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Diet Diva

Get advice on healthy eating, nutrition, and weight loss from expert dietitian Tara Gidus. 

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Dangers of Underweight

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I wrote yesterday about the Skinny on Being Thin and how people who are underweight or thin are not necessarily happy about being thin and don't always appreciate drawing attention to their weight. It is estimated that 8-9% of the population is underweight.

Just as there are many negative health consequences to being overweight, being underweight can also result in poor health.

Definition of underweight:
Body Mass Index less than 18.5 kg/m2. For example, a woman who is 5'6" would have to be less than 113 pounds to be considered underweight. A man who is 6' would have to weigh 137 pounds or less.

Personally I don't think BMI is the best measure. Looking at the example above, a woman at 18.5 is thin, but not that thin, but a man at 137 pounds and 6' is way too thin. The limitation with BMI is that is does not take into account body fat percentage.

Another way to think about underweight is with body fat percentage. Men have essential body fat of 2-5% where woman have essential fat between 12-15%. Anything lower than that for men or women can be associated with health risk.

Dangers of underweight:
  • Prone to infection, comprised immune system
  • Low muscle mass
  • Hair loss
  • Irregular hormone regulation
  • Osteoporosis
  • Anemia
  • Pregnancy complications (or inability to get pregnant)
  • Menstrual irregularities

Causes of underweight:
  • Genetics. Some people are simply genetically thin with a high metabolism.
  • Illness. Many illnesses have a side effect of weight loss including gastrointestinal illnesses, cancer, HIV, hyperthyroidism, and any illness that causes loss of appetite or medications that lead to loss of appetite.
  • Psychological. Eating disorders or depression can lead to weight loss.
  • Athletics. Many athletes maintain a very low body fat percentage to get a competitive edge over their competition
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About the Author


MS, RD, CSSD, LD/N

Tara Gidus is a nationally recognized expert and spokesperson on nutrition and fitness.

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