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I am 25 years old.
My height is 172.5 cm.
My weight as of today(26th Apr, 2013) is 54.3 kg.
I was 56.5 kg, 4 weeks back.
My BMI is 18.25.

I have been swimming really hard in this period, covered around 75 km (oh yeah, I am trying to become a good triathlete).
I have been eating a lot of vegetables, 2 eggs a day, lot of rice, nuts & sprouts but still I am losing a lot of weight.

Also I am feeling tired after every swim session and I sleep for 2 hours subsequently.

Is it really alright to lose this much weight rapidly?
How much will champion athletes BMI be (really curious on that)?

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The BMI is generally not a reliable test for champion athletes because their muscularity would give a false "overweight" value. The BMI is a quick and easy screening tool for large populations to help determine whether or not additional health parameters should be checked. It is generally not reliable for athletic or muscular people. –  BackInShapeBuddy Apr 26 '13 at 19:08
    
See this question fitness.stackexchange.com/a/7127/3778 for the BMI of world class female swimmers. If you put together a similar table for the best triathletes, I will put it in a similar diagram as an answer. –  FredrikD Apr 27 '13 at 9:52
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4 Answers

up vote 3 down vote accepted

Make sure your protein intake is high enough or you could be losing muscle. 2 eggs is only about 12g of protein. Maintenance levels of protein for a very physically active person are at least 1g of protein per lb of body weight (~2g per kg). Take this with a grain of salt, as it is subjective -- depending on your current physique, you may wish to lose muscle (for example: if you are a pro bodybuilder and wish to become a triathlete, significant muscle loss would be desirable).

You may also want to pick up some skin fold calipers to monitor your body fat. Your loss of weight should not be coming from lean body mass. Body fat percentage will be a much more helpful piece of information than BMI.

Ultimately (and most importantly) are you seeing quantifiable improvements? The essence of all training is progressive overload. The matter becomes: are you bettering your lap times? If so, continue until this begins to plateau, then start examining your diet more thoroughly.

One side note: feeling tired enough to sleep 2 hours after your swimming sessions seems to suggest to me that your carb intake may be a little low. You may want to add some sugary electrolytes (sports drink) to your diet around the time of your workout to keep your glycogen and electrolyte levels up.

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Essentially, what you are saying is that you've kept the same diet, and have drastically increased the amount of Calories you use. It's natural for you to lose weight under those conditions.

If losing the weight is undesirable, then you must increase the amount of food you eat to compensate for all the activity. Feeling tired after exercise is natural, and your body will need the rest to make the changes it needs to fit the new demands you've put on it. However, if you are always tired, it's likely you just don't have enough food.

Checklist:

  • Enough protein? As an active person a good ballpark would be 1g/lb or 2g/kg of lean mass minimum. You probably won't use that much directly, but it provides the building blocks your body needs to rebuild itself after training.
  • Enough Calories? If you are losing weight you are eating fewer Calories than you are using. If you are gaining weight, you are eating more Calories than needed. It's that simple.
  • How is your body changing? Tape measures and mirrors are probably going to be the best bet hear. If your shoulders are getting broader or staying the same while you are loosing belly fat, you are losing primarily fat. If the shoulders or arms are getting smaller, you are losing muscle. Typically the belly, hips, and for women thighs and arms are where fat tends to accumulate fastest and is lost first.

If it seems like you are loosing muscle, increase your protein. If you are loosing too much fat (you do need some), then increase your food.

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+1, thank you very much. Points taken. –  Freakyuser Apr 29 '13 at 14:16
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Is it really alright to lose this much weight rapidly?

Yes its fine. The important thing is to get a balanced diet with enough calories in it. You may find your weight bounces back up as you start to put on muscle. Just make sure you listen to your body and rest when you feel you need to.

How much will champion athletes BMI be (really curious on that)?

BMI is an awful metric for very fit people. It only takes into account height and weight so obviously someone with more muscle mass than somone of identical height will have a higher BMI. Its not uncommon for athletes to be "obese" (>27.5) on that scale.

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+1, for answering to the points specifically and the clarity of the answer. Thank you. –  Freakyuser Apr 29 '13 at 14:15
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I've seen several studies that suggested that a good target for weight loss is 1% of your bodyweight a week. Doing this helps you avoid arbitrary calorie deficit targets or unsustainable losses, plus allows your body to gradually get used to the shrinking you without provoking a defensive response (i.e. slowing your metabolism)

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Perhaps you can add some links/references to the studies? Google scholar is great. –  FredrikD Apr 28 '13 at 14:15
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