Girls are underweight due to a mix of genetics, high metabolism, physical activity, underlying health issues (like thyroid problems, diabetes, digestive conditions), mental health struggles (anxiety, depression, eating disorders), poor nutrition/food insecurity, or societal pressures for extreme thinness, all leading to low calorie intake or poor nutrient absorption, impacting bone health, fertility, and energy levels.
There are a few genetic factors that make some people naturally skinny. We often have thinner bones, fewer fat cells, faster metabolisms, smaller stomachs, and a different hormonal response to food. Sometimes, our organs work a little harder than usual, burning extra calories.
Women generally have a higher percentage of body fat than men.
Causes
Testosterone, the main male hormone causes men to have more dense bones and muscle mass, therefore gaining more weight (but not fat). Women, due to natural lack of testosterone in their bodies, dont gain such a muscle mass and such dense bones as men have.
Abstract. There is a global obesity pandemic. However, the prevalence of overweight and obesity among men and women varies greatly within and between countries, and overall, more women are obese than men.
Stress of managing a new household, new family makes you stress-eat. If you are not a working woman, sedentary lifestyle can make you gain weight. If you have doting in-laws, they might just feed you and feed you and feed you non-stop out of love.
Too Skinny: What BMI is Considered Underweight? From a clinical perspective, an individual is considered “too skinny” if they are deemed underweight. According to the Centers for Disease Control, an individual is underweight if their Body Mass Index (BMI) is below 18.5 [1].
In general, your plan may include:
The Complications of Being Too Thin
Registered dietitian Katherine Basbaum shares, “It's a jumping-off point to determine a healthy weight, but definitely not one-size-fits-all of healthy body factors." To get a rough idea, if your BMI is less than 18.5, you're considered underweight.
Abstract. There are large variations of size among humans but in all populations, men are larger on average than women. For most biologists this fact can be easily explained by the same processes that explain the size dimorphism in large mammals in general and in apes in particular.
Body composition differs between men and women. Men have more lean mass, and women have more fat mass than men.
2. A consensus statement from the American College of Sports Medicine emphasises that adult males are typically faster, stronger and more powerful than females of similar age/training status – due to factors such as greater muscle mass, larger heart and lung volumes, higher testosterone.
Body Shapes with the Highest Risk of Health Problems
Apple and pear body shapes tend to have the highest risk. Apples tend to be at greater risk for heart disease, diabetes, and strokes because they hold a larger amount of tummy fat.
Understanding the 3-3-3 Rule
Specifically, the rule suggests: Three balanced meals per day. Three hours between each meal. Three hours of movement per week.
You may be born naturally small and your low BMI can be due to your genes. You may have a very high metabolism and find it hard to put on weight, even if you eat foods that have a lot of calories. You may not follow a healthy, balanced diet because you forget to eat or cannot afford nutritious foods.
The bottom line for skinny girls to gain weight fast is to focus on a balanced diet with more calories, proteins, and healthy fats, combined with a workout plan that emphasizes strength training. Also, gaining weight should be a gradual process.
18 healthy foods to gain weight fast and effectively
If you weigh 107 pounds or less at this height, you are considered underweight with a BMI of 18.4. A healthy weight range for that woman would be 108 to 145 pounds. BMI is just one way to measure healthy weight. Some women have a low, but still healthy weight.
BMI <17.0: moderate and severe thinness. BMI <18.5: underweight. BMI 18.5–24.9: normal weight. BMI ≥25.0: overweight.
Do
Females tend to gain the most weight during two key periods: emerging adulthood (late teens to mid-20s) when life changes often disrupt habits, and midlife (around ages 45-55) during menopause due to hormonal shifts that decrease muscle and increase abdominal fat, although the rate of gain slows in later decades. While the 20s see significant overall gain, menopause brings distinct body composition changes and fat redistribution, not just scale weight.
“The research also suggests that men may be more likely to gain weight after marriage due to factors like increased portion sizes, social eating, and a decline in physical activity, whereas women perhaps remain more conscious of body weight due to societal pressures.
You burn fewer calories. This makes you more likely to gain weight. You may lose muscle strength and endurance, because you are not using your muscles as much. Your bones may get weaker and lose some mineral content.