Country-specific nutrient requirements & recommended dietary allowances for Indians: Current status & future directions

Salient features of Indian RDA

The expert committee of ICMR had the challenge of contextualizing RDAs in the background of comparatively less evidence in various areas. However, it had tried to cater to most of the major issues based on available evidence. The major challenge was to fix the reference body weights. For children under the age group of five years, the MGRS (Multicentre Growth Reference Study) mean body weights have been used4. However, a major deviation from the International definitions happened for the age group above five years including adults. The committee considered the body weights corresponding to 95^th percentile for Indians to be normal due to the following two reasons: (i) to ensure that the requirements derived are for healthy population due to a high prevalence of underweight in India, and (ii) even when the mean±standard deviation (SD) criteria are not followed strictly, the requirements reflect the upper tail end that the population is able to achieve. Although this judgment may not look ideal, it has been done to contextualize and have a nutrient requirement value which is close to real life situation and achievable for the normal healthy population at large. It was also considered that unlike other countries which have several values to reflect requirements, India had only one value (RDA value) to follow at the population level. While updating dietary reference values for Indians, this aspect needs to be carefully considered and adopt global harmonization of methodological approaches to establishing nutrient intake recommendations (Table II).

The general principles underlying RDA are as follows:

Factorial approach: The factorial approach employs an additive approach in calculating nutrient requirements. The nutrient requirements for different functions are fixed separately and added up to arrive at the total requirement. This has been used for arriving at the requirements of energy, protein during pregnancy and for calculating iron requirements2.

Obligatory loss of nutrients: Experimental data are used to understand the minimal loss of nutrient or its metabolic product through normal routes of elimination namely urine, faeces and sweat2. This is done by feeding a diet which is very low in the particular nutrient or is devoid of it. The data help in understanding the amount of nutrient to be consumed for replacing the losses (protein and iron requirement).

Nutrient balance or chemical balance: Nutrient balance is used for measuring nutrient requirement through the estimation of daily intake and excretion or losses (intake = output). The method is used for arriving at protein, iron and zinc requirements5.

Nutrient turnover: Stable isotope technique has been employed to understand the turnover of nutrients in healthy individuals67. Stable isotopes are relatively safer, and therefore, studies can be conducted among vulnerable segments of the population such as pregnant women and young children. The method has been in use for fixing the requirements of vitamin A, vitamin C, calcium, zinc, iron and vitamin B₁₂. However, the stable isotope technique is costly and requires expertise which is available only in a few centres in India.

Depletion & repletion studies: Response to feeding graded doses of a nutrient after depletion is studied. For this, volunteers were fed a diet low in the nutrient and subsequently the repletion was measured. The increase in biomarker and its equilibration was measured as the requirement of the nutrient. This has been in use for the requirement of several water-soluble vitamins.

Dose-response studies & randomized control trials: Functional outcome and biochemical measures are considered for the nutrient under consideration. For example, bone mineral density (BMD) measurement using Dual Energy X ray Absorptiometry (DEXA) for calcium and glutathione peroxidase for selenium and red blood cell folate status have been applied to derive the requirements of these nutrients.

Epidemiological & observed intakes: The functional outcome measure has been considered for deriving requirements (e.g. risk of fracture in elderly men and vitamin D deficiency). For many nutrients where sensitive biomarkers are not available dietary intake data have been utilized for computation of requirements (e.g. vitamin K).

