Why Can Not Gain Weight

While most people have a hard time losing weight, there are people that have a hard time gaining weight or have a problem with “unhealthy weight lost”.  Why would a person lose too much weight or can not gain weight if underweight?

While I’m not a medical professional, and there are different causes for different individuals. Some medical reasons for this may be…

  • Hyperthyroid autoimmune condition.  Thyroid producing too much thyroid hormone increasing metabolic rate too much.
  • Pancreas not producing enough insulin.  Type 1 diabetes (also autoimmune) is such as case.   There is also LADA (latent autoimmune diabetes in adults).  Zinc and vitamin B6 is needed to make insulin — may want to supplement if deficient.
  • Parasites or chronic infections

Note that while some individuals gain weight as they age and become elderly.  Other sets of individual lose weight can become frail as they age (possibly due to natural loss of muscle mass as we age).    WebMD says …

“Physically inactive people can lose as much as 3% to 5% of their muscle mass each decade after age 30. Even if you are active, you’ll still have some muscle loss.”

Resistance Training

One of the best things a skinny person can do to maintain health as they age is to maintain muscle mass.

If these and all other possible medical reason have been ruled out and you are perfectly healthy and not able to gain weight, consider the following to help maintain muscle mass…

  • May need more calories.  Some people overestimate what they eat and the number of calories they need.  Use this calculator to see how much calories you need per day.  And track your calories consumption with cronometer.com
  • May need more carbs if you are not having problem with blood sugar.   If you are very thin and have trouble with high blood sugar, look into Type 1 diabetes or LADA or other issues.   Did you start losing weight when going on a gluten-free, or low-carb diet?   Aa gluten-free diet will tend to be automatically lower in carbs.  Try adding rice as a non-gluten carb.
  • Weight training and exercise with short but high intensity, rather than long and light intensity.   Use progressive overloading style of exercise. The former helps build muscle if you give some rest days in between.  Whereas the latter burns calories.  Although the amount of calories burn is not that much.  And you still need light movement throughout the day instead of sitting all the time.

Zinc and B6

The New Optimum Nutrition Bible writes …

“Energy nutrient include vitamin B6 and zinc, which help insulin work”

And we need insulin to gain weight.  Zinc and magnesium are two minerals that are most deficient in people.

There are certain group of individuals with condition of pyroluria that require higher amounts of B6 and zinc — there are questionnaire and lab tests for this.  If you have poor dream recall, you may be deficient in either or both B6 and Zinc (read here for best form of these supplements).

Paleo Diet

Some thin people lose too much weight on the Paleo diet and want to gain weight.  Here is Chris Kresser’s answer…

Weight lose can be a concern if you are underweight.  He discusses some reasons why some people are underweight …

  • Fast metabolism
  • Gut issues, malabsorption, gluten sensitivity, celiac
  • SIBO (the bacteria get the food first)
  • Low stomach acid (not able to break down protein)
  • Leaky gut
  • Adrenal fatigue
  • Autoimmune diseases (such as type 1 diabetes, MS)
  • Sub-clinical hyperthyroidism
  • chronic infections

And some things to try…

  • Eat more calories and from all three macronutrients (not low-carb or not low-fat)
  • Increase starch and with each meal
  • Add fat (like coconut oil)
  • Smoothies (easier to drink more calories than eat them). One smoothie might be 600 calories.
  • Lift weights (heavier weights with less reps)
  • Dairy if you are not sensitive to it
  • White rice



Skinny’s Person Guide to Gaining Weight

Documentary: Why are Thin People Not Fat