There's a good chance someone you know has told you, "Low carb! I tried that, and I felt horrible." Further conversation will reveal that your friend switched abruptly from eating lots of carbs -- if not in the form of sugar, then definitely in the form of starch: bread, pasta, potatoes and so forth -- to eating virtually no carbohydrate at all. Whatever the ratio of protein to fat in the diet your friend jumped into, the resulting effects included lethargy, hunger, muscle weakness. Further conversation will, no doubt, also reveal that your friend did little reading, followed no particular author or doctor's plan, and simply winged it, with little information and less understanding about how and why reducing carbohydrates leads to fat loss, and what it takes to adjust your metabolism to this new regimen without suffering. Or even any understanding that a low-carb diet is healthier overall, not just a quick way to weight loss.
Or you may have heard that studies show the benefits of carb-loading for exercise, or that carbohydrate provides better fuel than dietary fat for exercise.
The key concept missing in all this is "keto-adaptation." It takes time to adjust after carbohydrate consumption is cut down. That means that studies lasting less than a week don't really show how well the body works in the absence of carbs; they only show how rocky the withdrawal phase can be.
Here's a paper that expands on this topics, explaining what the studies really show. It's easy to read and has lots of good info.
Stephen Phinney wrote this paper, which appears on the National Institutes for Health (NIH) Web site, and was published by Nutrition and Metabolism (London) in 2004. It appears on the NIH site courtesy of BioMed Central.
Thanks to Richard Tamesis, M.D, whose comment on this Protein Power post on a natural treatment for GERD led me to Phinney's paper.
Dr. Phinney was featured in the "Atkins Nutritionals Teleconference Call with Dr. Stephen Phinney and Dr. Jeff Volek in Episodes 170, 171 and 172 of Jimmy Moore's spectacular Livin' La Vida Low Carb podcast show. At the time of this posting, I am unable to access the podcast files from Jimmy's site, but if you contact me via the comments field, I'll do my best to get the MP3 files to you.