While intermittent fasting has been lauded for its health benefits, including promoting cellular maintenance and protecting against aging and neurodegenerative diseases, popular wisdom maintains that skipping breakfast is bad for you. Often labeled as the most important meal of the day, breakfast is said to “boost metabolism” and reduce hunger. But is this really true? Mounting evidence suggests that eating three meals a day may not be important for weight loss.
David Allison, director of the UAB Nutrition Obesity Research Center and senior investigator on a recent long-term study, commented:
The field of obesity and weight loss is full of commonly held beliefs that have not been subjected to rigorous testing; we have now found that one such belief does not seem to hold up when tested. This should be a wake-up call for all of us to always ask for evidence about the recommendations we hear so widely offered. (1)
In this article, we’ll explore the evidence for and against eating breakfast with all its nuances, including an ancestral approach, the problems with association studies, a review of the biochemistry of intermittent fasting, and relevant results from randomized controlled trials.
Did Our Ancestors Eat Breakfast?
For those of you who have been following my blog for a while, you know that I like to look at health questions through an ancestral or evolutionary lens in addition to looking at the latest scientific research. So, did early human anthropoids eat breakfast?
The most consistent daily eating pattern that is beginning to emerge from the ethnographic literature in hunter–gatherers is that of a large single meal which was consumed in the late afternoon or evening. A midday meal or lunch was rarely or never consumed and a small breakfast (consisting of the remainders of the previous evening meal) was sometimes eaten. Some snacking may have occurred during daily gathering, however the bulk of the daily calories were taken in the late afternoon or evening. (4)
It appears that the three-meals-a-day paradigm was not adopted until the Agricultural Revolution around 10,000 years ago. Frankly, the fact that we eat three times a day is somewhat arbitrary and seems to be based on when it was most convenient to eat during farm work and harvest. (5)
The Trouble with Association Studies
Most studies regarding breakfast consumption and obesity are association studies. And while there is undeniably an association between a lean body type and breakfast consumption, correlation does not imply causation, and many of these association studies have been inappropriately used to shape recommendations for weight loss.
I have talked before about the healthy user bias on my blog in the context of red meat. Because “eat breakfast” is such popular health advice, people who are committed to their health are more likely to eat breakfast. They are also likely to avoid smoking, manage stress, and eat more fruits and vegetables, all things associated with a healthier weight. Breakfast eaters tend to be leaner, but this doesn’t mean that they are lean because they eat breakfast.
Can you lose weight faster if you skip breakfast?
While the healthy user bias is difficult to avoid when designing a research study, there are also more intentional breaches of good science mixed up in this question. A recent meta-analysis looked at the proposed effect of breakfast on obesity (PEBO) and found that the PEBO was subject to two common practices used to distort scientific evidence. (6) The first was biased research reporting. Authors of breakfast studies were guilty of biased interpretation of their own results, misleadingly citing others’ results, and improper use of causal language. This is similar to confirmation bias or the “a priori” effect. When you presume something to be true, like “eating breakfast causes weight loss,” all of your research and interpretation flow from that belief.
The second was what they called “research lacking probative value” (RLPV). This is where experiments either continue to be performed about questions that have already been sufficiently answered or are designed in such a way that they simply cannot advance scientific knowledge about the question. As the authors put it:
… at some point, additional similar observational analyses will only trivially add to our knowledge regarding the PEBO. […] The association, but not the causal relation, between breakfast and obesity has been more than sufficiently established. (6)
Other scientific reviews came to similar conclusions: it’s abundantly clear that there is an association between leanness and breakfast intake. But to really determine if the breakfast–weight connection is causative, we need long-term, randomized trials. (7, 8, 9)
Breakfast vs. No Breakfast: Evidence from RCTs
Luckily, in the last few years, several research groups have sought to use randomized controlled trials (RCTs) to answer the question “does eating breakfast cause weight loss?” Let’s take a closer look at the studies and what they found.
In one of the first RCTs in 1992, researchers separated 52 moderately obese adult women based on their normal breakfast habit (whether they ate or skipped breakfast regularly) and then randomly assigned half of each category to a breakfast group and half to a no-breakfast group. In their results, they reported a trend suggesting that women who had to make the most substantial changes to their initial eating habits achieved more weight loss. Essentially, habitual breakfast skippers tended to do a bit better when they had to eat breakfast, and habitual breakfast eaters tended to do better when they had to skip breakfast. (10) Unfortunately, when this result was cited by other studies and the media, it was widely misconstrued. First, the researchers only observed a trend for this interaction effect, meaning that it did not reach the level of statistical significance (p < 0.06, for those familiar with statistics). Second, the study was widely reported in the scientific literature as having shown that eating breakfast led to weight loss, even though the authors never concluded anything of the sort. Unfortunately, poor reporting of this study shaped scientific and popular opinion for several decades.
