BMI, Overweight, and Longevity

     A recent article in the Journal of the American Medical Association (JAMA, 2013;309: 71-82) created quite a stir when it concluded that adults who were overweight but not obese had a 6% lower all-cause mortality rate. Nutritionists rushed to say that this did not mean that you should gain weight, although if the study had shown the reverse I am sure they would have unanimously recommended weight loss. This was not the first study to reach this conclusion, so I thought I should review it here, along with examining exactly what the BMI purports to measure since all obesity labels were based on the BMI (Basal Metabolic Index) results.
 
     The BMI was first defined by Quetelet in the 1800's as a surrogate number for estimating the amount of body fat. It is defined as mass(kg)/height (m) x height, i.e. the mass divided by the square of one's height. Since your mass increases with your total body volume, it scales with the cube of your height, so it is immediately obvious that only dividing by the square of the height will overestimate the total body fat of taller individuals. Similarly, muscular athletes have very little total body fat, but since muscle has mass, the BMI will  overestimate their total body fat as well. It is trivial to note that since most elderly patients have some degree of osteoporosis and therefore lighter bones, the BMI will mistakenly classify some of them as underweight.
I should also mention here that in 1998 the U.S. lowered its cutoff for the lower limit of overweight from a BMI of 27.8 to 25.0, in accordance with the WHO standards, thereby immediately reclassifying millions of Americans as overweight who the day before the change had "normal" weight. In addition, doctors in the US do not agree on the healthy lower limit of the BMI  for adult women, and the WHO does not have the classification of "underweight". And the 0.5 to 1.0 inches in height that you lose with age increases your BMI without you actually gaining any weight.

     The consensus in the US is that a BMI of 18.5 to 25 is normal weight, 25 to 30 is overweight, and greater than 30 is obese. Using these definitions, there have already been two published studies, one of diabetic patients and one of patients with ASCVD, and both demonstrated that overweight patients had a lower death rate than both normal weight and obese patients. No one has an explanation for these results, nor have I even seen theories that attempt to explain them. The JAMA study looked at 97 studies of all-cause mortality, encompassing 270,000 deaths and over 2,800,000 subjects, so it is fairly comprehensive.
We are left with the paradox that being overweight by BMI standards statistically increases your risk for high blood pressure, diabetes, ASCVD, stroke and some cancers, yet overall it is protective of mortality.

     In a previous blog I listed the 11 countries in order of longevity. I will repeat the list here adding the mean BMI of each country. Except for Japan, #1 and with the lowest BMI, there is no clear pattern:
Japan---22, Switzerland---25, Australia---26, Italy 23.5, Israel---25, Iceland---26, Spain---24.5, France---23.5, Canada---25.5, Singapore---22, and New Zealand---26.6.

     So once again we are left with "a beautiful theory ruined by an ugly fact" to quote a Nobel scientist. Perhaps we should re-normalize the BMI values and re-define normal weight. Or perhaps we should have a separate classification of "healthy BMI" and "unhealthy BMI". Or, more likely, there is a confounding fact of which we are unaware which would explain the results, but so far there are no candidates. It does make me feel better with my BMI of 27 because at 6'2" I doubt that I could get down to 185 pounds, even if I wanted to. I think we underestimate the genetic effects on us of our parents' diseases, especially parents of the same sex.

Diet, Health, and Longevity

     This blog was stimulated by a front page story in the New York Times of Saturday, October 6, 2012. It described the response of tens of thousands of high school students this fall to the new federal mandate that school cafeteria lunches be "healthy". The students are throwing the lunches away, or boycotting the cafeteria, or buying their food from vending machines. The same children who wouldn't eat their spinach as two year olds will not eat rice cakes and whole wheat pizza when they are sixteen. Come to think of it, neither would I (don't rice cakes really taste like styrofoam?). And the food pyramid that students are taught to guide their healthy eating habits in schools today is the inverse of what we were taught as students: What we were taught should be at the top of the food pyramid is now placed on the bottom, and our bottom is now their top.

