Forget complicated, celebrity-endorsed exercise plans, specialized diets and quick-start weight-loss programs. Bob Lehnhard says it all boils down to balancing the energy we take in against the energy we put out.

An associate professor of exercise physiology and biomechanics at the University of Maine, Lehnhard, 52, also is the author of a slender volume of no-nonsense advice and information about getting fit and staying healthy as we age.

At just 78 pages, “The Backside of 50” is a quick read that dispenses with popular myths about the diet and exercise industry, a commercial phenomenon Lehnhard says does more harm than good when it comes to counteracting the effects of the sedentary American lifestyle.

Counting carbs and loading up on protein? Forget about it.

“It’s only successful if the calories consumed are less than what you expend,” Lehnhard said in a recent interview.

Challenging yourself to run your 3-mile route instead of walking it in hopes of building up a healthy heart? You’re dreaming.

“You’re just going to get lame,” Lehnhard said. “At our age, we don’t have to run. We can walk. And the cardiovascular system will adapt.”

The simple principles of fitness and weight loss have been “convoluted,” Lehnhard says in his book, “made to appear complicated by those who stand to gain by keeping the rest of us slightly confused. Their hope is that our unsureness will make us dependent on their programs.”

Instead, in an astringent style that suits his straight-talk message, Lehnhard tells it like it is.

In his chapter titled “Work,” Lehnhard delivers the unwelcome, though hardly surprising, news that the typical working stiff in America does precious little actual work. Between the eight hours we spend sitting at our desks, or at the controls of our machinery, and the time we spend commuting, eating, watching television, using our home computers and sleeping, more than 22 hours of every day are spent sitting or lying down, he estimates.

Weekends aren’t much better with many Americans spending their leisure time riding the sofa – or else, in the name of outdoor recreation, a snowmobile, a four-wheeler or a Jet Ski.

“One of the biggest threats to our health … is leisure time,” said Lehnhard.

Compounding the problem, he writes, is the typical American diet. Even for reasonably health-conscious people, he notes, the temptations are great to eat much more than we need to maintain our slothful existences. But enjoy even as little as 50 extra calories a day – a half-slice of bread, half an apple, or a single, lonely Oreo cookie – and in a year you’ll be tucking an extra 5 pounds into your jeans.

Don’t get him started on chocolate, pizza, french fries or alcohol. (In a puzzling digression, he also takes a couple of pot-shots at the French.)

Drawing on his biochemistry training and employing his from-the-hip prose style, Lehnhard does a nice job of distilling important information about how the body metabolizes sugars and fats into the energy it needs to keep itself functioning and perform whatever additional tasks we ask of it. He assumes his reader has an intelligent interest in the contribution of the liver, for example, an elegant organ not generally given star billing in discussions of diet and fitness.

The liver, he notes, responds to a long-ago biological imperative and efficiently converts unneeded sugars into fat, storing that potential energy for times when food isn’t available.

“It’s a good system and would keep us alive and well if we still lived in caves and were running around trying to club our next meal to death,” Lehnhard writes. Now, though, the fat just stays put, with new fat packing in around it with every overgenerous meal and snack. Moving it out takes persistence and a long-term change in attitudes about food and exercise, he says.

While dietary principles remain more or less constant across the age span, fitness needs change as we age, Lehnhard writes. While younger people may seek bulging biceps, buns of steel and the endurance to party all night long, those in the second half of life should focus on building cardiovascular strength through walking or jogging and strong muscles and bones through moderate weightlifting.

The Ph.D. professor also makes a compelling argument for staying in shape – physically, mentally and spiritually – in order to fulfill not only our own ambitions but also our responsibility to shape the next generation. He gives credit to his elders for their dedication to leaving the world a better place and suggests that the baby boom generation has the same obligation.

“There’s no telling the people and situations you will influence in the future, if you’re able,” he writes.

Lehnhard said he was impelled to write his book to counteract “all the nonsense that’s out there.” Most people already know everything they need to know about weight loss, fitness and health, he said. Some may feel the need to buy thick, glossy books and exercise videos while others pay for special classes and personal trainers.

But in the end, it’s all about commitment, consistency and perseverance.

“People need to help themselves. It’s your own personal well-being. You can’t look to others to get it done for you,” he said.

“The Backside of 50” is available online for $14.95 from www.amazon.com. Lehnhard said he’s working on getting it some shelf space at Borders Books in Bangor, and it soon will be available at the University of Maine bookstore. Copies also are available by e-mailing Lehnhard directly at robert.lehnhard@umit.maine.edu.