Why do we need country-specific RDAs

While it appears to be an appealing task to unify the guidelines of nutrient requirements, it is imperative to understand why Indians need to put in a conscious effort to contextualize RDAs. The estimates of requirements are presented as the quantity of the nutrient that must be present in the daily diet, as consumed. Nutrient requirements are therefore, population specific due to variations in genetic environment and socio-demographic characteristics of the population. The estimates have accounted for the nature of the habitual diet and the dietary factors that affect the release and absorption of the nutrient. Region-specific phenomena such as the food pattern (dietary diversification and culinary practices), food matrix effects relating to inhibitory/promoting effects (therefore, bioavailability and bioconversion) are considered for nutrients. Scientific judgement based on ethnicity, environment (climate), deficiency and lifestyle, prevalence of disease in the area, physical activity and growth pattern, regional reference body weight changes are some of the other factors considered2. One classical example is India's habitual cereal - pulse-vegetarian diet which is considered to be low in protein quality, leading to a higher RDA for protein. The low iron bioavailability of Indian diet is another example, which considers >95 per cent of non-heme food sources of iron in Indian context leading to at least 50 per cent lower bioavailability8, while in Western world >75 per cent is assumed to be from heme source,9 and therefore, translates to a higher RDA for former and a lower RDA for latter. Same is the case of zinc, vitamin B12, vitamin D and provitamin A from food sources9.

First 1000 days (Pregnancy & Infancy)

Pregnancy and infants 6-12 months of age demand the maximum requirement of iron per kg body weight10. A pre-pregnancy weight of 55 kg and weight gain during the pregnancy period of 270 days of 10-12 kg has been considered for computation. Although average pregnancy weight gain may be much lower in India, the weight gain of 10-12 kg is considered to fix the requirements to facilitate normal pregnancy outcome. The total iron loss during pregnancy is about 760-860 mg and is the sum of iron needed to replace basal loss of 210 mg, for expansion of the maternal red cell mass of 450 mg, for the needs of the growing foetus 270-300 mg and for placental tissue of 50-90 mg. These figures have been arrived based on foetal tissue content of iron6 and the expansion of maternal red cell mass considering iron in haemoglobin of 3.47 mg/g of haemoglobin11. Trimester-wise iron requirement in mg/day is given in Table III.

Iron needs of infants during the first six months of age concords with the average iron intake of healthy infants who are exclusive breast fed121314. A full-term infant who weighs 3.2 kg requires about 0.23 mg/day of iron to replace the losses through excretion and to maintain haemoglobin at the normal concentration of 110 g/l. This computation considers an iron content of milk of 0.78 mg/l and an average milk volume of 650 ml/day and is based on isotope studies in lactating women from central India1213. Iron requirements increase markedly during later six months of life. Therefore, complementary feeding with iron-enriched solid foods should be practiced along with breastfeeding from 7 to 12 months of age. During the preschool years (1-3 yr), the total requirement is about 0.434 mg/day, which is based on the basal loss and average assumed weight gain observed in MGRS surveys3.

Lactation & women of child-bearing age

The total iron loss during lactation is 23 μg/kg/day (basal loss and iron lost in breast milk at the rate of 9 μg/kg/day). However, considering the huge losses during pregnancy, iron requirement is fixed similar to that for non pregnant non lactating (NPNL) women of 30 μg/kg/day which is the sum of the obligatory losses and the menstrual blood losses15. The iron requirement is about 1.65 mg iron per day2.

Children 4-9 yr & adolescents 10-18 yr

The salient feature of adolescent requirements is the allowance of iron storage being added to the requirement at this stage. For 4-9 yr, on account of growth leading to negligible iron accumulation, no allowances for iron stores have been considered. Iron requirement during growth encompasses the iron which is needed for the expansion of blood volume and increase in lean body mass16.

The distribution of iron in the human body with respect to different compartments is assumed to be haemoglobin 70 per cent, stores 25 per cent, muscle four per cent and transport one per cent. During growth spurt (10-13 yr), body mass increases to 2.8 kg/yr and the body store of iron build up and are maintained in girls until menarche. Considering iron accumulation in the body as a continuous and slow process, a conservative figure of about 50 per cent of that found in males (6 μg/kg/day) has been added to the total requirement during adolescence in girls (3 μg/kg/day)17. Therefore, allowances for basal loss, blood volume expansion, iron required for muscle mass and iron storage during this period are about 17 μg/kg/day. However, differences in iron requirements in girls and boys take place during 13-17 yr. The total iron requirement during this period for boys is 26 μg/kg/day and for girls 30 μg/kg/day2 (Table IV).

