Climate, altitude, and blood pressure

Human Biology, Aug 1999 by Hanna, Joel M

Abstract The effects of climate and altitude on casual blood pressure are examined from the perspectives of initial exposure, acclimatization, long-term residence, and birthplace. Hot arid and hot humid climates seem to have little effect on blood pressure, although a slight reduction may be found in some naturally acclimatized groups. Exposure of the total body to mild cold likewise has little apparent effect. Local exposure of the extremities to severe cold occasions significant increases in blood pressure during exposure but not at other times. Acclimatization reduces but does not eliminate that response. The effects of altitude on blood pressure are variable. There is initial hypertension, followed by gradual normalization. After years of residence at high altitude blood pressure may actually be lower than that observed among residents at sea level.

Homo sapiens is a polytypic species whose morphological and physiological variation arises from environmental sources and genetic adaptations. From the earliest days of scientific investigation through current experimental studies the possible role of climate in producing that variation has been of great interest (Count 1950; Marks 1995). With the accumulation of empirical data it appears that climate has played a role in our evolution. Climatic forces seem to have contributed to variations in skin color, body mass, body shape, and many metabolic processes (Frisancho 1993; Fregley and Blatteis 1996). Because maintenance and adaptive regulation of blood pressure are integral parts of such adaptations, it is logical to assume that climate may also play some role in the regulation of blood pressure.

Unfortunately, isolation of specific associations can be difficult because modern humans have often created microenvironments that shield them from the most extreme climatic forces. Living in a hot desert no longer necessarily means exposure to extremes of heat, solar radiation, or aridity. In addition, a host of nonclimatic factors, many of which are discussed in this special issue of Human Biology, also can modify blood pressure. Body fat, body mass, age, and sex as well as numerous social and lifestyle factors can play major roles in blood pressure variation (James and Baker 1995; James and Brown 1997). To fully understand climatic influences, we must consider these independent forces; however, it is a rare investigation that has attempted to match subjects on more than a few parameters.

The aim of this review is to examine the long-term effects of climate and altitude on casual blood pressure, because this measure is of great interest to human biologists, physical anthropologists, and medically oriented social scientists. The emphasis is on the results of acclimatization or long-term residence at high altitude in hot or cold environments. Special attention is given to studies of the natives of these environs, who are least acculturated or least exposed to Western cultures. Because climatic physiology is a vigorous and ongoing area of investigation, a thorough discussion of the regulation of blood pressure during limited exposures is beyond the scope of this effort. Emphasis is on research relevant to long-term residence or acclimatization. For those interested in a more comprehensive discussion of regulation, appropriate references are provided.

Climatic Rules and Blood Pressure

Environmental biologists studying animal populations have noted a relationship between body size or proportion and climate. These climatic rules have been applied to humans with some success. As originally stated, Bergmann's rule suggests that the members of a species living in the colder parts of the species range will be larger than those living in the warmer parts of the range (Bergmann 1847). This has been interpreted by human biologists to suggest a relationship between body mass and surface area (Roberts 1978; Frisancho 1993). Accordingly, larger organisms also have a lower ratio of surface area to heat-producing mass and are at some advantage in colder environments. A corollary, Allan's rule, suggests that extremities are shorter in the colder parts of the range, presumably because of thermal advantage. Although it has been argued that Allan's rule is a result of developmental acclimatization arising from a differential in blood flow, the determinants of Bergmann's rule remain unclear (Hanna et al. 1989).

It is known that among human populations temperature and body weight are highly correlated. Roberts (1978) found correlations of -0.6 for men and -0.8 for women when correlating mean annual temperature with weight. Roberts also reported a significant negative correlation between systolic blood pressure, diastolic blood pressure, and mean annual temperature in a large worldwide sample. It may be that this association reflects an elevation in blood pressure with increasing body size and with temperature. It is well established that blood pressure is positively correlated with body size, as estimated by the body mass index (BMI; weight/height2) (Seidell 1995). It follows that indigenous inhabitants of colder climates may then be subject to higher blood pressures simply as a result of their greater size. Ling's (1936) study of Chinese life insurance applicants suggests this possibility. Applicants from colder North China were taller and more massive and had higher average blood pressures and a higher prevalence of hypertension than applicants from temperate or tropical regions of the country. Although not conclusive, this finding does suggest that climate, working through body size, may play some background role in casual blood pressure.