We've reported on several studies showing vegetarians have a lower incidence of hypertension than consumers of the Western diet. This is because vegetables and fruits have a high potassium to sodium ratio. If you don't add salt to the vegetables, a vegetarian diet will have a high potassium to sodium ratio. You will be unlikely to develop hypertension. But there are occasional reports of eating more vegetables and fruits leading to hypertension, such as last week's report from Korea. When you read such a report you need to look deeper.
This week's post is about two more such reports. The first (1) reports that a non-vegetarian diet is more protective against hypertension than a vegetarian diet. The second (2) reports that an urban group of Indians (a group associated with more hypertension) ate approximately 50% more fruits and vegetables than a matched rural group (a group associated with less hypertension).
The reports are out of India, which is a country with more vegetarians than any other nation. Estimates are that between 30 and 50% of the total population are vegetarian.
India is involved at present in a large migration of its populace from rural to urban locations. Concurrent with this migration has been an increase in hypertension.
It's Not Just Migration
But the increase in hypertension has not been due only to the urban transition. In 1949 the prevalence of hypertension in urban populations was only 1.24%. It rose to 36% in 2003 (1). In rural India, the prevalence also rose. It went from 1.99% in 1958 to 21% in 1994 (1). There is something changing in both rural and urban India to account for this rise in both urban and rural hypertension.
Fried Fish Protective?
The first study (1) looked at the diet in urban Malda in eastern India. It concluded that being non-vegetarian protected against hypertension. The non-vegetarians had a lower prevalence of hypertension than vegetarians. The researchers felt fish consumption and the cooking oil used (mustard oil) may be why the non-vegetarians had less hypertension. However the researchers did only a limited investigation of dietary factors and food preparation. They did not consider sodium or potassium content at all.
The second study (2) compared the diet of urban and rural populations from several areas of India. They found that the urban group ate more fruits and vegetables. Like the first study they did not consider the sodium or potassium content of the diet.
The second study compared vegetarians to non-vegetarians (the group in the first study found to have less hypertension). The non-vegetarians ate only a minimal amount more meat and fish than the vegetarians. The non-vegetarians ate 19 to 28 g of meat daily and approximately 6 g fish daily. This is an average of only 2/3 to 1 oz. of meat a day and an ounce of fish every 4 to 5 days. The non-vegetarians were almost vegetarian.
A more likely reason for urban dwellers to be hypertensive is related to urban lifestyle. Urban lifestyle has been associated with an increased prevalence of hypertension in a large number of studies. These studies have found an association of hypertension with less physical activity and increased weight in urban migrants.
But the most glaring problem with the first and second studies is not looking at the sodium and potassium content of the study subjects' diets. Multiple studies have shown an increase in sodium intake and a drop in potassium intake when populations move from a rural to an urban location. This is especially important since salt is added to a great many dishes in India. The daily salt consumption varies from 5 grams a day to 30 grams a day among differing populations in India.
Other studies about Indian hypertension have looked at salt intake. A study (3) of sodium in the urine (and thus the diet) of research participants from a state in southern India showed a significant increase of sodium in the hypertensive group compared to the normal pressure group.
Tea Garden Workers
An especially revealing study (4) was done of tea garden workers in northeast India. It showed a much higher prevalence of hypertension than is generally present in India. This is despite the tea garden workers in the study being more physically active and thinner than other Indians, which is opposite the usual findings associated with hypertension.
Hypertension in these tea garden workers showed a prevalence of 61% among the adults. Other studies of tea garden workers, however, put the prevalence in the range of 46%. In either case, this is much more than the average rate in India, which a recent meta-analysis (5) put at approximately 33% of urban and 25% of rural adults.
30 Grams Of Salt A Day
What is unique about the tea garden workers is their high intake of sodium. In one study (6) they were found to consume from 20 to 30 grams of salt a day. This is compared to the average intake by Indians of approximately 10 grams a day.
Just as in Korea, eating more fruits and vegetables will not protect against hypertension when a great deal of sodium is consumed along with the fruits and vegetables. The protective aspect of fruits and vegetables is primarily their high potassium and low sodium content. Other factors such as polyphenols, antioxidants, and polyunsaturated fats may help. But the epidemiological studies and basic science studies of how these factors protect have not yielded the clear picture that the dietary potassium sodium ratio has.
Tables of potassium and sodium content can be found for high potassium fruits here and high potassium vegetables here. Links to tables for other foods can be found by clicking the Links To Food Potassium Tables tab at the top of the page.
1. Study of urban community survey in India: growing trend of high prevalence of hypertension in a developing country. Das SK, Sanyal K, Basu A. Int J Med Sci. 2005;2(2):70-78. Epub 2005 Apr 1.
2. Dietary intake and rural-urban migration in India: a cross-sectional study. Bowen L, Ebrahim S, De Stavola B, Ness A, Kinra S, Bharathi AV, Prabhakaran D, Reddy KS. PLoS One. 2011;6(6):e14822. doi: 10.1371/journal.pone.0014822. Epub 2011 Jun 22.
3. Relationship of sodium and magnesium intakes to hypertension proven by 24-hour urianalysis in a South Indian population. Chidambaram N, Sethupathy S, Saravanan N, Mori M, Yamori Y, Garg AK, Chockalingam A. J Clin Hypertens (Greenwich). 2014 Aug;16(8):581-6. doi: 10.1111/jch.12361. Epub 2014 Jun 17.
4. Hypertension and its risk factors in tea garden workers of Assam. Hazarika NC, Biswas D, Narain K, Kalita HC, Mahanta J. Natl Med J India. 2002 Mar-Apr;15(2):63-8.
5. Hypertension in India: a systematic review and meta-analysis of prevalence, awareness, and control of hypertension. Anchala R, Kannuri NK, Pant H, Khan H, Franco OH, Di Angelantonio E, Prabhakaran D. J Hypertens. 2014 Jun;32(6):1170-7. doi: 10.1097/HJH.0000000000000146.
6. Prevalence of modifiable cardiovascular risk factors among tea garden and general population in Dibrugarh, Assam, India. Mahanta TG, Joshi R, Mahanta BN, Xavier D. J Epidemiol Glob Health. 2013 Sep;3(3):147-56. doi: 10.1016/j.jegh.2013.04.001. Epub 2013 May 25.