A question of obesity
This is all about why losing weight is not going to be one of my New Year Resolutions.
The starting point is that I am obese. 😟
My BMI is 35.
Or 35.1 to be precise – but that’s just it, a BMI is anything but precise.
BMI was initially devised as a descriptor of population by a Belgian who was trying to describe “normal” in numerical terms. Lambert Adolphe Jacques Quetelet (1796-1874) was primarily an astronomer who turned his mathematical interests to measuring people – in fact he measured almost anything he could and then turned the results into equations, including counting crimes. To give him due respect he did realise that it was inappropriate to apply population statistics to individuals.
That error was made by the insurance industry of the early 20th century.
Then along came Ancel Keys, another polymath scientist, who, in 1972, renamed the Quetelet index as Body Mass Index. (Keys is also responsible for the Mediterranean diet, apparently) He too recognised that BMI was a rough measurement, a very basic way of considering risk for, specifically, heart disease.
In the next 20 years BMI acquired an air of scientific authority that was given the stamp of approval by WHO in 1995, when they defined obesity as a BMI greater than 30.
BMI does have units – kg per metre squared – but it has become one of those figures that even scientists accept without units.
The problem with BMI
BMI is an inadequate assessment of nutritional status or mean muscle mass in an individual. Everyone knows that someone with the body of Arnold Schwarzenegger is not inherently obese despite having a BMI well over 30.
There are other measures that might indicate degrees of fatness. For example the waist circumference is related to the amount of fat surrounding the internal organs and the waist:hip ratio has been shown to predict cardiovascular disease in the general population. Oddly though, these have been designated as “more complicated to measure” – personally I find using a tape measure easier than calculating the square of the height.
BMI is here to stay, however, and has the monopoly on obesity statistics.
Am I too fat?
Now, to the main paper that I want to bring to your attention:
Transplantation in the obese: separating myth from reality.
Khwaja and El-Nahas
Nephrol Dial Transplant (2012) 27; 3732-3735
This paper is a review covering over 40 studies with the aim of evaluating the outcomes of kidney transplant in relation to a patient’s BMI. Both authors are renal physicians from Sheffield, UK, and their department has authored many respectable articles on chronic kidney disease.
The paper begins with a clinical scenario, a story I have heard before, it probably applies to many ADPKD patients as they approach the need for renal replacement therapy and potential kidney transplant: a 50 year old woman on haemodialysis who has been suspended from the transplant waiting list for being obese with a BMI of 35, while she has a friend who underwent successful kidney transplant without complications when her BMI was 43 – “It is unfair!” she claims.
Is her claim justified?
The British Transplant Society guidelines:
“While obese patients (BMI>30) present technical difficulties and are at increased risk of perioperative complications, obesity is NOT an absolute contraindication to transplant.”
The European Association of Urology:
” Transplant provides better survival and better quality of life in overweight dialysis patients and there is not enough evidence to recommend exclusion based”
The situation in US varies but many centres restrict access to transplant for patients with higher BMI, either by clinical or financial limitations. (Segev, 2008)
To play Devil’s Advocate, obesity HAS been associated with increased risks of delayed graft function (ie the new kidney doesn’t work so well at first). In fact one study suggested this occurs in 50% of transplant recipients with a BMI above 30. In another study obesity was associated with more wound infections. So it could be argued that obesity is a potentially modifiable risk, that insisting upon weight loss before transplant is obligatory stewardship of a rare resource.
The Obesity Paradox
Registries of haemodialysis patients have consistently shown for dialysis patients:
- Higher BMI is associated with improved survival
- Higher BMI is associated with a lower mortality rate
This is the obesity paradox.
There are several studies with data supporting this unusual statistic – all are referenced in the Sheffield review.
It is a paradox because it goes against the grain of both common sense and an overwhelming societal dictum “lose weight= good”.
I am not going to try to explain it, merely to say that it is evident that overweight patients do not always do worse than their mesomorphic companions.
Long term dialysis or transplant?
Transplanted obese patients do better than listed obese dialysis patients who do not receive a transplant. A kidney transplant for an obese patient will improve the quality of life and reduce mortality compared to staying on dialysis. This is applicable up to a BMI of 40, above which the survival advantage is smaller.
The important factor here is probably that the surgical team should have experience of transplants in overweight and obese patients. But it does make some kind of sense, after a transplant you are probably more active and able to follow a healthier lifestyle.
