As was pointed out earlier, the energy that an individual takes in through food consumption will be spent to maintain body temperature and vital organ functions, as wellas for eating, sleeping, and essential hygiene. The remainder is available for discretionary use, such as work and leisure. It was also shown that the additional calories that became available in the wake of the Second Agricultural Revolution were used to engage in more energy-intensive tasks and increase labor force participation. Energy not used is stored, leading to weight gain. As such, the body mass index may be interpreted as a measure of net nutrition, which is defined as the excess of calories ingested over calories claimed for maintenance and discretionary use. Figure 2.1 documents the secular increase in body mass index for white men between 1864 and 1991.
The self-reinforcing cycle of greater body size and higher productivity
2.14
Modern Norwegian males
Union Army veterans
0.88
17 19 21 23 25 27 29 31 33 35
BMI
Figure 2.2 Relative mortality risk by BMI among men 50 years of age, Union Army veterans around 1900 and modern Norwegians (from Costa and Steckel, 1997). In the Norwegian data BMI for 79084 men was measured at ages 45–49 and the period of risk was 7 years. BMI of Union Army veterans was measured at ages 45–64 and the observation period was 25 years. Costa and Steckel (1997). Reproduced with kind permission from The University of Chicago Press. © 1997 by the National Bureau of Economic Research.
It has been shown that eliminating chronic hunger will strengthen the body’s defenses against infectious diseases, thus lowering the risk of contracting diseases and premature death. The relationship between weight, as measured by the Body Mass Index, and mortality was established empirically by Hans Waaler (1984) for Norwegian men aged 45–49 and confirmed for a sample of Union Army veterans measured at ages 45–64 and followed for 25 years. Figure 2.2 shows a U-shaped relationship between BMI and the relative risk of death for both samples. Among both modern Norwegians and Union Army veterans the curve is quite at within the range 22–28, with the relative risk of mortality hovering close to 1.0, which represents the average risk of death in the population. However, at BMIs of less than 22 and over 28, the risk of death rises sharply as BMI moves away from its mean value.
The gain in height
A larger and better survival diet allowed adult members of the generation that first witnessed the rise in agricultural efficiency to increase weight, and, consequently, to improve health and extend life. Better nutrition of pregnant women also improved the nutritional status of fetuses and infants. Access to sufficient amounts of calories and other vital nutrients in utero and developmental ages has been shown to affect the off-spring’s final height. Thus, whereas the immediate effect of the improvements in food
Economic and technological development and their relationships to body size and productivity
1.5
1.0
0.5
62 64 66 68 70 72 74 76 78 80
Height (inches)
Figure 2.3
Relative mortality risk among Union Army veterans and among Norwegian males. Author’s
Calculations
Supply was to raise the amount of energy spent at work and to boost body weight, the long-run impact over the course of several generations has been an increase instature. This conclusion is supported by the time series on mean final heights for various European populations, shown in Table 2.2. Waaler (1984) also identified the role of body height as a factor in uencing morbidity and mortality. Figure 2.3 plots the relationship between relative mortality risk and height found among Norwegian men aged 40–59 measured in the 1960s and among Union Army veterans measured at ages 23–49 and at risk between ages 55 and 75. Short men, whether modern Norwegians or nineteenth-century Americans, were much more likely to die early than tall men. Height has also been found to be an important predictor of the relative likelihood that men aged 23–49 would be rejected from the Union Army between 1861 and 1865 because of chronic diseases. Despite significant differences in ethnicity, environmental circumstances, the array and severity of diseases, and time, the functional relationship between height and relative risk are strikingly similar in the two cases. To gauge the relative importance of height and weight for an individual’s risk of mortality, an isomortality surface that relates the risk of death to both height and weight simultaneously is needed. Such a surface, presented in Fig. 2.4, was fitted to Waaler’s data. Transecting the isomortality map are iso-BMI lines that give the locus of BMI between 16 and 34. The heavy line transecting the minimum point of each iso-mortality curve represents the weight that minimizes mortality risk at each height. Since an individual’s height cannot be varied by changes in nutrition after maturity, adults can move to a more desirable BMI only by changing their weight. Therefore, the x-axis is interpreted as a measure of the effect of the current nutritional status of mature males on adult mortality rates. Moreover, since most stunting takes place before age three, the y-axis is interpreted as a measure of the effect of nutritional.
The self-reinforcing cycle of greater body size and higher productivity
Isomortality-risk curves Iso-BMI curves Minimum-risk curve
(0.7–2.2) (16–34)
1.95
1.90
1.85
1.80
1.75
1975
1.70
1.65 1870
1785
1.60
1705
1.55
40 50 60 70 80 90 110 100
Weight (kg)
Figure 2.4
Isomortality curves of relative risk for height and weight among Norwegian males aged 50–64 years, with a plot of the estimated French height and weight at four dates. Author’s calculations.
Deprivation during developmental ages (including in utero) on the risk of mortality at middle and late ages. Superimposed on Fig. 2.4 are rough estimates of heights and weights in France at four dates. In 1705 the French probably achieved equilibrium with their food supply at an average height of about 161 cm and BMI of about 18. Over the next 270 years the food supply expanded fast enough to permit both the height and the weight of adult males to increase. Figure 2.4 shows that the increase in available food per per-son translated mostly into weight gain during the eighteenth and nineteenth centuries. During the twentieth century the gains in calories per capita served mainly to increase height. Between 1870 and 1975 height increased at more than twice the rate that it did during the previous 165 years. Figure 2.4 implies that although factors associated with height and weight jointly explain about 90% of the estimated decline in French mortality rates over the period between 1785 and c. 1870, they only explain about 50% of the decline in mortality rates during the past century.
