A large study published on May 30, 2018 found that even if a woman has been metabolically-healthy for decades, if she is obese, she still has a higher risk of developing cardiovascular disease compared to women of normal weight. 
The research proves that there is no such thing as “fat but fit.”
The “fat but fit” theory suggests that being obese doesn’t put a person at risk for health problems if they don’t have high blood pressure, high cholesterol, high blood sugar, or Type 2 diabetes.
Researchers analyzed data on 90,257 women in the United States from 1980 to 2000, culled from the Nurses’ Health Study, which has tracked adults aged 30-55 since 1976. They found that obesity is a risk factor even if a woman lacks common metabolic diseases, such as high blood pressure or Type 2 diabetes. What’s worse is that even if a woman is metabolically-healthy, the odds are that she will become metabolically-unhealthy in the future.  
In a statement, lead study author Dr. Matthias Schulze, of the German Institute of Human Nutrition Potsdam-Rehbruecke in Nuthetal, said:
“Our large cohort study confirms that metabolically healthy obesity is not a harmless condition, and even women who remain free of metabolic diseases for decades face an increased risk of cardiovascular events. What’s more, we observed that most healthy women are likely to develop Type 2 diabetes, high blood pressure, or high cholesterol over time, irrespective of their BMI, putting them at much higher risk for cardiovascular disease.”
Obesity is defined as a body mass index (BMI) over 30. As many as 1/3 of obese people might be metabolically-healthy.
A person is considered metabolically-healthy if they don’t have 3 metabolic risk factors: Type 2 diabetes, high blood pressure, and high cholesterol.
“Fat but Fit” is Still a Myth
Every 2 years, study participants filled out a questionnaire updating their BMI and metabolic health status, lifestyle habits, health behavior, and medical history.
Researchers adjusted for age, diet, smoking status, physical activity, alcohol consumption, ethnicity or race, highest education level, menopausal status, aspirin use, and family history of heart attack or diabetes.
There were 6,306 new cases of cardiovascular disease, including 3,304 heart attacks and 3,080 strokes, at follow-up, which was 24 years, on average.
- Women of normal weight who were metabolically-unhealthy were found to be 2.5 times more likely to develop cardiovascular disease compared with women who were or normal weight and metabolically-healthy.
- Women who were metabolically-healthy and obese had a 39% greater risk of cardiovascular disease.
- Over 20 years, 84% of women who were obese but metabolically-healthy became metabolically-unhealthy. Among women who were of normal weight and metabolically-healthy, 68% became metabolically-unhealthy.
- Even obese women who managed to stay metabolically-healthy faced a 57% higher risk of developing cardiovascular disease compared to normal weight, metabolically-healthy women. 
Carl Lavie, of the University of Queensland School of Medicine, wrote in an accompanying commentary:
“Drastic efforts are needed to prevent obesity in the first place and, especially, to prevent conversion to more severe degrees of obesity and the metabolic syndrome. Public health policies aiming to increase cardiorespiratory fitness through physical activity and exercise will further contribute to improve people’s health. It is prudent to remind ourselves that an ounce of prevention is better than a pound of cure.” 
“Our findings highlight the importance of preventing the development of metabolic diseases, and suggest that even individuals in good metabolic health may benefit from early behavioural management to improve their diet and increase physical activity in order to guard against progression to poor metabolic health.”
The findings are published in The Lancet Diabetes & Endocrinology journal.