age.

Arctic mammals, including humans, tend to have more brown fat than those living farther south. Very young offspring of species that raise their helpless babies in nests, warmed by the mother, generally have less brown fat than the offspring of species whose newly born offspring are furry and ably to move about (like caribou). The Inuit of Greenland have a special gene that facilitates formation of brown fat; it’s a good guess that the Native peoples of northern Alaska and Siberia do, too.

Birds are not known to have brown fat; they have to shiver during a winter night. They have a kind of fat that apparently looks a lot like brown fat, but it seems not to function in the same way.

Plants use fats and oils as energy-storage products too. Notably, the seeds of some species contain a high percentage of fat (by weight)—and the caloric value is generally correlated with the oil and fat content. For example, walnuts and hickory nuts contain more than 50 percent of their weight as fat (the values vary among information sources), and they are also quite large. That’s a big contrast with millet, sorghum, and corn, which hold about 5 percent fat. Red oak acorns (18 to 25 percent fat) have more fat than white oak acorns (5 to 10 percent). Closer to home, Sitka spruce and western hemlock seeds contain 30 to 40 percent fat (but not all conifers do so), although they are quite small. I think that the great variation in fat content of seeds may be related, in part, to the amount of energy needed for the germinated seed to get established and the seedling to begin to grow, but I have not found any data about that idea. In addition, there are undoubtedly constraints on the physical size of the seed (for example, small, light seeds can disperse on the wind, but large, heavy seeds cannot), which in some cases would make it especially advantageous to store energy compactly, in fats. (Other factors are involved also, but that gets to be a long story…)

There are oils in the leaves of plants too. If the leaves fall into a stream, in the process of decay they release organic molecules called surfactants; these break the surface tension of the water, which results from the tendency of water molecules to cling together. Breaking the surface tension allows the fats and oils of the decaying leaves to intermix more easily with water, and rapidly moving water churns the mixture into fatty air bubbles. That’s what creates the thick piles of white or brownish foam we often see caught on the upstream sides of logs.

Thanks to Jonas Lamb, University of Alaska Southeast public services librarian, for helpful discussion.

• Mary F. Willson is a retired professor of ecology. “On The Trails” appears every Friday. Her essays can be found online at


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