Bioequivalence and Bioavailability Forum

Dear MiLi!

❝ […] bioavailability studies in nutrition health. I have many difficulties to find a definition of bioavailability appropriate for nutrition.

First it's important to differentiate between nutrition, nutrient, and dietary supplement.

As an entry point visit FDA's CSFAN (Center for Food Safety and Applied Nutrition), and the search feature there.
For European regulations see 2002/46/EC and 2006/37/EC.
Bioavailability is not mentioned in these regulations; maximum levels are set 'based on generally accepted scientific data' - whatever that means. If you really want to study bioavailablity - the definition is not different from the one used for drugs: the amount available in the central circulation - I would consider this a rather difficult task:

• a run-in period of low-content food
  
• a suitable baseline measurement
  
• single and/or multiple doses of the supplement
  
• measurement of plasma or urine concentrations

For some (most of?) supplements such an attempt will be futile, because the body's pool is that high and you will never see any differences compared to baseline (e.g., the calcium and phosporus pools are in the kilogram range, iron about 3 grams)…
We once performed a study on magnesium (body pool about 35mg) and the results went straight into the garbage pin.