Objective Measures of Dietary Intake

Aim of the Session

The session focuses on the following objectives: understanding the meaning of objective assessment in dietary intake, listing different types of biomarkers with relevant examples, recognizing the difference between dietary intake and nutritional status, and identifying the limitations of biomarkers when used as dietary assessment tools.

Laboratory-Based Dietary Intake Studies

Laboratory studies involve participants consuming only the foods provided by investigators under controlled conditions. This approach ensures that the energy or nutrient composition is precisely determined, allowing researchers to accurately estimate the amount of energy and nutrients consumed by each individual over a designated period. Participants may reside in a lab or return home for meals, which provides a comprehensive understanding of their dietary intake. However, this controlled setting has limitations, including compliance issues and a lack of data from free-living individuals.

Definition of Biomarkers

Biomarkers are defined as biological markers that indicate measurable alterations at the cellular, biochemical, or molecular levels within biological media, such as human tissues, cells, or fluids. They can reflect dietary intake and nutritional status, offering objectivity in dietary assessments.

Nutrition Biomarkers

Dietary biomarkers serve as biochemical indicators of dietary intake or nutritional status and can represent both recent and long-term consumption. An ideal dietary biomarker is specific, sensitive, and applicable across diverse populations, accurately reflecting true dietary intake levels.

Types of Biomarkers

There are several uses for biomarkers, including the validation of dietary instruments, acting as surrogate indicators of dietary intake, and providing integrated measures of nutritional status. Biomarkers can be classified into three main classes: Recovery Biomarkers, which are biologically specific products directly linked to intake with minimal influence from individual metabolic differences (examples include the doubly labeled water (DLW) method, urine nitrogen, and potassium); Concentration Biomarkers, which correlate with food or nutrient intake but cannot be accurately converted to actual intake levels (such as blood lipids and serum vitamins); and Predictive Biomarkers, which demonstrate a stable yet time-dependent correlation with intake, offering intermediate estimates of dietary consumption (Frequent examples include urinary sucrose and fructose used as markers for sugar intake).

Specific Biomarkers Examples

• Recovery Biomarkers

  • Doubly Labeled Water: Measures total energy expenditure through the differential elimination of deuterium and oxygen isotopes after water consumption, providing accurate energy expenditure figures, albeit in a relatively invasive and expensive manner.

  • Urinary Nitrogen: Used as a marker for protein intake validation, closely correlating with dietary nitrogen intake.

  • Urinary Potassium: Reflects potassium levels necessary for cell metabolism and fluid balance, which helps guide dietary adjustments.

Advantages and Limitations of Biomarkers

The advantages of using biomarkers include avoidance of biases commonly associated with self-reported dietary methods, such as recall bias and social desirability, providing independent error measurements related to dietary records, and ease of specimen collection without significant burden to participants. However, limitations exist, including high costs and potential invasiveness of some biomarker methods, difficulties in identifying specific food sources linked to nutrient biomarkers which complicates dietary advice, and variability due to genetic, environmental, and lifestyle factors that can influence biomarkers and their correspondence with dietary intake.

Future Directions

Future research efforts are essential for addressing the identified gaps in nutritional biomarkers. The Institute of Medicine stresses the need for new dietary biomarkers that are sensitive, specific, cost-effective, and non-invasive to enhance dietary assessment accuracy.

Conclusion

While biomarkers provide valuable insights into dietary intake and nutritional status, further research is crucial to solidify their role in dietary assessment, particularly regarding the complex relationship between nutrient intake and biomarker values.