The belief that breakfast is important for weight loss prevailed, despite a few smaller studies that found that skipping breakfast had no effect or even a potential beneficial effect on weight loss. In 2013, Cornell researchers performed a randomized crossover study in 24 undergraduate students and found that skipping a meal did not result in energy compensation at later meals and that it might even be an effective means to reduce energy intake in some people. (11) In 2015, researchers in the UK performed a similar study with a week-long intervention in 37 participants and concluded that “there is little evidence from this study for a metabolic-based mechanism to explain lower BMIs in breakfast eaters.” (12) However, these studies were both relatively short-term compared to the 1992 study and didn’t receive as much attention.
In 2014, as part of the Bath Breakfast Project in the UK, 33 obese adults were randomly assigned to a breakfast group or no-breakfast group for six weeks. (13) The breakfast group ate slightly more calories but was also a bit more physically active. The no-breakfast group ate fewer calories over the entire day but was also slightly less active and had slightly more variable glucose levels in the afternoon and evening at the end of the trial. Body mass and fat mass did not differ between the two treatments, and neither did indexes of cardiovascular health. Contrary to the popular notion that breakfast “boosts metabolism,” resting metabolic rate did not differ between the groups. Breakfast also did not provide any significant suppression of energy intake later in the day. It seemed like the evidence was mounting against popular belief.
Finally, in the largest long-term, multisite clinical trial to date, researchers attempted to settle the debate once and for all. They randomized 309 obese adult participants to a breakfast group or no-breakfast group for 16 weeks. They reported in the American Journal of Clinical Nutrition:
A recommendation to eat or skip breakfast for weight loss was effective at changing self-reported breakfast eating habits, but contrary to widely espoused views this had no discernable effect on weight loss in free-living adults who were attempting to lose weight. (14)
Over 92 percent of subjects complied with the recommendation they were given, but it had no impact on weight loss. They also separated individuals based on their baseline breakfast habit and found no interaction between initial breakfast habit and success of the intervention. This is directly contrary to the near-significant interaction found by Schlundt and colleagues in 1992 and was a much larger study.
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What Type of Breakfast Were They Eating?
But wait, does a bowl of cereal and toast with jam have the same effect as an egg omelette, greens, and a sweet potato? Food quality matters more than food quantity, right? Yep. When “breakfast” is lumped into one big category, there’s not conclusive evidence for or against it, (15) as we saw in the previous section. But researchers have looked at different types of breakfast and weight loss as well, with some intriguing results.
In 2015, a study in China found that obese teenagers ate less at lunchtime if they had an egg breakfast compared to a bread breakfast. The egg breakfast was reported to increase levels of satiety hormones, keeping them full for longer. The egg breakfast group also had significantly more weight loss. (16) Sounds pretty good to me! Unfortunately, there wasn’t a “no-breakfast” group in this study, so it’s hard to know how the egg breakfast would have compared to intermittent fasting.
Researchers in Missouri performed a randomized trial in 2015 with three different groups. They randomly assigned 57 breakfast-skipping teens to a cereal-based breakfast (13g protein), an “egg-and-beef rich” breakfast (35g protein), or to continue skipping breakfast. They found that the egg-and-beef breakfast led to voluntary reductions in daily food intake and reduced reported daily hunger. It also prevented fat mass gains over the 12-week study. (17)
The truth is, most of the studies above (that found no effect of breakfast) were likely based on a typical high-carbohydrate breakfast, a la the Standard American Diet. It would be very interesting to see the metabolic response to breakfast omission in a group of healthy individuals eating a nutrient-dense, evolutionarily appropriate diet.
Skipping Breakfast and Exercise
What about fasting in relation to exercise for weight loss? In the fed and fasted states, we preferentially oxidize (“burn”) different substrates to produce energy. Could exercising in one state or the other provide benefits for weight loss? In 2012, researchers in London performed a crossover study, monitoring food intake and energy expenditure in 49 participants during one week with breakfast and one week skipping breakfast. They found that total energy intake, energy expenditure, and activity levels did not differ between conditions. (18)
A study in Japan in 2014 used a randomized crossover design with eight male subjects, all of whom were habitual breakfast eaters. The subjects were instructed to eat or skip breakfast, and the researchers measured their energy expenditure during the day. Interestingly, they found that breakfast skipping did not affect energy expenditure, fat oxidation, or the thermic effect of food if you looked at the entire 24-hour period (similar to the previous study), but it did change the rhythm over the course of the day. When people skipped breakfast, energy expenditure was lower during the morning but higher during the evening and sleep than those who ate breakfast. Breakfast skipping increased fat oxidation and reduced carbohydrate oxidation in the morning relative to breakfast eating and increased carbohydrate oxidation during the evening. (19)
Following up on this study, a crossover study in Korea in 2015 tracked 10 obese male college students. For one week, they ate before their morning workout. The second week, they ate breakfast after their morning workout. Their results? The fasted workout caused the men to burn more body fat, but it also increased levels of the stress hormone cortisol after exercise relative to the fed workout. (20) While burning body fat is beneficial to weight loss, large rises in cortisol are not. It should be noted that these participants were not adapted to fasted exercise and that “fat-adapted” people might have a smaller cortisol response to fasting. I’ll explore this idea more in the next section, when we dive into the biochemistry.
