Warren Buffett, through his mega-conglomerate Berkshire Hathaway (NYSE:BRK-A) (NYSE:BRK-B), owns 400 million shares of Coca Cola (NYSE:KO)

That's just north of $15 billion for those of you keeping score at home. By dollar value, it trails only Wells Fargo (NYSE:WFC) in Berkshire's publicly traded investment portfolio. 

But before you jump on the Buffett bandwagon and scoop up some KO shares for your long-term portfolio, you may want to consider this concerning fact:

Growth at Coke is slowing down. 

Breaking down the numbers
The backbone of Coca Cola sales has been and remains in the United States and Latin America. But the growth story over the past 20 years has been overseas. For Coke bulls, the thesis for the next phase of growth is increasing per capita consumption.

In the chart below, we can clearly see that Coke has plenty of room to grow in Eurasia, Africa, and the Pacific. 

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Both Eurasia/Africa and the Pacific fall below the worldwide per capita consumption average. And that's despite phenomenal growth over the past 20 years. 

From 1992 to 2012, per capita Coke consumption increased 104% worldwide. It increased 174% in the Pacific, and, wait for it, 338% in Eurasia & Africa!

Last week, I wrote about the four countries most critical to Coke's future growth, "critical" being the choice word. 

Your dentist always warned you to avoid sweets
One region has bucked the per capita growth trend over the past 10 years: North America. The primary reason, in my view, is health related. North American consumers are increasingly turning away from the sugary deliciousness of soft drinks in favor of healthy alternatives. Fellow Fool Ted Cooper does an excellent job explaining the risks here.

The fact still remains that Coke and other Coca Cola brands are exceedingly popular in the U.S. and Canada, even if consumer preferences are evolving to health-conscious alternatives. As ubiquitous as Coke is, it's not altogether surprising that the market would be saturated and experience a flat or slightly negative growth trend. 

But the same cannot be said for emerging markets. With so many, including myself, looking to Africa, Eurasia, and the Pacific for growth, it's hard to explain away the declining growth rates in these key markets. 

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With North America as the only exception, per capita growth is positive in all of Coca Cola's global regions. But, as this chart makes it clear to see, that growth rate is declining.

Even more troubling is that the decline is not limited to just the U.S. or Europe; it's evident across the board (Latin America being the exception that makes the rule). Particularly so as our investment thesis hinges on the large and upwardly mobile populations of Asia, Africa, and elsewhere.

The one stock I'm buying for my grandkids this Christmas
Recently, I named Coca Cola as the one stock I'm buying my grandkids this year. I made that decision in spite of the risks discussed in this post. What specifically mitigates the declining per capita growth in the emerging and developed markets?

Coca Cola has a product in demand, one of the strongest brands in the world, and room to grow -- even if that growth is marginally slower over the next 20 years than it was in the past 20. 

And don't forget how the once-in-a-generation financial crisis and recession that began in 2007 affected businesses and consumers around the world. For the emerging markets, Coca Cola is more of a luxury than it is in the U.S., likely amplifying the deceleration visible in the chart above.

For example, last year I traveled to Phnom Phen, Cambodia, and routinely paid US$1 for a can of Coca Cola. So despite Cambodia's per capita GDP being approximately 52 times less than that in the U.S., Cambodians still pay essentially the same price as Americans for a can of Coke. When times get tough, people cut back on luxuries before essentials. 

As the world economy continues to recover from the recession and financial crisis, I anticipate Coke's per capita growth to resume its upward trajectory. So much so that even with stagnant growth in the U.S., for the reasons illustrated by Ted, I'm as confident as I can be in Coke's future prospects.

Fool contributor Jay Jenkins has no position in any stocks mentioned. The Motley Fool recommends Berkshire Hathaway, Coca-Cola, and Wells Fargo. The Motley Fool owns shares of Berkshire Hathaway, Coca-Cola, and Wells Fargo. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.