Intermittent Fasting Pros and Cons

Intermittent Fasting Pros and Cons

Intermittent fasting has become something of a fad. Its proponents claim not eating for a period of time can improve vitality, support healthy weight and even confer a longer lifespan. Its opponents claim that it disrupts normal metabolism, doesn’t often the promised benefits and is potentially dangerous. Although the debate has been going on for some time, the recent popularity of the Eat Stop Eat program brings it to the forefront.

Eat Stop Eat is perhaps the most popular of several approaches to intermittent fasting. But why is anybody considering intermittent fasting anyway? After all, humanity has spent most of history trying to get enough food for everyone to eat. Fasting was left to mystics and religious zealots. And starving people who couldn’t find food.

Aside from the fact that we need to eat to live, doubters point to several physiologic consequences of fasting they find objectionable.

Intermittent Fasting and Metabolism

For one, many people try intermittent fasting in an effort to lose weight. Yet we know that chronically reducing calories lowers our metabolic rate, making subsequent weight loss more difficult. This is a great survival mechanism, but not so great if you’re trying to get rid of excess fat. It is also why “crash diets” always fail. If anyone can possibly force themselves to stay on it nailed me to get to the point where their metabolism is so slow they can’t lose weight without risking severe malnutrition.

The answer to this is in the word “intermittent”. Supporters of its imminent fasting claim the relatively brief periods of going without eating aren’t long enough to induce this type of metabolic slowdown.

Ketosis vs Ketoacidosis

Opponents also object to inducing ketosis. Ketones are produced when the body runs out of carbohydrates and mobilizes fats to meet metabolic needs. The opponents consider it dangerous because untreated Type 1 diabetics can develop ketoacidosis which is in them, in fact, dangerous.

However, as we’ll see elsewhere, it’s not entirely fair to compare diabetic ketoacidosis to the production of ketones in a fasting nondiabetic. There is definitely a difference between diabetic ketoacidosis and nutritional ketosis in an otherwise healthy person with a normal ability to produce insulin.

There are lots of other aspects of this debate will discuss later. But let’s return to the question about why anyone started looking at intermittent fasting as a healthy way to eat.

Early Studies on Calorie Restriction

As with a lot of things, observations in animals contributed at least in part. Back in the 1930s researchers noted that rats on a very calorie restricted diet that still maintained adequate levels of vitamins and minerals and other micronutrients had twice the life expectancy of rats on a regular rat diet. This led to the concept of calorie restriction as a longevity tool.

While experiments in animals and, to a lesser extent, observation in humans suggest there is some benefit to calorie restriction, it hasn’t caught on.

For one thing, it’s difficult to adhere to. Chronic caloric restriction can feel like chronic starvation. And it takes careful dietary selection to maintain adequate nutrition while still keeping calorie intake consistently low.

There are people who advocate chronic, low calorie diets as a way to increase longevity, but they are in a definite minority.

Intermittent Fasting as a Form of Calorie Restriction

One way to think of intermittent fasting is trying to get the benefits of caloric restriction without the downsides. Again, animal studies provide some support to this concept.

One interesting study looked at several groups of mice. One group could be freely. Another group was put on a diet. They only got 60% of the calories that the freaking group got. A third group was fasted for 24 hours and then was allowed to free feed for a period of time before another period of fasting.

Both the fasting group and the restricted calorie group had lower blood glucose and insulin levels than the control group, yet the fasting group consumed more calories on average than the restricted calorie group.

( “Intermittent fasting dissociates beneficial effects of dietary restriction on glucose metabolism and neuronal resistance to injury from calorie intake“. Proceedings of the National Academy of Sciences 100 (10): 6216–20)

It seems unlikely that the debate will be settled anytime soon, which somehow feels familiar. After all, the high carb versus low-carb battle has been raging for decades.

Yet, as with any of these dietary arguments, there are common sense underlying principles we can adhere to.

For example, the obesity rates make it pretty clear that too many of us are eating too much, or at least too much of stuff that isn’t good for us. Also, I think we’d all acknowledge that eating nutrient rich fruits and vegetables is probably a healthier decision than scarfing down packaged junk food.

Sugared sodas? Aren’t they the definition of “empty calories”?


In my mind, there’s no final answer on intermittent fasting just yet. However, while remaining somewhat of a skeptic, I am finding the approach interesting. Eat Stop Eat brought this debate to my attention. I put together this site to make it easier for you to look at some of the information I’ve collected so you can make your own decision about whether you want to give this program a try.



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