Exercise is great, but should someone who is underweight (approximately 168cm and 49kg for a BMI of 17.4) focus on gaining weight (until a healthy weight is reached) before beginning strenuous exercise?

My friend has no history of anorexia or bulimia (that I am aware of) or any health problems. There are some famous female endurance athletes (runners, etc) who are similarly underweight, however she is an exercise novice without their level of cardiovascular fitness.

I am concerned (knowing her personality) that she will push herself too hard and and risk injury.

What are the health risks associated with too much exercise while underweight?

  • I am not underweight. This question is being asked on behalf of a friend
    – Javier
    May 24, 2016 at 12:44
  • How underweight are we talking here?
    – Dark Hippo
    May 24, 2016 at 13:21
  • 2
    BMI is not a good measurement, put your friends height and weight instead. Plus the weight they want to get up to.
    – Recycle
    May 24, 2016 at 14:03
  • 1
    BMI is an actuarial classification originally. It is not a great chart to use.
    – JohnP
    May 24, 2016 at 14:23
  • @Recycle BMI is calculated based exclusively on height and weight, so those two parameters still wouldn't be very useful. A low BMI could be a result of very low body fat or very little lean body mass.
    – G_H
    May 24, 2016 at 15:20

2 Answers 2


The first thing to find out is if there's underlying medical conditions for the lack of body mass. It might be unlikely if there's no constant problematic weight loss, just a general long-term low body mass, but only a medical practitioner is in a position to determine this. Any conditions could affect the safety of exercise.

In the absence of such conditions, I'd say it's quite the opposite. Resistance training is an outstanding way of gaining body mass; muscle hypertrophy, increased bone density and, if wanted and a large caloric surplus can be tolerated, an increase in body fat. It must be understood that gains in lean body mass will be slow and require a lot of time, as well as a lot of food.

What possible risks I can think of are attempting a training load that exceeds the body's capabilities for supporting (either because of skeletal muscle or bone density and structure), or a volume or progression that exceeds the capacity for recovery. But that's what any decent beginner routine is for. Start every exercise at a weight with which it can be performed with good form and used to learn that form in the first place. Then focus on linear progression in realistic increments while respecting the rest days and getting the sleep and nutrition required for driving such progress. For an underweight female trainee the increments might need to be small, but linear progression can still be done and should yield good results for someone who is untrained.

What is probably most important to avoid injury, whether resistance or endurance training is used, is to make it clear a workout shouldn't be considered "good" on the basis of being exhausted at the end or working to failure. Start with manageable weights, volume or (for cardio) time/distance that feels easy enough. Increment a bit per workout, no too dramatic jumps. The point at which workouts start getting challenging and spur adaptation will manifest itself soon enough. I'd say for increasing body mass, strength training would be the best choice, combined with eating like you mean it.

  • Thank you for the informative response. She had a physical exam not long ago and I have the impression she is in perfect health other than being slightly underweight. I will find some appropriate training plans for her and emphasize the importance of form over weight load for lifting. I feel pretty well qualified to get her started with the type of aerobic work she has expressed an interest in (running).
    – Javier
    May 24, 2016 at 15:47
  • @Javier Beginner programs like StrongLifts 5x5 or Starting Strength are popular (and for a reason). No need to have a woman do anything else than a man, other than maybe the absolute load and increments. Form is important but like Rippetoe said, "perfect is the enemy of good". No need to artificially reduce progress either. In the end she has to enjoy what she does, so if she would like running, then that's fine all the same.
    – G_H
    May 24, 2016 at 15:58

Since G_H already gave a pretty detailed answer, I'll keep it short. If it is getting injured that you're worried about, I would suggest starting off with very basic and fundamental bodyweight exercises such as push ups, body weight squats, dips, pullups, chinup. Reason being, it is much harder to injure yourself with your own bodyweight, provided you have somewhat sufficient form. These, combined with a caloric surplus containing sufficient protein intake (minimum around 1g/lb of bodyweight), are enough to "prepare" this individual's body for more strenuous exercises to come.

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