In questo contesto si potrebbe tradurre così
Measuring usual dietary intake in free-living human subjects,
besides being difficult to accomplish with precision,
can in some instances represent a contradiction of terms.
For instance, in quasi-experimental studies that provide
controlled diets to subjects living, but not eating, in their
natural environments, the generalizability of any findings to
the subjects’ own diets may be limited. Another problem
with this type of study is that strict compliance to
experimental regimens often poses a major problem, e.g.
when subjects are living in their usual environments and
have access to other foods. Finally, in the special case of
ad libitum intake studies, while food intake can be
measured easily and precisely in controlled situations,
the quantity and quality of the food selected is likely
to be altered as a consequence of the controlled condition
itself
http://journals.cambridge.org/production/action/cjoGetFullte...

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Note added at 20 min (2009-02-06 11:49:18 GMT)
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oppure, appunto "in condizioni di vita normale"