In this volume, Spiegler and coworkers present their work on very low-birth weight infants (VLBW), they analysed at what age parents start complementary food in these infants, they determined risk factors for early introduction of complementary food and they analysed whether the age at introduction of complementary food influences height or weight at two years of age. The study is an important contribution to infant nutrition. But there is more to the study: the study was performed at the crossroads between nutrition and auxology.

Studying nutrition and nutritional intervention on growth involves measuring length/height and weight. We measure height in centimetres and weight in g or kg, and quite intuitively consider these units appropriate for describing growth. Terms referring to absolute weight like low birth weight and VLBW infants are ubiquitously used in modern neonatology.

But absolute measures are not unambiguous. What does VLBW (<1500 g) mean? In a group of infants born in the 34th week of gestation, <3% weigh <1500 g; when born in the 30th week of gestation, ~50% are below this weight; but ~97% when born in the 28th week of gestation.1 A VLBW sample of infants born in the 34th week of gestation will consist of selection of hypotrophic infants, whereas a VLBW sample of infants born in the 28th week of gestation is just an appropriate for gestational age sample of prematurely born infants.

Height and weight measures become clinically relevant when compared with an appropriate reference. Growth references are statistical summaries of anthropometry, conditioned (usually) on age and sex. References describe how children do grow. They usually provide mean values and s.d. scores or Z-scores, that is, relative measures for an individual child’s height, weight or body mass index. Z-scores refer to the difference between the individual measurement (Xi) and the age- and sex-specific mean values (Xmean) of the reference population, divided by the standard deviation.

Relative measures track growth within the reference group. Children tend to keep their Z-score throughout development, that is, those who start tall already early in life, tend to stay tall; those who start short, tend to stay short.

Complementary to growth references that describe actual growth, growth standards prescribe optimum growth. As groups of infants and young children from diverse ethnic groups whose nutrition, health and care needs are met were claimed to grow similarly,2, 3 a single set of so called growth standards has been developed to prescribe how children should grow. WHO growth standards are used across countries to assess the growth of children up to 5 years of age.4

The idea of growth standards is intriguing, not only biologically, but it pleases our modern belief that all humans be similar, and most differences in growth be due to differences in the environment and will disappear in a world where everybody is happy and healthy. But is this idea true? Is it possible to interpret all individual growth data in the light of one international growth standard?

Indian newborns are light and short5 when compared with WHO standards. We are used to explain anthropometric birth data from India by unfavourable maternal conditions. But also Indian infants born to modern upper class women are shorter and lighter than WHO standards, and do not reflect maternal wealth and caste (Table 1).

Table 1 Indian unisex birth weight5

Khadilkar and Khadilkar6 state: ‘The disadvantage of using charts such as these (WHO charts) is that they are likely to over diagnose underweight and stunting in a large number of apparently normal children in the developing countries such as India’. Not only wealthy Indian children are shorter and lighter than these standards. Figure 1 illustrates the Japanese situation. The average Japanese growth curve for height also falls below WHO standards. Not only historic Japanese cohorts are significantly shorter particularly early in life, but also modern Japanese children. Japanese growth patterns differ. The two Asian examples may underscore that the simple idea of one single set of globally applicable international growth standards needs to be questioned.7

Figure 1
figure 1

Height difference between modern and historic Japanese children14, 15 and WHO standards/references.4

Spiegler and coworkers8 used German references. This is an important detail in their study, and we should be aware that Spiegler’s approach is the exception. The majority of nutrition reports published world wide do not refer to national, but to the international WHO standards and references.

Meanwhile the discussion has become even more confusing: prominent research groups have started again questioning the use of Z-scores and published controversial opinions. Responding to a critical review by Prentice et al.9 on the timing of growth faltering, Leroy et al.10 suggested that rather than using relative measures to track growth faltering in groups of infants and young children, absolute ‘height-for-age differences’ should be used instead. Lundeen et al.11 added that ‘an important area for future research is determining which metric, changes in height-for-age Z-scores or changes in height deficit over time, is better at predicting different outcomes’.

It is misleading to assume that good reasons to quit the concept of analysing relative growth are in sight. Instead, it is ‘obvious that there are many instances in biology and in public health in which absolute and relative scales provide apparently contradictory results’,12 and no evidence has been provided so far that such contradictory results are clinically relevant. Children tend to track along their Z-scores—first drawings showing this characteristic human growth pattern were published by Bowditch in 1872 (cited after 13, p 475), long before the term centiles and Z-scores were inaugurated.13 Thus, transforming length/height and weight into Z-scores, describing growth in relative, rather than in absolute terms, and analysing patterns of growth on the basis of a valid local reference, must still be considered appropriate. And this is what Spiegler and coworkers did.