Ariana Green was an ambitious forty-five-year-old realtor in San Francisco. She was new to the business, which meant working seven days a week to build her reputation and client list. She was up for the challenge, though, and over time, her business grew. Ariana was also a beautiful woman who stood five feet ten and had blond hair, blue eyes, and high cheekbones. Her mother was a fashion model and Ariana inherited her good looks, which certainly had its advantages.

As Ariana marched through her late forties, she started gaining weight. At first she attributed this to the stresses and erratic eating habits that came with the new career. Then came a knee injury that slowed her down for a while. In the past, she’d always been able to shed weight easily. However, this time was different. Now fat appeared out of nowhere and seemed determined to stay. Her clothes became snug so she would go to a bigger size, and then it would happen again six months later. She was putting on weight like never before, and feeling depressed and confused about it. Finally, at fifty, something happened that shocked her: Ariana saw a snapshot of herself standing in the water in Cancun.

“I was absolutely obese,” she says. “I hadn’t realized it had gotten this far.”

On a different continent but also in the same physical predicament was Mike Hanson, a forty-eight-year-old software engineer living in Sydney. His job was demanding and entailed traveling from Australia to northern California and China. Mike was no stranger to hard work. He had competed with Silicon Valley tech jocks for years and could hold his own. But now, in middle age, things seemed different. He was getting a spare tire around his waist and feeling sluggish. The punishingwork schedule was bad enough, but when Mike turned fifty, his wife left him. His stress level shot through the roof. Suddenly, he was in a depression, and the spare tire was joined by fat elsewhere on his body.

What were Mike and Ariana doing wrong? They made the mistake of aging.

With age comes hormone decline and stress, and our bodies change in many ways. Among the most frustrating is that we accumulate fat more easily, and lose it only with difficulty. Worst of all, fat begins to appear in new and strange locations that we never had to worry about before.

Fat takes on different responsibilities at different ages. The younger we are, the better our fat behaves. When we are infants, a good proportion of our fat is the brown type, which burns calories and produces heat. At this stage, fat’s primary functions are to keep us warm and safe as we leave the womb and enter an uncertain world. Babies have more brown fat, percentage-wise, than any other age group. Baby fat also serves to cushion us from falls and injuries. As we grow out of infancy, the proportion of brown fat decreases and white fat increases.

In our teenage years, fat changes function again and plays a key role in sexual maturation. It helps trigger puberty by telling the brain that we are sufficiently well fed to bring offspring into the world. Without the proper level of body fat, sexual development is delayed. One way fat controls maturity is by secreting leptin, which aids in producing menstruation in girls. Another way fat regulates puberty is by producing estrogen, also critical for development. As their bodies get ready to bear children, girls will start to pack on more fat compared to boys.

Once the childbearing years arrive, it is baby fat, part two. Fat and the estrogen it produces are needed for women to get pregnant. Females must have the right amount of fat—not too much or too little. They will continue to gain fat in pregnancy, some of which will be used to produce milk for lactation. For nursing mothers, fat is used to foster the next generation.

Fat-wise, at this stage all seems right in the world. Then we hit middle age. And everything changes. As we approach our forties, production of the three sex hormones—estrogen, testosterone, and progesterone—which have heretofore been plentiful in our bodies, begins to wane. And, not coincidentally, our body fat suddenly becomes troublesome.

It begins to shift from places where it once seemed so appealing to locations where it is anything but.

In men, it now accumulates in the belly, the lower back, the nape of the neck. Women’s fat settles in the belly, too, and on the thighs, buttocks, and breasts.

As we age, our fat mass peaks. Between fifty and sixty, we are typically at our heaviest and have the most difficult time keeping fat in check. Many people who were thin since childhood suddenly struggle with weight. “What is going on?” they ask.

We know that fat can talk. It sends out messengers in the form of chemical signals like leptin to our brains, bones, and reproductive system.

But just as fat can talk, it can also listen. This extraordinary property of fat was actually noticed decades before researchers knew it could talk.

In 1969, Dr. Pedro Cuatrecasas at the National Institutes of Health ran experiments in which he combined fat cells and insulin and noticed that the insulin made fat cells act differently. When insulin was present, fat cells would increase their conversion of glucose to fat.

Cuatrecasas refined his experiments to understand how insulin was having such an effect. After some searching, he determined that fat cells had receptors on their surface that were uniquely designed to bind to insulin. And once insulin was bound to a receptor, the behavior of fat cells would change to produce more fat. Receptors are like “ears” on the cell’s surface that pick up incoming messages from the body. They are part of a two-way communication path whereby fat talks to the body (by emitting hormones such as leptin and adiponectin), and the body talks back to fat (by sending hormones to fat). In the case of the insulin receptors on fat cells, these “ears” would “hear” insulin (coming from the pancreas) on the cell’s surface and signal to fat cells to absorb more glucose and produce more fat.

Soon, other receptors were located as well. Dr Thomas Burns at the University of Missouri School of Medicine and his team found that fat cells also had receptors that could bind adrenaline, which communicates to adipocytes to release fat into the system for energy. If you see a bear, adrenaline tells your fat: “Don’t hoard energy for later. Use it now! Run!” Fat hears that signal and starts to release free fatty acids into the system for energy.

In the decades to come, it was discovered that fat has receptors for our most potent hormones – thyroid hormone, growth hormone, estrogen, testosterone, and progesterone. All these hormones tell fat when it is time to liquidate and release energy into the system.

When we are young, we have an abundance of these hormones. They work to grow our tissues, activate our reproductive systems, and keep our energy and metabolism high, which helps young people lose weight faster and keep it off more easily. But when we approach middle age we no longer need to activate our reproductive systems. Biologically speaking, we’ve outlived our usefulness. At this point, the production of most of those hormones decreases, which means the messages to our fat to dissolve itself are less powerful. With our bodies burning less adipose tissue through hormonal messaging, we inevitably get fatter.

At the same time, another hormone, cortisol, increases with stress and age. Cortisol is released from the adrenal glands in response to ongoing stress and is correlated with higher abdominal fat. All these hormonal changes together make it easier for fat to grow. We see it happen before our eyes; even though we may not be eating more than when we were young, fat now sticks to us more easily.

Women especially experience this weight gain as they approach menopause. During this period their hormone levels plunge as their ovaries head for retirement and produce less estrogen, progesterone, and testosterone. Lower estrogen levels cause increased appetite, reduced fat burning, and a redistribution of fat to the belly area, not to mention hot flashes and decreased energy. Furthermore, as the ovaries produce less estrogen, the body begins to rely more on fat’s ability to manufacture the hormone. Fat becomes a dominant source of estrogen in postmenopausal women.

It is hypothesized that this dependence is one reason women have a more challenging time reducing their fat compared to men.

Progesterone also declines significantly, altering the progesterone-to-estrogen ratio and causing a condition known as “estrogen dominance.” This can lead to irritability, depression, sleep problems, water retention, a bigger appetite, and sugar cravings. It’s like premenstrual syndrome, except that it lasts for years.

Testosterone, which is critical for both sexes, also decreases, causing a reduction in lean body mass and energy, ultimately leading to slower metabolism. Though we tend to think of testosterone as the male hormone, there is more of it in a woman’s body than estrogen at most times of the month, and certainly during the perimenopausal or postmenopausal years.

Excerpted with permission from The Secret Life of Fat: The Groundbreaking Science On Why Weight Loss is So Difficult, Sylvia Tara, Aleph Book Company.