If I were to play the word-association game, and my prompt was "fat," the first thing I would think of is "frustrating." I was a toddler with a cute, round tummy and it's never quite gone away. I exercise vigorously almost every day and eat a very healthy diet. But losing fat is much more complicated than that.

Most of us who have put time, energy and money into fighting our fat would like to be able to zap it away, but there's a good reason it's difficult to lose. Fat isn't just padding. Fat is actually an important organ — yes, an organ — and it impacts our health as much as our kidneys, liver, skin, intestines or any other organ.

The Secret Life of Fat book cover"Thousands of research studies from around the world are now revealing that fat is not just fat — it is a dynamic and interactive endocrine organ that has life-or-death influence over us," writes Sylvia Tara in her new book, "The Secret Life of Fat."

Tara — who has a Ph.D. in biochemistry — was frustrated by her own battles with the bulge, which she has successfully tackled with a diet and exercise program that would intimidate most bootcamp sergeants. In writing the book, she dove deep into the history of how we understand fat, culturally and scientifically. She spent five years, spoke to more than 50 doctors and looked at hundreds of research papers to learn what we really know about fat. She learned that fat is incredibly important to our survival.

In a case of "nature knows best," it's probably wise that we can't totally control where on our bodies fat is. What we might prefer might not be best for our overall health or longevity. First off, our body fat is really important for managing the varying amounts of time that might pass between meals — our bodies (and for humans especially, our brains) need a constant flow of glucose, vitamins and minerals. "Body fat allows us to absorb energy from food now and retrieve it later, so that we can think of other things than eating," writes Tara.

So, what else does fat do for us?

"Fat produces hormones that our body depends on. It affects our bone health, our immune system and wound healing," Tara told MNN and explains in detail in the video above. Fat is related to how well newborn babies thrive, when girls and women menstruate (the connection wasn't known until Dr. Rose Frisch discovered it in 1974) and it affects young men's maturity as well. Having enough fat is not only important for physical growth in both sexes, it affects psychology: "Fat, through leptin, turns on the switch that allows our transition to adulthood, both physically and psychologically," writes Tara. "If only our body-conscious teenagers — so eager to jump into adulthood — knew how important their fat was!"

Like many things when it comes to personal health, fat is about balance. While too little fat causes a host of problems, from hormonal to developmental, too much fat can also knock those hormones out of whack. So how much is too much? While general guidelines are useful, it really seems to come down to the individual. Whether someone looks "fat" is not a solid indicator of health.

That's because "not all fat is equal," says Tara. "There are different kinds of fat. Visceral fat is the most dangerous kind — that's the kind that's under the stomach wall, and it leads to cardiac issues and diabetes. Subcutaneous fat is the kind in our arms, legs and butt. If you’re going to store fat, that’s the place to put it, " says Tara, because that fat, while not what you might like to look at, isn't going to lead to the same kind of health problems as visceral fat. "Brown fat (found on the back) burns calories to produce heat," says Tara. "There's even beige fat that can turn brown when we exercise and are exposed to cold."

The middle of Tara's book digs deep into the genetic aspects of how much fat we carry. Some people have a gene variation where they have less body fat but more circulation of fat in blood. "Another [genetic] variation clears the blood of fat, and these people are thinner but depositing it in places where it shouldn’t be. They are thinner but not healthier," says Tara. So if you eat fat and it ends up on your hips, that might be healthier overall than if it circulates in your blood, where it will more likely stick to blood vessels or end up as dangerous visceral fat.

"Genetics plays a huge role. Viruses and bacteria have a role. Age and gender as well," says Tara, who devotes a chapter to the differences between how men and women consume, store, and burn fat — finally explaining why a man and a woman can eat the same and have the same exercise routine, but the woman will end up with a much higher body-fat percentage.

Fighting fat ... and when fat fights back

woman lifting weights Exercise is a crucial element to health — and much more important than what the scale says. (Photo: MaxShutter/Shutterstock)

It's almost impossible to cheat fat. It "fights to stay on you," Tara writes. Even removing fat via liposuction doesn't really work. In a study cited in the book, 36 women had lower-belly liposuction. Then the women were split into two groups: One group was given a workout plan that included a five-minute warmup, 30 minutes of strength exercises and 30 minutes of cardio, three times a week. The other group did nothing. At the end of four months, both groups had maintained the loss of the subcutaneous fat. But in the non-exercising group, the fat was actually gained back — under the stomach wall in the visceral area. The non-exercisers also had measurably slower metabolisms — the body was fighting every which way to hold onto fat in other ways and places, even though it had been surgically removed.

Throughout the book, exercise comes up often. It can be a way to keep fat off, to some extent, but as Tara points out, there are many other biochemical reactions that affect your metabolism — all driven by your genes. Whatever your level of fatness, exercise leads to healthier outcomes, especially as we get older. "As we age, we lose growth hormones and fat-busting hormones. When they decrease we get more fat and it distributes differently. Women get more fat in their hips and thighs. And men get the belly. The way to counter that is to exercise, which boosts those hormones," Tara says.

We've all heard the "secret tricks" to losing fat, from hucksters as well as real scientists. The truth is as complicated as fat itself. Our bodies are all different. Body fat can and does vary from person-to-person — even when people live in exactly the same way. Ancestry, lifestyle, metabolism, gender, type of exercise, eating habits and other unknown factors all combine to determine how much fat we carry. Some of it we can control; some we can't.

The science agrees on one thing: Whatever your age, exercise is always a smart move. "You can be fat but fit," says Tara. So making judgments about others — or yourself — based on appearance isn't really accurate. And it's definitely unhealthy.

Starre Vartan ( @ecochickie ) covers conscious consumption, health and science as she travels the world exploring new cultures and ideas.