Can fasting help you burn off those vacation pounds?

Vacation's over, and with it comes the end of the french fries, ice cream, and other delicious but fattening foods that are hard to resist. "I'm on vacation," we tell ourselves, "I can diet later."

Well, it's later now. How about fasting? Is that an effective way to take off the weight?

Fasting has been growing in popularity in recent years. One of the latest fads is intermittent fasting, in which you fast at regular intervals and then eat all you want in between. There's the 5-2 fast, where you eat for 5 days and fast for 2, every week. Or there's alternate-day fasting, where you eat one day and fast the next, for as long as you can. Or you can fast every day for 16 hours and get all your eating done in the other 8.

Here's the thing: eating is easy, and fasting is hard. If you're going to fast to lose weight, you probably will lose a bit, but the evidence is that most people gain the weight back fairly soon once the fasting diet is over. So fasting might provide a quick reward in terms of weight loss, but the loss will be fleeting.

Suprisingly, though, fasting may be a good idea. A recent review by Stephen Anton and colleagues, in the journal Obesity, found that intermittent fasting may come with a variety of health benefits, including reducing inflammation, improving the ratio of lean tissue to fat, improving cognitive function, preventing type 2 diabetes, and possibly even prolonging life span.

How does fasting produce these benefits? Professor Valter Longo of USC, one of the leading researchers on fasting and longevity, hypothesizes that fasting forces your body to recycle many of its immune cells, particularly white blood cells. Then your body works hard to replenish its white blood cells, essentially re-setting parts of your immune system. Longo is also the inventor of the fast-mimicking diet, where you eat a special diet for 5 days every month, one that makes your body think you're fasting even though you're getting adequate calories and nutrients. (See Alice Walton's story in Forbes for more about that.)

Another effect of fasting is a change in metabolism. As Anton's article explained:

"the key mechanism responsible for many of these beneficial effects appears to be flipping the metabolic switch." 

This happens when the body runs out of its normal fuel, glucose, and begins to burn fat, which means it's converting fat to fatty acids, which in turn produce ketones. The body then uses ketones instead of glucose.

So how much fasting do you need to flip the switch from glucose to ketones? Anton et al. write that

"The metabolic switch usually occurs between 12 and 36 hours after cessation of food consumption, depending on the liver glycogen content and on the amount of exercise during the fast."

This doesn't tell the whole story, because once the switch occurs, you need to burn ketones for some time to gain any benefits. Back in 2014, I wrote about new evidence (from Valter Longo's research) that a longer fast, such as 3 days or more provide significant long-term health benefits. Three days is a long time to fast, but Longo has said that you don't need to do it more than a few times per year.

If you do decide to try a fast, don't expect it to be easy: you're going to get really hungry, and fasting can also interfere with social obligations in an inconvenient way.

I should emphasize that the evidence isn't yet clear for any of these strategies, and there are multiple studies going on now that may provide a clearer picture. Nonetheless, despite the current fad of fasting strategies and diets, fasting really does seem to have some potential health benefits.

(A final caveat: fasting can be harmful, especially for people who have other health problems. If you’re seriously thinking of trying this, you should consult your doctor first.)

Is being a little bit obese unhealthy?

Everyone knows that being obese is very bad for your health. But how overweight do you have to be before you should worry? A new study covering millions of people attempts to answer this question.

The short answer: being a little bit fat isn't so bad, especially if you're already a senior citizen, but the fatter you are, the shorter your life expectancy. Let's dive into the details.

The new study, published in The Lancet, is a combined evaluation (a meta-analysis) of 239 studies that included over 10 million people from four continents: Asia, Australia, Europe and North America. All the studies followed their subjects for a long time, averaging nearly 14 years of observation. The authors (a large consortium called "The Global BMI Mortality Collaboration") wanted to exclude people who might have already been sick, so their study only looked at people who (a) had never smoked, and (b) who lived at least five years after the study began.

This left them with nearly 4 million people, of whom 385,879 died at some time during the course of the study. From this large data set, the researchers computed the risk of death as a function of body mass index (BMI).

[Aside: BMI is a simple function of your height and weight. For example, someone who stands 5'11" and weights 170 has a BMI of 23.7. A height of 5'6" and weight of 150 gets you a BMI of 24.2. You can calculate your own BMI using this calculator.]

The study divided people into six groups:

• underweight, BMI 15–18.5
• normal, BMI 18.5–24.9
• overweight, BMI 25–29.9
• obesity grade 1, BMI 30–34.9
• obesity grade 2, BMI 35–39.9
• obesity grade 3, BMI 40 or above

The main outcome that they studied was mortality (death) from any cause. Of course, one can argue that this is too simplistic, since if someone dies from, say, an auto accident, it probably wasn't due to their weight. But the results were consistent across all four continents, which argues that the study design was probably good. Here are the main findings for each group:

• BMI 15–18.5: 47% increased risk of death
• BMI 18.5–24.9: no increase (normal)
• BMI 25–29.9: 11% increased risk of death
• BMI 30–34.9: 44% increased risk of death
• BMI 35–39.9: 92% increased risk of death
• BMI 40 or above: 171% increased risk of death

Another way to describe these hazard ratios is this: with a BMI above 40, people are 2.71 times as likely to die during any particular time period as people with a normal BMI. 

If these numbers seem scary, keep in mind that this is relative risk, not absolute risk. So an 11% increase in risk might mean that your chance of dying increases from 1% to 1.11%; it certainly doesn't mean you have an 11% risk of dying. 

To put some real numbers on this risk, consider this comparison: out of 1,075,894 people with a near-optimal BMI between 22.5–25.0, 98,833 died during the course of the study, or 9.2%. (Remember that these data come from 239 different studies, and the average length of followup is 14 years. So fewer than 1% of this group died per year.) Compare this group to people with obesity grade 1, or BMI from 30–35. That group had 330,840 people, of whom 37,318 died, or 11.3%. After various adjustments, this translates into a 44% relative increase, but the actual mortality rate, per year, was about 0.66% versus 0.81% in the two groups.

One mildly positive note: if you're already 70 or older, having a BMI from 25–30 has almost no effect; in other words, it's okay to be a little plump when you're older.

A word of warning to men: the ill effects of obesity are much stronger in men than in women. The study breaks down those hazard ratios by sex, and in each range the risk is higher for men than for women. So for example, if you're a woman with a BMI of 30–35, your hazard ratio is 1.37 (37% higher risk of death), but for men it is 1.70 (70% increase). 

There are many, many more details in the study, including a breakdown of how BMI is associated with four major causes of death (heart disease, cancer, respiratory disease, and stroke), and if you're interested in those, you should read the study. 

Of course, one can think of many caveats to these findings: people die from all sorts of illnesses, and many of them are not caused by being overweight. For any individual case, we might not be able to say whether someone's weight had anything to do with their illness. Nonetheless, this very large study shows clearly that the more obese you are, the greater your risk of dying. That's precisely what we would expect if obesity was causally linked to mortality.

So is it okay to be a little bit fat? The answer is probably yes: people with a BMI of 25 might view themselves as "a bit" fat, even though they are not overweight. But very high BMIs (and very low BMI, below 18.5) are definitely unhealthy. Unfortunately, no one has an easy answer to the problem of losing weight (despite what you might have heard from Dr. Oz), but if you do have a dangerously high BMI, reducing it will likely be good for your health.