( 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 1Â½-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 do n'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 ca n'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 do n'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 was n'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 did n'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 wo n'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 is n't nervous anymore . `` I 'm excited for it , '' she says . `` I 've taken 40 into my hands . '' Share your weight loss story on iReport .
Michelle Jackson got nervous about her weight as her 40th birthday approached . Women 's metabolisms slow in the premenopausal years because of hormone changes . Weight training and eating a balanced diet is important to stay healthy after 40 .