For some nutrients, requirements change across the lifespan in association with physiological changes that are assumed to occur at various average ages. For example, the AI for vitamin D is higher for adults older than 50 years than for those younger than 50 years, and the recommendation for vitamin B₁₂ is that individuals older than 50 years obtain most of their vitamin B₁₂ from fortified foods or supplements. For these nutrients, the changes in recommendations are associated with age-related changes in vitamin D metabolism and in gastric acidity, respectively. These changes do not occur abruptly at age 50 and it could reasonably be suggested that average dietary requirements would be increased at the upper end of the 51- through 70-year age range.

In other situations the physiological changes that result in different requirements occur over a shorter time or can be identified by individuals. An example would be iron requirements of women. The requirements for women ages 31 through 50 years are intended to cover losses associated with menstruation whereas for women older than 50 years it is assumed that menopause has occurred. Onset of menopause, then, rather than age, is the physiologically significant event.

Although the EARs for intake of thiamin, riboflavin, and niacin are not set based on energy intake (IOM, 1998b), it may be appropriate to evaluate intake of these vitamins as a ratio to energy intake for some populations.

The DRI report on the recommended intakes for the B vitamins (IOM, 1998b) notes that no studies were found that examined the effect of energy intake on the requirements for thiamin, riboflavin, or niacin and thus these EARs and RDAs were not based on energy intake. Despite this lack of experimental data, the known biochemical functions of these nutrients suggest that adjustments for energy intake may be appropriate, particularly for individuals with very high intakes (such as those engaged in physically demanding occupations or who spend much time training for active sports). Adjustments may also be appropriate for healthy people with low intakes due to physical inactivity or small body sizes.

For thiamin, riboflavin, and niacin, an energy-adjusted EAR may be calculated as the ratio of the EAR to the median energy requirement for an individual or population. Because DRIs have not been