Should dialysis patients be encouraged to lose weight?
So what of the overweight (BMI 30-35) or obese (BMI 35-40) patient on dialysis? Should they be on calorie restricted diets in addition to all the usual dialysis restrictions? (Fluid, salt, potassium, phosphorus etc)
One study of over 14,000 dialysis patients (Molnar, 2011i) concluded that those who lost more then 5kg in weight had a death hazard ratio of 1.5 when compared with those who did not lose weight. These hazard ratios are not quite the same as relative risk but essentially the study suggests that some people who lose weight on dialysis are more likely to die than those who maintain their starting weight.
Analysis of the USRDS (US renal data system) shows that pre-transplant weight loss has no positive impact on graft survival or post transplant morbidity. This is observational data, it does not tell us anything about cause and effect.
The hypothesis is that weight loss in what is often a sedentary population leads to loss of muscle mass (called sarcopenia) more than loss of fat. Another study (Molnar, 2011ii) demonstrated that patients with low muscle mass do less well after a kidney transplant.
So there is a risk of losing muscle mass if you pursue an aggressive weight loss regimen when you are on dialysis. That is not to say “once obese always obese”, but it does indicate that you will need advice and support on how to lose fat and build up muscle.
Data from Florida transplant registries show that of the obese patients needing a kidney transplant, less than 5% manage to lose the required amount of weight. This leads to the question as to whether obesity is truly a modifiable risk factor.
After the transplant
In the longer term, there is no relationship between the pre-transplant BMI of the patient with survival of the new kidney (Chang, 2007). However, it has been shown that transplanted kidneys can be slow to start functioning in the plumper patients.
In fact, a low pre-transplant BMI is associated with an increase in post-transplant morbidity. In particular, patients lacking in muscle mass seem to have a lower overall survival after transplant.
The bottom line here is that obesity is comparable to other risk factors (such as diabetes, cardiovascular disease, antibody mismatching, kidney ischaemia time) when it comes to predicting outcome. In some ways insisting a patient has to lose weight before being considered for a transplant is almost as daft as insisting they get rid of the diabetes before they can go on the list. Nobody would condone denying kidney transplants to diabetic patients as a group, but doctors still consider denying obese patients as a group. Obesity is just one aspect out of many that can lead to complications. Of course it has to be taken into account in balancing the risks and there is evidence that morbid obesity (BMI>40) is associated with less favourable outcomes, but are the current guidelines too strict or is it that they are being applied over-enthusiastically?
Conclusions – from the paper
1. Obesity can increase the risk of complications but selected obese patients can achieve good outcomes after transplant.
2. There is no evidence that intentional weight loss before transplant improves the post-transplant outcomes.
3. It is not clear whether weight loss on dialysis is safe.
4. If counselling a person to lose weight before a transplant it needs multidisciplinary support to ensure that weight loss is achieved in a safe manner, avoiding the loss of muscle mass.
Conclusions – my own
I don’t think it is fair to use BMI as a single pre-screening factor when it comes to assessing patients for the transplant waiting lists.
BMI itself is an imprecise measure of both obesity and surgical risk.
Rather than try (and probably fail) to lose weight this New Year I will focus my effort on improving my muscles – spinach and Popeye here I come!
Chang SH, Coates PT, McDonald SP. Effects of body mass index at transplant on outcomes of kidney transplantation. Transplantation 2007; 84: 981–987
Eknoyen G: Adolphe Quetelet (1796–1874)—the average man and indices of obesity. Nephrol. Dial. Transplant. (2008) 23 (1): 47-51. doi: 10.1093/ndt/gfm517
Khwaja and El-Nahas, Transplantation in the obese: separating myth from reality. Nephrol Dial Transplant (2012) 27; 3732-3735
Molnar MZ, Streja E, Kovesdy CP et al. Associations of body mass index and weight loss with mortality in transplant-waitlisted maintenance hemodialysis patients. Am J Transplant 2011; 11: 725–736 (i)
Molnar MZ, Kovesdy CP, Bunnapradist S et al. Associations of pre- transplant serum albumin with post-transplant outcomes in kidney transplant recipients. Am J Transplant 2011; 11: 1006–1015 (ii)
Segev DL, Simpkins CE, Thompson RE et al. Obesity impacts access to kidney transplantation. J Am Soc Nephrol 2008; 19: 349–355