(CNN) -- Thirty was not a good year for Michelle Jackson. A few weeks before her birthday, she lost her job. Then her husband left her -- with a 1½-year-old at home.
So it's understandable that as her 40th birthday approached, the Milwaukee resident was nervous.
Sure, she had a new job that she loved and her husband had returned. But the medical data analyst was more than 150 pounds overweight, despite working every day with experts who stressed the importance of staying healthy.
Around her 38th birthday, Jackson came across an article about weight loss. It discussed aging and how middle-aged women have to work out three times a week just to maintain their weight.
After 40, Jackson remembers reading in horror, metabolism slows and the pounds don't come off as easily.
Unfortunately, says CNN's diet and fitness expert Dr. Melina Jampolis, that's not a myth.
Jampolis says she sees evidence of it all the time in her practice -- women over 40 who are attending spin class every day and restricting calories, frustrated because they can't drop five pounds.
There's nothing magic about the age of 40, Jampolis says. But in the premenopausal years, a woman's sex hormones start to change. The body produces less healthy estrogen, and more estrone -- a type of estrogen that comes from your body's fat tissue.
More estrone leads to insulin resistance and a loss of muscle mass. And with their hormones going crazy, women start to crave carbohydrates and sweets.
"It's like PMS on steroids," Jampolis says. "The best thing to do is a preventative attack in your 30s. It's going to be much, much harder to lose the weight in your 40s."
Jackson decided to take control of her upcoming milestone.
"At the time I was 38. (I thought), 'I'm going to turn 40 regardless. Why don't I accomplish something in two years instead of being in the same place?'"
She launched her mission: "Fit By 40." But starting a new lifestyle was easier said than done.
"I'm regarded as a pretty great home cook and take a lot of pride in that," Jackson wrote in her iReport submission. "I've also never been any good (with) sports -- physical sport typically conjured feelings of embarrassment. It was daunting to consider eating differently and exercising regularly because I was contemplating something larger: losing the compliments on my cooking and facing embarrassment at the gym."
Always data-obsessed, Jackson started using SparkPeople.com to plot her weight along a goal line. She cut her calories to 1,600 a day and started logging 30 to 45 minutes at the gym three times a week.
Jampolis says weight training needs to be a priority for women before and after 40. More muscle leads to a higher calorie-burn -- essential when your metabolism decides to limp along.
Also important is eating a healthy diet, Jampolis says. Vegetables, fruits and grains should be spaced out between several servings of lean protein.
"Women are super busy in their 30s; they're usually raising kids or kicking butts in their career," she says. "They really need to put (the focus) back on themselves to make a smooth transition to their 40s."
On the new plan, Jackson's weight dropped at a steady rate. Since March 2011, she's lost more than 100 pounds. Along the way, she realized she had always known how to get healthy. The problem wasn't a lack of information. The problem was motivation. "If you had asked me when I was at my heaviest how being heavy had impacted me, I would have said, 'not at all,'" Jackson says. "I was comfortable in my own skin."
She didn't consider herself unhealthy at 330 pounds. Yet after losing weight, she stopped snoring, went off her asthma medication, felt less pain in her knee while walking and threw out the acid reflux medication she used to take before bed.
"Michelle is a driven, determined and strong-willed force," her friend and co-worker Krissy Fischer says. "When she committed to weight loss, there was no doubt she would succeed. Michelle was never shy, but she has become even more confident, outgoing and self-assured as she has lost weight."
Jackson won't reach her goal weight of 164 pounds -- a healthy body mass index for her 5-foot-8 frame -- by her birthday in February. She could add more intense workouts or drop her calorie intake, but she wants to maintain a lifestyle she can keep up for the rest of her life.
As her big birthday approaches, she isn't nervous anymore. "I'm excited for it," she says. "I've taken 40 into my hands."