I agree that weight loss, obesity, and generically health issues are far more taboo than I wish they were. I would love for it to be a more open topic, but it really isn't, and that makes it difficult to discuss.
If you want to compare obesity to substance addictions, that's fine as far as the similarities go. There are some differences though that make it a whole 'nother kettle of fish.
"Interventions" and confrontations with family members regarding substance abuse is riskyand messy business. Many professionals believe that interventions and confrontational discussion of the substand abuse should be done only with the presence and supervision of a trained expert (and without that supervision the confrontation tends to backfire, even making the situation worse in many cases). Intervention and confrontation tends to work out positively only when the person agrees to go immediately into in-patient treatment (the goal of most interventions). Some experts argue that interventions that don't immediately result in inpatient treatment, tend to make the substance abuse worse, not better.
In that regard, there's almost no option for that kind of help. There are very few inpatient treatment for obesity and weight loss (and the treatment programs that do exist, are almost never covered by insurance), so even the substance abuse model doesn't work very well here. Finding and getting outside help is a lot harder for weight issues than for substance abuse issues.
Having had family members that were constantly on my *** about my weight loss, I didn't find it helpful. Mainly because everyone had an opinion on what I was doing wrong, but they had very little idea of how to be helpful in fixing the problem (most of their advice sucked ***, even more so when it came from people who had similar or other behavior control issues - I felt "fix yourself, before you try to fix me.")
I think it is possible to talk about taboo subjects with friends and family members, but it's difficult, and so difficult to do well, that it almost always backfires. Many people are willing to start the fire, but no so willing or able to hang around to deal with the fire and help put it out. Instead folks dive bomb you with criticism, and then get mad at you when you don't take it with a smile and thank yous, and really get pi**ed if you share some of your own criticisms of them (they can dish it out, but they can't take it).
If you're not prepared to deal with the possible backlash, which can include anger, rage, dispair, feelings of betrayal... it's best not to open the discussion. This is as true about alcohol and drug abuse as it is eating disorders and obesity. You have to not only weigh the potential consequences, but be prepared for any and all likely responses. Are you prepared to dash someone's world to pieces AND are you willing to stick around to help them pick up those pieces. Or are you expecting to criticise, maybe make a few suggestions, and expect the person to be grateful and able to run with your suggestions with no further help or input from you (it's a pretty safe bet, to say that's NOT going to happen).
I would not bring up what I saw as a friend's alcohol abuse either, unless I was prepared for the backlash. Am I willing to lose the friend? Am I willing for the person to verbally attack me and hate me forever, because I told them what they didn't want to (and may or may not have needed to) hear from me. Bringing up a friend or family member's substance abuse shouldn't be a casual subject either. It should be well thought out, and the consequences need to be weighed. It's not generally a topic for a casual conversation, and it's rarely a pleasant or even civil one.
I think most overweight people realize they're overweight (I think most alcoholics realize they have a problem with alcohol). I don't think comments or confrontation ever helps unless the person is already on the brink of making the changes on their own. (You can't drag a person into change, at best you can nudge them in a direction they already were heading).
Unlike food addiction and obesity, a person can be committed against their will to alcohol or substance abuse treatment (it's not easy, but it at least theoretically can happen). That option isn't available with obesity. Voluntary treatment (inpatient or outpatient, or even self-help groups) aren't always easy to find, at least not affordably. So it's a little riskier to push a person over the edge when there's no commonly available or agreed upon way to pick up the pieces.
I think "I'm worried about your health," is about as far as you can go without potentially making the situation (and your relationship with the person) worse.
You have to decide whether saying something is more important to you than avoiding the negative backlash likely to occur. In any case, if you decide to say something, I'd recommend choosing your words very carefully, if you want them to have a positive impact. Being positive without sounding cruel and judgemental is very difficult, and if you can't do it, it may be better to say nothing (but I think that's true of substance abuse issues also. Saying something only works out positively when the person is in the right frame of mind for change already - otherwise it just sounds like judgemental nagging).