CLIMATE

(Prepared by FAO)

1. Introduction

Since the last FAO and WHO Expert Committees met in 1956 and 1963 a considerable amount of work has been reported on the effect of changes of environmental temperature and humidity on human energy requirements. Not so much new data has been found on such environmental factors as they affect protein requirements. As most of the new information has been derived from studies on male subjects this paper is directed towards the energy requirements of adult man.

2. Reference Man

The FAO reference man (1957) is 65 kg, aged 25 years and lives in the temperate zone at a mean annual temperature of 10°C. For moderate activity he is assumed to require for an average of the entire year 3200 kcal/day or 49 kcal/kg body weight. His energy requirements should be decreased by 5% for every 10°C of mean annual external temperature above the reference temperature. His requirements should be increased by 3% for every 10°C of mean annual external temperature below the reference temperature of 10°C.

The U.S./NRC reference man (1968) is 70 kg, aged 22 years and lives in an environment with a mean annual temperature of 20°C and is presumed to wear clothing compatible with thermal comfort. For moderate activity he is assumed to require 2800 kcal/day or 40 kcal/kg body weight. Below 14°C an increase of 2% to 5% in energy requirements is recommended for the effects of cold weather clothing and footgear. For physically active men an increased kcal allowance of 0.5% is made for every degree of temperature rise between 30°C and 40°C. The U.K. allowance (1969) for men 18-65 years of age weighing 65 kg is 3000 kcal.

The differences of approach to the subject adopted by the FAO Committee in 1957 and the U.S./NCR in 1968 reflect to some extent the more recent knowledge of the extent to which man protects himself against cold even at extreme levels and the observation that in very hot environments an increased energy expenditure is required (a) for maintenance of body temperature and (b) for carrying out standard tasks. The literature on the subject has been reviewed admirably by Consolazio (1963, 1969) and the Committee will be aware of his conclusions.

3. Resting Metabolic Rates

Considerable amounts of new evidence have been acquired since the 1957 Expert Committee which confirms that resting metabolic rates are lower in inhabitants of many parts of the tropics, including visitors from temperate regions after a few months' residence. That this is probably not a racial factor but is due to climate is suggested by the finding that Asian visitors to Scotland have resting metabolic rates at European levels (Durnin and Passmore 1967).

However according to Davis (1964) "man can be said to be a tropical animal requiring for his absolute comfort a combination of environmental variables which will allow his skin temperature to average out at 33°C". These variables are environmental temperature and humidity, air movement and availability of clothing, shelter from weather, housing and so on. He reports that "metabolic thermoneutrality for nude resting subjects is from 24°C to 35°C. The comfort zone is between 28°C and 32°C and is characterized by vasomotor, pilomotor and sudomotor neutrality and BMR exactly equals heat loss to the environment". Davis' data would appear to show that for the resting nude man at temperatures below 24°C the metabolic rate is raised by about 25% at 20°C, by 100% at 10°C and by about 150% at 0°C. At temperatures over 35°C he suggests that the resting metabolic rate is raised by about 30% at 40°C and by about 50% at 43°C.

These data of Davis may not be inconsistent with the view that resting metabolic rates are lower in the tropics than in temperate zones for he does not give base-line data. They could be consistent with contemporary fact at subsistence levels of existence. The small number of the world's community living in sub-arctic or arctic regions protect themselves against cold by suitable clothing, housing and by developing an insulating layer of subcutaneous fat. In tropical zones covering the greatest part of the world's developing population, a state of near-nudity is the rule and housing is adapted to provide shade and encourage air movement. But few of these peoples live at an average annual environmental temperature over 35°C.

4. The Effect of Environmental Temperature on Energy Requirements for Work

The conclusions reached by Consolazio in 1963 (and not much modified by his 1969 report) regarding man's energy requirements in different temperatures and at different levels of work are summarized below. His conclusions derived from studies of the energy expenditure and food intakes of men while living at different levels of controlled temperature and carrying out standardized tasks. His figures, given in kcal/kg body weight/day, have been calculated as the daily requirement for men of 65 kg body weight and compared with FAO allowances for moderate work, at lesser extremes of temperature, for 65 kg men as follows:

The important difference between the FAO figures for moderate work and those of Consolazio is for hot climates. If the FAO formula was applied at 37.8°C the difference would be even greater.

