Every time he disembarked at O'Hare airport, it seemed, he bumped into a family of five oversized creatures waddling gently towards the jetway clutching foot-long hot dogs and giant drink cups. To his moderate English eyes, Americans looked less like human beings than livestock.
The official statistics, such as they are, tend to confirm my friend's perceptions. Americans are the fattest people on the planet. We are also growing fatter by the moment. Not content merely with winning this particular contest, we are busy running up the score.
The last government report on American fatness claimed that cases of obesity were up by half in the past decade.
We have expanded so fast that we have all but squeezed the breath from the few brave souls who not very long ago dared to apologise to the thin world on our behalf.
It's not our fault, they said, we Americans are merely cursed by our genes. Clearly this was a fiction. No "fat gene" can explain the speed with which we have swollen. If we are growing fat so fast, it is because we eat more and exercise less than we used to do. End of story.
And so the debate on fat is now shifting - away from causes and towards effects. Authors are now writing whole books about the social consequences of obesity. Two such authors are now waged in a delicious pitched battle in Slate, the Internet magazine.
Richard Klein, the author of Eat Fat, argues that the prejudice against fat people is far more malign than the fat itself, and that fat people should be left to enjoy their gigabuckets of buttered popcorn.
Michael Fumento, the author of The Fat of the Land: The Obesity Epidemic and How Overweight Americans Can Help Themselves, argues that fat is not merely the cause of 300,000 untimely American deaths each year, but a dry rot set deep in the American soul.
Mr Klein attacks Mr Fumento for being yet another fat bigot. Mr Fumento, in turn, attacks Mr Klein for being unscientific - and fat.
Readers are encouraged to grab a family-size bag of Nacho Cheese Doritos and watch them fight it out. Both men are worthy of attention.
But in spite of their fierce contention, or perhaps because of it, neither makes the incontrovertible economic point: our fat is a great source of prosperity. If the American economy continues to expand, it is at least in part because we Americans are expanding too.
The most immediate contribution to economic growth made by fat people is their demand for foodstuffs. The average American consumes something like a thousand calories more each day than his European cousin, and each of those unnecessary calories must be bought and paid for.
It is true that in extreme cases where people literally eat themselves to death, their consumption is thereby curtailed.
But a rough back-of-the- candy-wrapper calculation suggests that their lifetime consumption still far exceeds what it would have been had they chosen instead to remain thin. The nation's fat pours profits into the pockets of giant corporations, and creates jobs for people who might otherwise go unemployed: fat people.
But the consumption of surplus grub is just the starter button on a powerful fat-driven economic machine. The people who buy the food required to become fat are then made to feel guilty about being fat. Consumption in America has become an energetic cycle of fluctuating falls and redemptions; our consumers have learned to pay coming and going.
The guilt they are made to feel for their surplus consumption underpins an industry of weight loss programmes and diet pills and late-night television wisdom. (Where else in the world would two books about fat simultaneously find a receptive market?) The excess consumption of these in turn increases demand for physical and mental health care.
And that is not all. The presence of many hugely fat people stimulates the imaginations of those who are not fat. They come to view fat not as something remote that lands on other people. Fat is a perpetual threat, squeezed tightly into the seat beside them on every aeroplane they ride.
Their terror motivates them not merely to abstain from overeating but to consume the antidotes to overeating - health clubs and exercise equipment and a whole range of fat-free foods. (Where but in America would food companies be employing droves of people to come up with fat-free cookies? What is the point of a fat-free cookie, anyway?)
I could go on. Once you turn your mind to the ripple effects of fat you cease to fear it. If you fear anything you fear its opposite. The challenge for the leaders of the 21st century may well be to find ways to continue our current expansion. Fat as we are, we must grow.
q Michael Lewis is the author of 'Liar's Poker', the best-selling account of life on Wall Street in the 1980s.