     Do we really know what "healthy" food is? Do we really know which diet is "best" for us? What should be the nature of our diet to maximize our life span? Do we know if optimal health requires us to eat three meals a day rather than two meals or six meals? And at what time of the day should we eat our largest meal?  Is salt really bad for you, or sugar? How much or little fluid should we drink, and of what kind? Is it safe to fast for 24 hours?  What do we do with the fact that human skeletons from 9,000 B.C. when humans were hunter-gatherers and ate mostly meat  show no signs of tooth decay, while skeletons from 6,000 B.C. when humans began to cultivate grain and ate carbohydrates show extensive tooth decay and jaw abscesses? (saliva turns starch to sugar in your mouth, but only can break meat down into its proteins) What about the many benefits of coffee (described in an earlier blog), or the benefits of moderate alcohol consumption or dark chocolate? Should we eat until we are no longer hungry, or stop when we are only partially filled? Is the Japanese diet responsible for their high rate of stomach cancer or for their  longevity? or both? or neither? Isn't the reason that everyone likes ice cream due to the fact that it is really flavored frozen mother's milk, rich in sugar and fats?

     I will now list the top countries for life expectancy, first from birth, and then from age 40. There is a slight variation because infant mortality enters into the total life expectancy from birth: For instance the Unites States rates 49th in life expectancy from birth, but 33rd in life expectancy from age 40. If diet affects total health (as opposed to the amount of food or calories eaten at a single sitting) then we would expect a clustering of countries from the same area of the world who consume approximately the same diet, and we will see that this is not the case.
     Life expectancy from birth in descending order: Japan, Singapore, Australia, Canada, France, Sweden, Switzerland, Israel, Iceland, New Zealand, Italy.  Life expectancy from age 40: Japan, Switzerland, Australia, Italy, Israel, Iceland, Spain, France, Canada, Singapore, New Zealand . I have included 11 countries rather than 10, because I am not certain if Singapore should be classified as a city rather than as a nation.

     Japan and Australia rank #1 and #3 in both lists, and they have totally different diets and eating habits. The French paradox pops up, of course. Israel and Canada both rank high, and again they have radically different dietary habits. So if we look at total longevity, it is difficult to draw any conclusion about the benefit of any particular diet. I should also mention that the longest lived people born in America are Asians, so it would seem that genetic heritage might be more important than diet in determining longevity. As far as I am aware, no substantial research has been done on the country-wide genetic contribution to longevity, but we do know that there are many genetic factors which are common to people of a given country. As a trivial example, all 100% Inca indians from Peru have type O blood, but very little research has been done on the effect of a blood type on one's health. We know that almost every female Pima Indian from the American southwest has severe gall bladder disease and a cholecystectomy by age 18. The Parsees in India, who are descended from the Zoroastrians (who believe in a God of good and a God of evil) who were expelled from Iran and Iraq, have an exceptionally high rate of breast cancer. Much more research has been done on disease incidence and risk factors than on the incidence of health and longevity factors, for obvious reasons: Disease is immediately obvious, but the absence of disease is more difficult to measure, and longevity requires waiting for a life span to ensue.

     The problem in recommending a "healthy" diet is one of insufficient information. We only have biomarkers which are surrogates for health and longevity. A white laboratory rat reproduces every 30 days, so in six months I will have studied six generations of rats and have a fairly good idea of which diets are beneficial or harmful for them. Humans reproduce every 25 years (on the average) and the longest detailed interventional diet study was 5 years, with a few tracking studies of 10 years and the Framingham study also available. So we just don't have enough diet data to advise healthy people (except the trivial advice to not get fat, not to smoke, and to have a glass of wine three days a week). And we have gotten burned with misplaced advice: Vitamin E supplements were shown to increase the rate of heart disease, and  the anti-oxidant beta carotene was shown  to increase the incidence of lung cancer in smokers. Finally, there is absolutely no evidence that diet is more important than the genetic makeup in a country's overall longevity.