Hi there Shai13 and thanks for posting this question!
Topic: Is total Body Weight a good measure of health?
Yes and no. Although you are really close to the lowest recommended weight range you can actually gain few pounds and be healthier in body weight distribution.
Normally when there are recommended weight ranges you should stay in the middle as average. However the good news is that if you are expressing concern is because you want to gain some muscle mass that will help you achieve a more optimal weight.
You are 125lbs or 57kg
According to your height of 5' 8" your ideal healthy weight is 147 pounds. Your recommended weight range is between 131 and 164 pounds.
This was calculated using Hamwi formula, a simple formula commonly used by many clinicians.
These are recommended weights. Talk with your doctor about what weight is best for you and before undertaking any new diet or exercise programs.
You can learn more about BMI and see what it is for your current weight by checking out our Body Mass calculator.
Steps to Gain weight:
Metabolism: Eat more Frequently
If you have a small appetite, try to eat more frequently and add “Good calories” For example, add mixed almonds to your breakfast or as a snack . Some protein shakes can help you as well to increase your weight in a safe way. This increases your fat intake and calories without increasing the volume of the food you consume.
You need to select The Right Foods
Do not consume foods that are high in saturated fat, calories and sugar increasing the risk on your arteries and organs. Include more fresh fruits, vegetables and lean meats in every meal.
Add More Liquids
Incorporate more calorie-laden, healthy liquids like milk, orange or juice into your diet. Avoid sugary drinks like sodas and sweetened teas. The website Ask The Dietician recommends you not drink liquids with meals because it can fill you up and lower your caloric intake. Try adding liquids mid-morning and mid-afternoon to increase the number of calories you take in.
Focus on strength training as part of a workout regimen. You need to make sure you don’t expend too many calories in each workout session. Therefore Light cardio is OK but strength training is better. Work all of your major muscle groups increasing ressitance and allowing long periods of rest like 10 repetitions and 2 minutes rest for those heavy loads. Otherwise you should body weight exercises that can improve your resistance training although with this type of training you need to be careful since your heart needs more calorie burning so i will suggest to add whole body movements and add lengthy resting periods of time. Muscle weighs more than fat and strength training promotes the development of lean muscle.
Ask The Dietician: Underweight
Mayo Clinic: Underweight? See How to Add Pounds Healthfully