I've been doing a lot of research about ketogenic diets and have been finding mixed reviews. People on the diet report problems ranging from lethargy and mental sluggishness to a disillusioningly slow weight loss (zero to two pounds a week). I would expect something exceptional from a radical dietary change. Otherwise, wouldn't a healthy, medically advised dietary shift effect the same changes? Is this diet worth it? --Desperate

Well, you've certainly posted some interesting questions. Let me address them one by one.

First, let's be clear that the Shape Up program is not a ketogenic diet. Ketosis is achieved by reducing carbohydrates of all kinds to a very low level, low enough that it forces the body to use incompletely metabolized fats (ketone bodies) as fuel.

The prototype of this kind of diet is the Atkins diet, though there are many others. Although this kind of diet has gotten a bad rap from the traditional dietitians, I personally don't think that in most cases ketogenic diets for short-term use are particularly dangerous (there are exceptions), and they may be very helpful.

Two, there are many reasons why some people initially feel a bit tired as their bodies adapt to the changeover in fuel sources, but it usually doesn't last. An equal number of people report having more energy than ever before, which just supports my view that no one diet is perfect for everyone.

In addition, many people think they're following a ketosis-inducing diet but in fact are not. The only way to tell for sure is by constant monitoring of the urine with keto-sticks.

Three, I personally don't think it's necessary to go into ketosis in order to get the weight loss and health benefits of low-carb eating, although, as I said, for some people in the short term it may be a good strategy. As Robert Crayhon says, you can "flirt" with ketosis and get many of the same benefits.

And finally, you're right, for most people weight loss is a laboriously slow, gradual process, as is getting healthier and getting fitter. It doesn't happen overnight, and generally a couple of pounds of fat a week are a pretty reasonable expectation, barring the occasional blip upwards or downwards. In a year's time, that's 100 pounds, a pretty substantial amount, and more than most people need to lose.

Now, is it worth it? You tell me. I can tell you this: If by "worth it" you mean is it worth changing the way you eat to get healthier, feel better and lose weight, well, only you can make that call. If by "worth it" you mean will you lose weight really fast, the answer is probably not. But what's the alternative? I can promise you this: No method that promises anything faster, quicker or more dramatic works in the long run.

Got a question or comment for Jonny? Post it on the Shape Up message board.