Dietary non-heme iron & absorption

The iron density of a typical cereal-pulse vegetarian Indian diet is around 8 mg/1000 kcal2. Non-heme iron contributes about 90-95 per cent of total daily iron. Heme iron consumption is negligible in India. Majority of Indians obtain their iron requirement from non-heme sources such as cereals, pulses, vegetables and fruits18. Regulation and guidelines on introduction of iron-fortified staple foods including iron-fortified iodized salt have been implemented in the country and are expected to provide 30-50 per cent of RDA of iron. However, the major concern appears to be in improving iron absorption and bioavailability from staple foods.

Meeting iron requirements in adolescents

Adolescence marks the second growth spurt and the onset of menarche in girls. Both increase the requirements for dietary iron. The expert committee derived an increase in body mass of 4.3 kg/yr for boys and 4 kg/yr for girls2. The increment in body weight was based on National Nutrition Monitoring Bureau (NNMB) surveys17. An increment in haemoglobin to the extent of 20 g/l in boys and 10 g/l in girls was also fixed1920. The exact role of sex in modulating iron absorption is not clear.

Consequent to these assumptions, the iron requirement of adolescents which is 1.05 mg/day in boys and 1.33 mg/day in girls is estimated to increase to 1.60 mg/day in boys and 1.36 mg/day in girls (Table IV). At this stage, the dramatic increment in requirement among boys would account for growth which after factorial addition, overrides the iron demands of menstruation in girls21. Previous studies among Indian adolescents did not observe any sex difference in iron absorption. Apparently, the iron requirements during this period need to be met by dietary means lest may lead to a negative balance1920. Thus, both girls and boys in this age group are equally prone to increased physiologic demands for iron and consequent deficiency.

The absorption of non-heme iron is mainly regulated by meal composition and dietary factors. There are only two recent studies that assessed the bioavailability of iron in Indians using the stable isotopic method. The study among 18-35 yr old women has reported iron bioavailability ranging from 5.2 to 9.4 per cent in non-iron-deficient and 15.6-19.7 per cent in iron-deficient from a typical rice meal22. The study in adolescent boys and girls of 13-15 yr reported an iron bioavailability of 9.7 per cent in girls and 8.6 per cent in boys23. The figures were not much different from what had been reported for adult women22.

Strategies to double iron bioavailability

Dietary modifications with fruits rich in ascorbic acid (>75-100 mg/100 g) such as guava and papaya have been shown to double iron absorption (>20%), suggesting that creating food synergy as part of regular meals are promising approaches to improve dietary non-heme iron absorption. One effective and sustainable option is the popularization of locally available, ascorbic acid-rich foods as part of everyday meal922.

Implications of harmonizing RDA

By unifying the recommendations, the population has to strive incorrectly to achieve the higher cut-off of challenge underestimating the impact of various programmes. RDAs also take into consideration, region-specific programmes and policies, such as in the case of folic acid, vitamin B12 and vitamin D. Western countries do have a mandatory food fortification programme in place which leads to their increased intake of folate and vitamin D. When median intakes of healthy individuals are quantified for RDA purpose, this will automatically translate to a higher median intake and therefore, higher RDA as compared to countries where such a programme is not initiated. The bottom line is that though nutrient requirements in the broadest sense are similar, the dietary allowances should be by and large based on country-specific challenges and in the long term should contribute to promoting good health among its individuals without any adverse effects.

Fixing the indices

There needs to be a careful consideration to continue with the single value or harmonize with the internationally accepted values14 and adopt the methodologies to compute all the four values i.e., estimated average requirement, RDAs, AIs and tolerable upper limits (TULs). The advantage is that using these requirement constructs, not only can the risk of inadequate intakes be calculated (using the probability approach) but the risk for excessive intakes that cross upper limit can also be determined25. Of the four, TULs have been fixed by the ICMR expert committee recently, based on the deliberations of ICMR committee (https://foodsafetyhelpline.com/?s=ICMR+committee). This can be included as part of the next revision of nutrient requirements. The TULs have been arrived at based on the available literature and is a metric used to define unsupervised intake.