There's plenty of obesity at certain levels of society in what you call 'third world' countries, jft. Mostly among the ruling classes who keep the rest of their nationals in the poverty we'd all like to see wiped out. Obesity doesn't actually mean eating well, or even eating a lot. It means eating too much of the wrong foods, drinking sugary pop or beer way too much, and exercising far too little. What you forget, perhaps, is that the people raking in the cash from well-meaning charities, from governments, from UN relief agencies, etc., are the people who are in positions of trust (ministers, clerks, regional governors, the police, the army, etc., etc.) who should be trying to raise their own country's standards. But they won't and they don't - how many examples of thieving from aid do you need? Go right back to the days of the Golden Throne of Ghana in the 1960s onwards, and you'll see that thriving, economically-healthy, and just plain HEALTHY African countries were rapidly drained of their financial foundations by the venal and often despotic rulers who were elected (usually under threat of force) to guard them.
As for places like the Maghreb countries, don't delude yourself a minute longer in thinking they're 'third world', because they don't, and they're not! From elsewhere, Eastern European and Asian countries are providing us with not only cheap labour, but the side-effect of extortionists, gangsters and pimps along with the genuine workers. Charming, not.
Your anger and concern would be better placed, may I humbly suggest, in lobbying hard for the rulers of the worst-affected countries to stop making idiotic and vicious tribal or religious wars, and for the largesse of the better-behaved world to go DIRECTLY into the hands of those most desperate. Unfortunately, believe it or not, it usually can't, due to the stranglehold on airfields, main roads and railways by the warring factions, who merely grab it for themselves and to sell on at huge profits in a very healthy black market. The UN doesn't have a mandate to invade these countries and bash a few heads together to knock some sense into them, but sometimes I think someone should.
You're right, of course, that we shouldn't be buying 25 varieties of iPod, a new car every year, the latest stupid trainers, designer labels and millions of over-packaged food every day in THIS country while elsewhere babies die of dehydration - try telling that to the population, though, and they'll say it's up to other countries to get up to our standards, not for us to drop back (as we have been there before!) to theirs. In order to accomplish anything near that, though, the corruption and repression in those countries has to be stopped. Your mission, should you choose to accept...