"Recently they came out with the new lifespan estimates for people and at most we've had a 10-percent bump, but 20 years ago, dogs died at around age 10," Steve Thompson, director of the Pet Wellness Clinic at the Purdue University School of Veterinary Medicine, in West Lafayette, Ind., told UPI's Animal Tales.

"But, they now live to age 14 to 15. That's a 50-percent increase in a relatively short period of time."

For most of the thousands of years that dogs have been companions to man, they've been given table scraps, but according to the Pet Food Institute, the first commercially prepared pet food was a dog biscuit introduced in England about 1860.

"The first dog food was developed for the Admiral Byrd expedition to the South Pole in 1933 because they needed to take something with them to feed the sled dogs," veterinarian David Carey, who works on research and development for IAMS, in Dayton, Ohio, told UPI. "They didn't understand about all nutrients but they came pretty close."

Since then, pet foods have expanded to include canned, dry and semi-moist foods, although some pet owners still give their dogs and cats table scraps.

"I don't recommend giving pets tables scraps. They're usually fatty, salty, spicy bits that we do not want ourselves," Nancy Irlbeck, Colorado State University College of Veterinary Medicine in Fort Collins, Colo., told UPI. "If they are not good for the human body, why do you think they are good for a dog or cat?

"It is much better to give them fresh veggies, or bits of human-quality meat scraps for treats but always in addition to a premium balanced diet."

However, while the nutrients needed for a pet's diet are not in question, how to obtain them often is, just as in human diets.

"We know pets have 42 nutritional requirements," Carey said. "The Association of American Feed Control Officials has two expert committees, one for dogs and one for cats, and they determine the nutrient profiles for dogs and cats."

For example, the nutrient L-arginine is an essential amino acid but it can be obtained in grains or meats.

Corn is 75 percent starch, a carbohydrate, and 6 percent to 7 percent protein -- the small white piece in a kernel of corn.

"Some pet foods use corn as its protein source, IAMS has always used an animal protein source, but unless the dog is very active the dog's body does not use it as an energy source right away it's stored as fat," Carey said. "If the dog is not very active, eating protein from corn can lead to the dog's body having less muscle and more fat."

Rice is absorbed quickly in the blood as glucose and it is high on the glycemic index, requiring insulin to be secreted to balance out the rise in blood sugar, but barley is absorbed more slowly in dogs and sorghum in cats, according to Carey.

"Just like in people, pets don't need a high sugar, high fat diet," Carey said. "Dogs are omnivores and while they can handle produce, animal protein works best with them, however, cats are carnivores and they must have animal based protein."

Omnivores cannot digest some of the substances in grains or other plants that do not produce fruit, but they can eat fruits and vegetables.

Irlbeck, who wrote the textbook, "Nutrition and Care of Companion Animals," recommended that pet owners look for diets with poultry meal or sometime of meat meal as the first ingredient -- not beef or chicken because they contain up to 70 percent water.

Cats are obligate carnivores and must have meat or they will catabolize their own tissues. Scientists have now linked grain/plant protein diets in cats to increased incidences of diabetes and obesity -- which can often be remedied by feeding high meat diets, according to Irlbeck.

"In general, I would not buy pet food in a grocery store or a discount store -- cheap means cheap ingredients and in most scenarios, more expensive means human quality and thus better," Irlbeck said. "Cheaper diets are full of low-quality ingredients and fillers."

A side benefit to higher-end pet food diets is that it results in less poop, according to Thompson.

"A key factor is that dogs don't need to eat quite as much high quality food," Thompson said. "If the pet food label lists a lot of fillers there's a significant increase in poop and the number of times a dogs needs to poop -- on a high end diet they can often go once a day."

Barring a trauma, bigger dogs do not live as long as smaller dogs, but as animals age, they, just like people, do better with some supplements.

"As an animal ages, it loses immunity and we find that it's better to add supplements to help modify the loss such as Vitamin E, and Lutein (for eye health)," Carey said. "IAMS research in 1994 found the optional ratio for the essential fatty acids of omega 6 and omega 3 was five to one."

Because omega 6, known as Linoleic acid, found in plant protein such as corn, and omega 3 or Linolenic acid, found in fish, fish meal or flax are metabolized using the same pathways, so the ratio is important.

Fatty acids help in establishing a lipid barrier in the skin to block irritants and infections. The liver and in animals can make many of the fatty acids required, but dogs and humans cannot make omega 6 and omega 3, they need to have a dietary source. Consuming too much or in an incorrect ratio could lead to problems and relative deficiencies of Vitamin E and other vitamins.

The North American diet has gone away from fish and whole grains but it has lots of foods with omega 6.

"It also reduces inflammation potential which can help the heart in both dogs and people -- IAMS has been putting omega 3 and omega 6 in dog food from 1991," Carey said. "In the wild, wolves get their omega 3 from prey animals that grazed on grass, but livestock fed corn and soy in feedlots have very little omega 3."

For people, fish contains an excellent source of omega 3 but it often is fried in vegetable oil and therefore is covered in omega 6 putting the ratio out of whack, according to Carey.

"One way to check if a dog is getting enough omega 3 and omega 6 is to visibly check that his fur is healthy looking and his skin is not dry and flaky," Thompson said. "It's one of the reasons it's a good idea to have the dog see a vet every year."