Indications of CR status: Blood pressure versus BMI

[GenGenimney]

Working on my theory that we already see plenty of CR (but very little CRON) in the wild, where is the point that BMI results in much lower blood pressure, and at what BMI is there no longer a positive effect, understanding that there are confounding variables in population studies?

 

It's hard to find studies that don't just go "BMI <25," but I did find one that not only showed values for every BMI point but was also a study of lean populations (in Africa and the Caribbean) AND went down to a BMI of 17!  http://hyper.ahajournals.org/content/30/6/1511.full

 

 For men, BP had a positive correlation with BMI all the way down to a BMI of 17.   For women, it leveled off at a BMI of 21, mostly due to much higher BPs for the lower BMIs at ages above 45, but the correlation under the age of 44 is, if anything, even more dramatic than that of the men's, so I'm tentatively suggesting lifestyle factors at play here (maybe increased sarcopenia/increased fat percentage after age 44?) but also likely a BMI-dependent floor on BP, with any further decrease dependent more on, say, nutrition.

 

The authors said:

 

 

We have demonstrated nonlinearity (threshold) in the relationship between blood pressure and BMI for women in a pooled population of individuals from low-BMI populations in Africa and the Caribbean. The proposal by Bunker et al14 of a BMI threshold at approximately BMI=21 was confirmed in this study, although only for women. In the case of men, we observed nothing that would contradict published suggestions of a continually decreasing mean blood pressure at lower BMI values.12Furthermore, we have documented a consistent pattern of steeper BMI slopes for men than for women, with a pooled value for the male/female slope ratio of roughly 2.0. Some of this effect can be explained by the nonlinearity in the relationship for women, which underestimates a slope that is restricted to the linear form. When we consider the pooled slope value above the threshold point for women, the male/female ratio reduces to 0.90/0.57=1.58, for example.

 

THey pooh-pooh the idea that BMI itself is to credit/blame:

 

 

Physical activity can have a substantial effect on blood pressure.36 We have recently shown a strong inverse correlation between energy expended in physical activity and fat stores in this same Nigerian population using the doubly labeled water technique.37 BMI should therefore be viewed conceptually as a proxy for other causal exposures; whether it is diet, hormone changes, physical activity, or other factors that link increasing relative weight to rises in blood pressure cannot be determined at the present. It is clear from the data in this study, however, that obesity, per se, is not required for this association to be manifest, since for men at least BMI values <21 kg/m2 appear linearly related to blood pressure.

 

But while PA EE and leanness are traditionally linked to lower BP, I don't think we should discount lower BMI due to CR versus heavier people.  The PA EE in studies I've seen is quite a bit lower for men below a BMI of 18.5 in Africa specifically (sorry, don't remember the studies).

 

So there's no answer here...but maybe there's a suggestion that "CR" probably shouldn't be thought of as a switch but rather as a continuum with the extremes obesity on the other end.  There is no flat then asymtoptic curve here, even though the study included moderately underweight people, into "lower PB."  Instead, it was a gradual, linear slide (at least until a BMI of 21 for women.)  They speculate that the overall BMI/BP graph would be sigmoid, and that surely seems the most likely.  The BMI of 21 for women may be that point under the nutritional conditions of those populations.

[GenGenimney]

Another one, which is kind of crappy.  Normal weight men and women who become underweight achieve lower blood pressure in spite of increasing age:  http://www.nature.com/ijo/journal/v29/n6/fig_tab/0802944t4.html#figure-title
 
This is part of the Nord-Trondelag Heath Study (Norway):   The researchers examined changes in weight with changes in BMI.  Again, it's one of the few that looks any anyone who is underweight.  There may or may not be an inflection point at a BMI of 21 because "overweight" and "underweight" are such broad categories.  I'm not sure how many people were in the underweight category at any point....  This is pretty rough and ready but interesting, nevertheless. 
 

 

Our result supports an independent effect of change in BMI on change in SBP and DBP in both women and men, and that people who increase their BMI are at increased risk for hypertension. .... Additinally, (sic) to change BMI category (WHO's categorisation) from the first to the second survey had a strong effect on SBP and DBP, which was independent of initial and attained BMI category.

 

Source:  http://www.nature.com/ijo/journal/v29/n6/full/0802944a.html