In a previous blog post, I discussed Intermittent Fasting--both what it looks like and how it works. In this installment, we will discuss what the research says about the relative merits of Intermittent Fasting (IF) as opposed to Multiple Small Meals (MSM) a day. There are several presumptions that are floating around, and we’ll try to tackle them one by one.
1. We must eat frequently to maintain our metabolic rate. This is the one that prompted me to do the research. There is an old adage that if we skip a meal or fast then our bodies will go into “starvation mode” and hold on to calories tighter, making it harder to lose weight. I have even used this same advice with my patients after hearing it somewhere and just assuming it was correct.
We often use the term “starvation mode” to denote the phenomenon whereby the body reduces caloric expenditure to match reduced caloric intake. It is more properly termed “adaptive thermogenesis” and is a well-known natural physiologic response.
However, this response is not nearly as dramatic as we think, as this large meta-analysis showed. In this study, it was found that our daily energy expenditure decreases by about 5.8 calories for each pound of weight loss. In other words, losing 50 pounds would naturally result in us burning, on average, 290 fewer calories per day. This would occur as a result of long-term calorie restriction and weight loss, regardless of whether that calorie restriction was the result of IF or MSM (or any other method).
There is a concept called the Thermic Effect of Food, which basically means that we actually burn calories when we eat and digest our food. This accounts for as much as 10% of our total daily caloric output and explains why cold water and food leads to more weight loss than warm food and drinks. Using this concept, one could make the argument that eating more frequently leads to burning more calories. However, the Thermic Effect of Food is based on total daily caloric intake, so it doesn’t matter if one eats six 500-calorie meals (3000 calories) or three 1000-calorie meals (again, 3000 calories), the Thermic Effect of Food is the same.
As it turns out, several studies like this one have demonstrated that increasing or decreasing meal frequency does not affect total calories burned. Likewise, this study showed no improvement in weight loss between patients who followed more frequent (6 meals per day) feeding schedule and those who ate three meals a day.
So, while we’ve all heard that eating small frequent meals increases one’s metabolism, there is scant evidence in the scientific literature that it helps with weight loss.
On the other hand, there is some evidence that IF increases metabolic rate, at least in the short term. This study indicates that resting energy expenditure increases (as the result of norepinephrine increase) for the first few days of an extended fast. Basal metabolic rate does tend to go down, however, if the fast goes on for too long, as shown here.
2. We must eat very low carb to achieve ketosis. Ketosis refers to the physiologic state where we use fatty acids and ketones for energy as opposed to glucose and its storage form glycogen. The beneficial effects of ketosis are multiple and widely agreed upon. These include fat and weight loss, increased concentration, increased energy and increased insulin sensitivity.
Those of us familiar with typical ketogenic diets have long believed that achieving ketosis is a process that takes several days to accomplish. This study showed that, at least for some people, we can train our bodies to be “dual-energy efficient”. In other words, we can cycle into ketosis (after a 12-16 hour fast) and then out of ketosis again on a daily basis and train our bodies to use both glucose/glycogen and fatty acids/ketone bodies for fuel. This is easier to accomplish for people who are already fairly healthy and it’s important to note that people with Insulin Resistance or obesity often take longer (several days) to achieve ketosis.
So what about combining IF and ketogenic diets together? Certainly, this would lead to quicker ketosis, But what about safety? While there are no randomized controlled trials that I could find in the literature, there are several anecdotal reports that indicate that this practice is “probably safe”, with a few caveats. Those who are pregnant or nursing should avoid Intermittent Fasting. Those with active diabetes or heart disease should consult their doctor before starting this (or any other) dietary change.
3. Skipping meals makes you overeat later. “Breakfast is the most important meal of the day.” Right? Well, for some people skipping meals could lead to increased calorie intake later, but not for everyone. In a study published in 2014 in The American Journal of Clinical Nutrition, 283 people were randomized into either “breakfast skipping” or “breakfast eating” groups. Both groups were attempting to lose weight. At the end of 16 weeks, there was no difference in the average amount of weight loss between the two groups.
However, another study in the same journal, this one published in 2005, demonstrated that most people who achieve and maintain long-term weight loss are indeed breakfast eaters.
It seems that the jury is out on this one. I suspect that there is a great deal of biochemical individuality at play and that skipping meals is perfectly fine for some people while not being effective for others. If you feel shaky or fatigued after not eating for several hours Intermittent Fasting may not be for you.
4. Multiple Small Meals a day is better for your health. Many people believe that Multiple Small Meals daily is simply better for your health. The research shows that higher calorie, frequent eaters are at an increased risk for fatty liver disease.
Moreover, Intermittent Fasting has been associated with inducing a process called autophagy, which is the recycling of old and damaged cellular parts for energy. Autophagy has been associated with lowered cancer risk, as well as lowered risk for Alzheimers. In addition, even longevity seems to be impacted by our choice of when we eat as much as the choice of what we eat. At least in animal studies like this one, IF has been associated with increases in life span.
Finally, some people say that IF increases mental clarity. That is likely due to the fact that fasting for several hours increases something called Brain-Derived Neurotropic Factor (BDNF). While not perfectly understood, BDNF seems to just make the brain work better and increases resistance to neuronal damage.