Quite remarkably, not only is Whole Foods (NASDAQ:WFMI) managing to grow as other retailers struggle, but also its rate of growth actually seems to be accelerating. As the company turned in another strong quarter yesterday, CEO John Mackey stated that the natural foods grocer had hit a "tipping point" in terms of its growth trajectory, and the company broke into the Fortune 500, making its debut in the 479th position.

For the fiscal second quarter, sales increased 20% to $1.1 billion, comparable-store sales were up 11.6%, and net income was up 22% to $42 million. The results are particularly impressive given the tough year-over-year comparisons, including a record-breaking 17.1% increase in comparable-store sales in the second quarter last year.

The key question for investors is whether this growth trajectory, which is reflected in the current stock price, is sustainable. As I have written in the past, there is no doubt that Whole Foods is a great company. But at more than $100 per share, the stock trades at an enterprise value-to-revenue ratio of 1.6. That's pricey relative to other leading retailers -- twice as expensive as Wal-Mart (NYSE:WMT), which trades at 0.8 times revenue and four times as expensive as Costco (NASDAQ:COST).

My worries about valuation have increased in recent quarterly earnings calls, since I hear Wall Street analysts gushing with praise for company management and thanking them for another great quarter. There's no doubt that the stock is hot on Wall Street, which is a clear red flag for me when it comes to valuation.

When I recently wrote that I sold my shares because of valuation concerns, Mackey sent me an email articulating the growth story for the stock -- if the management team can continue to deliver against its growth estimates and if future growth prospects remain strong, the stock price should continue to grow at the same rate as the underlying cash flows. With cash flows growing at 20% or more per year, it's a pretty compelling story. He also gently pointed out that people like me have been saying the stock is overvalued based on fundamentals for years, but the stock price has continued to increase.

But for me, there is still too much risk at this price point to justify recommending the stock. I'm hoping that there's a hiccup, that the stock falls out of favor with Wall Street, and that the opportunity to buy a piece of this great business at a decent price comes along. Given the most recent quarterly results, however, it just doesn't look like that's going to happen any time soon.

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Whole Foods is a prior Motley Fool Stock Advisor recommendation, and Costco is a current one. To find out what else Tom and David Gardner are looking for, you can subscribe today with the benefit of a six-month money-back guarantee.

Fool contributor Salim Haji lives in Denver, where he shops at the Whole Foods near his house. He also owns shares of Costco. He does not own shares in any other of the companies mentioned.