5. Energy Requirements for Normal Life in Tropical Climates

A comparison has been made between Consolazio's data and FAO's recommendations for energy requirements with the energy consumption of adult male Nigerian peasant farmers (Nicol, 1959). They have been classified in two groups, one living in the Sudan and Sahelian savannah zones and the other in the high forest zone. In the savannah the climate is very hot and dry by day and cool at night (10°C to 45°C) except for a short humid period before and at the beginning of the rains. In the high forest the humidity is high throughout the year and the temperature does not fluctuate much from about 25°C except for a short period of cooler nights during the heavy rains. The savannah tribesmen can be called moderately active, walking considerable distances, herding cattle and working hard cultivating sorghum and millet, cotton and tobacco for at least six to eight months of the year. In the high forest yam farming involves only about 30 to 40 working days per year. In other words they were engaged in light work. The adult men in both groups had a mean weight of 56&+-;4 kg and were approximately the same weight at the beginning and end of the year's investigation. The savannah group were taller and leaner than the forest group. The comparison is expressed as follows:

Energy requirements or intakes of 56 kg men (kcal/day) in hot climates

The measured energy intakes are not far off the Consolazio's requirements for moderate and light work in hot climates, particularly if the fact that his data were obtained under higher conditions of temperature is taken into account. As the FAO method of calculation takes into account only body weight and environmental temperature it is obvious that it cannot allow for differences of activity. But it is very difficult to know what is the average environmental "effective" temperature when it has wide fluctuations.

6. The Effect of Environmental Humidity

Both Consolazio (1969) and Durnin (quoted by Consolazio) have reported that, compared to a temperate environment, energy expenditures in hot environments were increased by 5-9% with no significant difference between hot/wet and hot/dry conditions. The temperature levels studied were 22°C (temperate), 37.8°C (hot/wet 53% RH) and 44.3°C (hot/dry 25% RH).

7. The Effect of Altitude

It appears that energy requirements may be unchanged at elevations of 3475 metres but that they are increased at elevations of 4300 metres (Consolazio, 1969). The extent of this increase depends upon acclimatization and is not easy to quantify.

8. The Effect of Temperature on Protein Requirements

The FAO/WHO Expert Group on Protein Requirements (1963) felt that it was not in a position to decide to what extent sweating influenced protein requirements. While it was understood by that Committee that up to 3 g N per day could be lost in sweating at very hot/wet temperatures (37-39°C 65-73% RH) it was not known to what degree acclimatization took place to N loss in sweat among populations living in tropical environments. The writer does not know of any studies which have been undertaken to help solve this problem but members of the Committee may well do so.

9. Conclusions

1. Since the last Expert Committee on Calorie Requirements met it has been realized that there is no good evidence that cold increases the energy requirements of humans at a given activity level. The heavy clothing required to protect against very cold climates may increase the energy cost of standard tasks by 2 to 5% (Consolazio, 1963; Durnin and Passmore, 1967). In extremely hot climates the energy required for standard tasks is increased but this increase is not large (Durnin and Passmore, 1967).
2. The newer knowledge of energy requirements at extremes of temperature has been derived from healthy subjects living under standardized conditions and performing standard tasks. Under normal living conditions it is difficult even to decide what is the "effective temperature" of the environment. Our knowledge of energy expenditure under normal living conditions is very imprecise and this is a much more important factor in determining energy requirements and allowances than is climate. Therefore, and as only a very small percentage of the world's population lives under the extreme conditions described above, the Committee may wish to consider if it is important to make any adjustment at all for the effects of climate upon energy requirements or allowances. If however it is felt necessary to include adjustments for environmental temperature, the 1957 FAO Requirements (Allowances) should be adjusted towards those of the 1968 NRC Recommended Allowances to make allowance for increased energy requirements for work in hot environments. The U.K. recommended intakes do not make allowance for variation in temperature nor do they state an average environmental temperature.
3. The effect of humidity upon energy requirements at any level of temperature is apparently negligible.
4. It is possible that the energy requirements of populations living permanently at altitudes over 4000 metres may be increased for specific tasks but the question of adaptation needs further investigation.
5. More studies of adaptation to protein requirements under tropical conditions are required, particularly in areas of extreme heat.
6. The Committee may wish to recommend that studies of the activities of groups of men and women customarily living in the tropics, in cold environments or at high altitudes, who are healthy and have the means to follow active recreations if they wish, should be carried out.

References

Consolazio, C.F. Energy Metabolism and Extreme Environments (Heat, Cold and High Altitude), 1969 Report to the VIIIth International Congress of Nutrition. Prague, Czechoslovakia.

Consolazio, C.F. et al. J. Nutrition, 79, 399. 1963

Consolazio, C.F. World Review of Nutrition and Dietetics, 4, 55. 1963

Davis, T.R.A. Am. J. Public Health, 54, 2051. 1964

Durnin, J.V.G.A. and R. Passmore. Energy, Work and Leisure. Heineman, London. 1967

FAO. Second Expert Committee on Calorie Requirements. FAO Nutritional Studies No. 15, Rome. 1957

Nicol, B.M. Brit. J. Nutr. 19, 293 and 307. 1959

U.K., Recommended Intakes of Nutrients. HMSO, London. 1969

U.S./NRC on Recommended Daily Allowances, Publication 1694, National Academy of Sciences, 1968 Washington, D.C.