Altitude effect of health in the USA

In the UK and in Europe overall there is a strong latitude effect in life expectancy. and the incidence of a variety of diseases. As the distance from the equator increases, life expectancy decreases. 

Multiple sclerosis shows a latitude effect.  The world's highest incidence is in Scotland, and the incidence decreases with residence towards the south-east of the UK, and dramatically so south of the European Alps. 

Coronary heart disease (CHD) is similar, and again Scotland has the world's highest incidence. There is a UK gradient with the lowest incidence in the south-east, gradually increasing to the north-west. This suggests a climate effect, disease incidence increasing as sunshine intensity and hours per annum diminish.

UK : life expectancy, greatest in the south-east

UK : Sunshine, greatest in the south-east

We can see in another Post that the health of the population of Belfast, Northern Ireland, is very much worse than that of the population of Toulouse, France. It could only be explained by the difference in latitude, Belfast being about 1000 km north of Toulouse, with resulting lower energy of sunlight at ground level. The UVB component of sunlight is almost entirely (about 98%) filtered out by the atmosphere. It is this UVB that is vital in the production so vitamin D, itself essential for immunity. At close to sea level at the latitude of Belfast, it is only when the mid-day sun is less than 45 degrees from the vertical that the atmosphere will allow sufficient UVB to enable vitamin D production.

In the USA, CHD mortality shows a similar but much weaker latitude effect. It is different in the UK because in the UK there is very marked gradient of weather as well as just latitude. The north west is under the influence of cold arctic air and warm water from the Gulf Stream giving rise to a great deal of cloud cover and rain. The south-eastern parts of the UK have a much more continental climate with much less cloud and therefore more hours of sunshine per year.

However, age-standardised deaths from CHD and CVD (cerebrovascular disease - ie stroke) in the USA show a powerful longitude effect. 

The latitude of between 37 and 38 degrees north of the equator is a convenient latitude to choose. We find Washington DC on the east coast, Kansas City Denver, and then San Fransisco on the west coast. 

USA, 37–38 degrees north of the equator

The land mass profile is as follows:

USA, land mass profile

The next stage is to look at death rates, the combined death rates in men from coronary heart disease (CHD) and stroke, cerebrovascular disease (CVD).

USA, death rates from CHD and CVD

Putting them together emphasises the reciprocal relationship.

USA, land mass and death rates from CHD and CVD

It is quite clear that living at a high altitude in the USA gives a survival advantage, reducing risk of death from CHD and from CVD.

It is important to note that these are age-standardised death rates. Crude death rates might show low death rates in "young" cities such as Seattle, but this is not useful to medical science. Data must always be standardised.

New Mexico

The same can be seen by examining death rates in various cities in New Mexico, where there is a wide range of altitude. We see the lowest death rates in cities at the greatest altitude.

New Mexico, inverse association between altitude of residence and death rate from CHD 

There is a negative correlation between altitude of city of residence and death rates from coronary heart disease. Living at a high altitude is protective, good for health and life expectancy.

Once again we are dealing with observational data. If robust the observations are real but the explanation of mechanism becomes conjectural. It has been suggested that the low atmospheric pressure at high altitude is protective, but th is not really plausible. There is no physiological reason why low atmospheric pressure should be good for health and lengthen survival.

Perhaps the simplest explanation is that at a higher altitude the air is indeed thinner, and this leads to a greater intensity of ultra-violet light and therefore a greater production of vitamin D in the skin. The blood level of vitamin D is the only natural index of exposure to the sun.

It has been known for a long time that living at a high altitude is beneficial to health. This is explained in the Heidi story. Healing of tuberculosis at high altitude in the Alps led in the early years of the 20th century to the recognition of the importance of the sun in human health, and the development of the sanatorium movement, especially in Switzerland. It also led to the identification of vitamin D synthesis in the skin. 

The observations of the altitude effect in the USA strengthens the importance of the sun and vitamin D, particularly in respect of cardio-vascular disease.

References:

Fabsitz R, Feinleib M. Geographical patterns in county mortality rates from cardiovascular diseases. Am J Epidemiol 1980; 111: 315-328.

Mortimer EA, Monson RR, MacMahon B. Reduction in mortality from coronary Heart disease in men residing at high altitude. N Engl J Med 1977; 296: 581-585.