Peter:
"my personal observation would suggest a link between abdominal obesity and diabetes"
No question about it. Abdominal adiposity is without doubt a risk factor for metabolic syndrome and type 2 DM. Abdominal adiposity is much worse than adiposity elsewhere. In fact, there is evidence for protective effects of adiposity elsewhere. It is the stomach fat that is the killer. That's why I suggested (following the advice of prominent researchers) using waist circumference or waist:hip ratio as much better target numbers than weight or BMI. The former are REAL risk factors; the latter are not, or not much, except at the extremes (BMI upwards of ~35).
"I am somewhat doubtful about the claim that you can have a high BMI but still be healthy, because I have literally never seen it in my office."
I believe it, and the likely reason is that they've never tried. It takes real WORK to get healthy, and the typical overweight person may not have ever undertaken that work. But it is quite possible to get healthy while still heavy, and it has been demonstrated repeatedly in the literature. Some weight might be lost, incidentally, in the course of getting healthy, but the main focus is getting healthy, not weight loss. Let the scale do whatever it is going to do that is consistent with health. More vegetables, more exercise, less refined food, less animal products, more fiber, optimal sleep, magnesium supplements, less TV, etc., etc.
"let's just say I'm open minded about it but sceptical."
As you should be.
"An alternative explanation for the 'I have a high BMI but I am healthy' argument, is that the high BMI person would like to believe that this is true and is deceiving him or herself."
I am certain that this happens a lot, and it is the danger of the HAES concept. But let's not overstate it. The HAES concept – the idea that one CAN be both heavy and healthy – does not suggest that every overweight person IS healthy. Health is determined, insofar as we can determine it, by a number of measures. If those measures are out of line, then you are unhealthy or you are at risk of disastrous health outcomes.
"If a morbidly obese person has their stomach reduced in size or bypassed, they lose weight rapidly."
Yes, of course. No one denies this. The question is: what is the health benefit? Always keep in mind that health cannot be cleanly inferred from weight. Some of the literature on bariatric surgery reports rather shocking loss of lean mass along with fat mass; hence "weight" is lost, including a great deal of muscle, vital organ and other functional tissue. Just how healthy are you going to be after that? I don't know. I suppose it might vary by case.
Bariatric surgery might be very useful for selected morbidly obese cases. What I object to is the assumption that "they lost a ton of weight, therefore they must now be healthy". Not so.
I am in favor of whatever improves people's health. If gastric bypass improves health, then I am for it. Does it? Maybe it does. But let's keep in mind that it is a very radical, risky and expensive way to improve health, if it does improve health.
"… this for me sets alarm bells ringing, because it sounds suspiciously like another variation of the above "wishful thinking" ideas: the obese person finds it hard to lose weight, so gives up trying to lose weight and justifies this to him or herself by saying it's OK to be obese."
Please turn off the alarm bells. Yes, it might be OK for them to be obese. That's the point. Why not build health first, and find out? It might be OK, and it might not be OK, but why not work on building health as the first goal? That is your role as a physician, after all: to treat people's states of ill-health, and help people become healthy, not to treat abstract numbers. Weight and BMI correlate only poorly with health. Other numbers correlate much more strongly – so let's treat THEM.
This is, I believe, the appropriate approach to the patient: "let's work on your health first, and then see if you need to lose additional weight or not". Maybe they will need to restrict calories and lose some more weight; maybe they won't. We don't know until we try. But we do know from the literature that huge health improvements are quite possible with minimal weight change.
I agree that the HAES movement – the political movement – goes too far in some respects. It overlaps with the "fat acceptance" movement, which is a valid civil rights movement, opposed to discrimination and bigotry which are rampant. It is important to embrace the civil rights aspect while at the same time attending to the real medical/biological issues.
Personal aside: I was obese, technically, for at least 10 years, but I have great numbers: BG, BP, lipids, all of it. Actually, a few of my numbers were slightly off, years back, but I corrected them, while still at a BMI of ~30 ("obese"). I eat a great diet. I train hard, often. I take megavitamins and many supplements. I am in great shape. Just recently I dropped some pounds and am at ~28, so now I am just "overweight", not "obese". But who gives a damn about my BMI or weight? Does it matter? NO! It does not matter. It has zero medical significance. I seek to improve my health and appearance, and weight and BMI are largely irrelevant to those goals. I want to gain 10-20 pounds of muscle, and I have a plan to do so. No doubt I will lose some fat in the process, so my weight and BMI will likely remain about the same. Still "overweight". Big deal. Who cares?