Methods for monitoring the functional status of the circadian system in dietary surveys studies: application criteria and interpretation of results.

To evaluate the circadian system status of the subject may be of special interest in nutrition. Particularly for those studies related to the assessment of diseases related to malnutrition, as it is the case of most of the degenerative diseases such as obesity, cancer, or cardiovascular diseases. For this purpose, one of the approaches consists to measure a) the external synchronizers of the internal clock, such as light intensity, and changes from fasting to eating and from resting to activity. Indeed, "chronodisruptors" have been defined as "exogenous and endogenous exposures or effectors which are chronobiologically active and can thus disrupt the timing and order. Another approach to assess the circadian system health is to measure the b) outputs of the internal clock (circadian marker rhythms). Among such outputs, the rhythm of body temperature, motor activity, melatonin, cortisol and clock gene expression are the most commonly used. From the genetic perspective, we are now able to measure failures in the internal clock, in order to assess c) the genetics of the molecular clock. Indeed, new nutrigenetics techniques are giving us the opportunity to measure the association between different genetic variants of our clock genes and several illnesses such as obesity, cardiovascular diseases, diabetes or cancer. In addition to these techniques, self-reported questionnaires based in the morning-evening preferences have been developed as complementary procedures to assess human chronotypes.

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