Is it just me, or is the Atkins/South Beach, low-carb phenomenon beginning to resemble the Nasdaq around the time it peaked? By that, I mean there is no shortage of empirical or statistical evidence supporting the claim that the very foundation of low-carb dieting is beginning to show signs of cracks that could possibly lead to collapse. Yet casual dining chains, fast food restaurants, grocery stores, and food manufacturers are ignoring the warning signs and continually rolling out new products or dishes designed to cash in on the popularity as long as it lasts.

To be sure, there are still millions of Americans devoted to the low-carb lifestyle. Also, as someone who could live by bread alone (and has at times) my personal views on this subject are somewhat partisan and get more biased each time I see a new display of "carb-friendly" products at my neighborhood Wal-Mart (NYSE:WMT) or Kroger (NYSE:KR).

Nevertheless, the popularity of low-carb dieting is clearly on the decline, and just as the Nasdaq meted out punishment to those who piled in shortly before it fell, companies that arrived to the low-carb party a little too late may also soon have regrets. Products such as Hershey's (NYSE:HSY) soy crisps and Heinz's (NYSE:HNZ) low-carb ketchup seem questionable to me, and one analyst has dubbed the Coca-Cola (NYSE:KO) C2 and Pepsi (NYSE:PEP) Edge mid-calorie colas as "either slow builds or something closer to dead on arrival."

More than half of former low-carb dieters have fallen off the bandwagon, and the percentage of Americans still watching their carbs has fallen into the single digits, welcome news to a company such as Panera Bread (NASDAQ:PNRA). This morning the bakery-café operator posted a 14% increase in second-quarter earnings to $6.5 million ($0.21 per share) on revenues that rose 31%. Though quarterly systemwide same-store sales edged up only 1.9%, the number rises to 2.7% over the past month and is expected to be as high as 4% during the second half of the year.

It is far too early to begin writing the obituary of the low-carb craze, but it doesn't take an intuitive leap to draw the conclusion that interest is clearly waning. One thing is for certain: The low-carb trend will play a role in many investors' portfolios, regardless of the direction it takes, and it is never too soon to assess the impact it might have on yours.

Fool contributor Nathan Slaughter would like to note that the word carb appears 11 times in this article. Can you find them all? He owns none of the companies mentioned.