What do these studies tell us? Well first, the thermic effect of food in the morning, a common argument for why we should eat breakfast and “boost our metabolism,” is a myth. Over the total course of a day, total energy expenditure does not change. They also suggest that morning fasting might be a great time for a fat-burning workout, as long as it’s not too stressful on your body.
Fat-Adapted vs. Carb-Adapted: Fasting Biochemistry
When you eat, the hormone insulin is released from your pancreas to the bloodstream and shuttles glucose (carbohydrate) into muscles and other tissues, where it is used for energy production. Excess glucose is converted to fat and stored in the adipose tissue. When you fast, the hormones glucagon and cortisol stimulate the release of these fatty acids from adipose tissue into the bloodstream. The fatty acids are taken up by the muscles and other tissues and broken down (oxidized) to produce cellular energy. In this concerted manner, the body switches from utilizing carbohydrates to fats as its primary fuel and ensures a constant source of energy to the body.
This is all good and rosy, as long as the body can actually make this metabolic switch. In the scientific literature, this is called “metabolic flexibility,” (21) though you may be familiar with it as “fat-adapted.” People who are “fat-adapted” are more accurately “metabolically flexible,” meaning that they can easily switch from oxidizing carbohydrates in the fed state to oxidizing fat in the fasted state, and vice versa. On the other hand, people who are said to be “carb-adapted” are “metabolically inflexible,” meaning that they are constantly burning carbohydrates and have trouble switching to fat oxidation. These people still release fatty acids from adipose tissue to the bloodstream but have lost the capacity to oxidize fatty acids in the muscle and other tissues. The accumulation of lipids due to reduced fatty acid oxidation has been hypothesized to cause insulin resistance, (21) and a low ratio of fat to carbohydrate oxidation has been identified to be a good predictor of weight gain. (22)
The phenomenon of metabolic inflexibility may explain some of the results of breakfast studies. Most of the participants in these studies were individuals eating an evolutionarily inappropriate Standard American Diet with large amounts of refined carbohydrates three times a day. If, all of a sudden, you instruct these “carb-adapted” people to skip breakfast, you’re asking for a blood glucose crash and insatiable hunger by lunchtime. In reality, most people who want to try intermittent fasting transition do so gradually by slowly increasing the time between meals, allowing the body to adapt and restore metabolic flexibility.
This may explain why prior breakfast habits have an effect in some studies. Researchers at the University of Colorado studying 18 overweight women found that the adverse effects of skipping breakfast were restricted to habitual breakfast eaters. While habitual breakfast eaters who skipped breakfast had increased blood lipids, insulin, and free fatty acid responses at lunchtime, habitual breakfast skippers who skipped breakfast had none of these effects. The authors concluded that meal skipping may have enhanced effects in habitual breakfast eaters due to entrainment of metabolic regulatory systems. (23)
So, skipping breakfast might not cause weight loss in the short term, but if over the long term it allows your body to “reset” and restore metabolic flexibility and insulin sensitivity, you may ultimately see some weight loss benefit. This is especially true if you’re also improving the overall quality of your diet. A low-fat diet reduces your body’s ability to release fatty acids from adipose tissue and oxidize them in the muscle, (24) while a high-fat diet increases the ability to use fat for energy in muscle and thus improves metabolic flexibility. (25, 26)
Summing It Up: Should You Fast, or Break-Fast?
If you’re overwhelmed by this quantity of research, you’re not alone. Researchers have been struggling to find consensus on this topic for decades. If you glazed over some of it, here are the major takeaways from this article:
- Hunter–gatherers probably only ate one large meal later in the day.
- You cannot trust association studies. Correlation does not equal causation!
- When all breakfast is lumped together, skipping or eating breakfast has no apparent effect on weight loss.
- If you separate out different types of breakfasts, a protein-rich, fiber-rich breakfast seems to confer the most benefits.
- Eat before or after exercising depending on your health status and goals. Skipping breakfast will optimize fat metabolism during your morning workout, but it may also spike your cortisol levels.
- Most of the individuals in these studies were “carb-adapted” individuals eating a Standard American Diet. It would be interesting to see how the results might differ in “fat-adapted,” metabolically flexible individuals eating a nutrient-dense Paleo diet.
And that’s it! If anything is clear from this consortium of research, it is the need for individualized nutrition. I’ve written several articles and spoken on my podcast previously about why intermittent fasting (IF) may not work for everyone. If IF works for some people (they lose weight) and is detrimental to others (they gain weight), and these people are all lumped together, we’ll see a net zero change in weight.
So how do you know if intermittent fasting is right for you? Try an n=1 experiment: eat or skip breakfast for a period of time, and notice how it affects your weight, mood, productivity, gut function, and other factors. Transition slowly if necessary, by eating your first meal of the day later and later each morning. There are some predictors of success with fasting, but only you can really know if IF works well for you.
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