For more years than I care to remember, we nutritionists have known that, to a calf, consumption is life. A client can vaccinate and/or treat his calves with whichever vaccines or antibiotic he wants. But if calves don't eat, they die.

All these years, we have assumed that the value of consumption is that nutrition is essential for the immune response. Specifically, antibodies are primarily composed of protein as well as dependent upon certain vitamins and trace minerals. We assumed that without the nutrition, the body doesn't have the building blocks to form antibodies and, therefore, we see a depression in the immune response.

It's turned out to be more complicated than that. A recent discovery in human nutrition has also shed light on this depression in immune response we see in calves. This discovery concerns a hormone secreted by fat cells. Known as leptin, this hormone radically changes what we have known about the physiology of hunger.

Hormone Controls Eating A commonly held fallacy in human nutrition is that obesity is due to differences in metabolism. People with high metabolisms don't have weight problems; for those with low metabolisms, everything they eat "turns to fat."

Those of us in animal nutrition have long known this is false. With all the intense selection of livestock for "efficiency of gain," we have yet to see an animal that can fatten on the same amount of ration that will only maintain others.

New research indicates that obesity is related to the secretion of a newly discovered hormone called leptin. Although the details are not fully known or accepted, it appears leptin is secreted whenever fat cells are growing.

Circulating via blood, when leptin reaches the brain it sends the signal that the body is in a positive energy balance. This sends a signal to stop eating.

It is theorized that obese people either have less leptin production and/or the brain is less responsive. Some also theorize that for moderately obese people, the social aspects of continuous over-eating may tend to lessen the response to leptin.

Leptin Affects More Than Appetite At any rate, it has been discovered that leptin is a powerful hormone that affects more than just appetite. Specifically, medical researchers have discovered that leptin affects the immune system. They found leptin receptor sites on what are known as T cells, the cells used to create antibodies.

Conducting an experiment with mice, they found that when starved for 48 hours, a marked reduction in immunity resulted. When injected with leptin, however, a more normal production of immune cells continued. This indicates that it isn't just the lack of nutrition that shuts off the immune response.

Future Applications This is big news. I believe it will revolutionize the way we treat sick calves in the next millennium. It seems obvious that synthetic or bio-engineered leptin will become available for injection.

In addition, however, there are other means to achieve the same goal: specifically, compounds that can be given to stimulate appetite. (By artificially stimulating intake, the body would be induced to secrete leptin naturally.)

Twenty years ago a compound known as Elfazepam was shown to be capable of causing cattle to eat beyond what would be normal satiety. Similar to diazepam (Valium), the widely prescribed tranquilizer for humans with anxiety problems, the parent company for some reason never sought a clearance.

These are narcotic compounds, and whether or not they will ever be released for animal use is anyone's guess. However, in Third World countries they're available for veterinary use and are recommended primarily for use in sick animals. While I have overseas clients who have used them, we have not approached their use with any type of scientific protocol.

With this new information on the value of leptin and the immune response, I'll ask clients in countries where these appetite stimulators are legal to try and document their value in the sick pen. That way, if they ever become legal in the U.S. (or Canada), we'll have a background of experience. Stay tuned.