You’ve probably heard many things about intermittent fasting, ranging from being a weight loss miracle to not working at all. Let’s cut through the confusion and take a close look at whether it works and whether or not you should try it.
Overview of popular intermittent fasting methods
The basic concept of intermittent fasting is simple – a repeatable pattern of intentionally extended periods of not eating.
This can be implemented in many ways, including the following:
Alternate-day fasting: You limit yourself to ~500 calories every other day. On the other days, you eat without restriction.
5:2 fasting: Similar to alternate-day fasting, you limit yourself to ~500 calories on certain days, but only 2 days per week.
Time-restricted eating: Instead of limiting calories, you restrict the time window during the day you eat. And instead of an alternating-day schedule, you do it every day. In addition to being the safest and most practical form of intermittent fasting, the duration of the eating window can be adjusted based on your circumstances.
The science (and common sense) behind intermittent fasting and fat loss
It doesn’t take much searching to find extreme opinions about weight loss, like whether calories are all that matter, or if they don’t matter at all in light of other important influences like food quality, insulin, metabolic health, etc.
Sadly, many of these opinions are nothing more than speculation, but are misleadingly expressed as fact. Even in the scientific literature, there’s a lot of debate about the significance of calories versus other factors. But fortunately, there are some basic principles that we can rely on.
In some ways, each side of the debate is correct. You need to eat fewer calories than you expend to lose weight, but many things influence that balance. Genetics, food choices, and other lifestyle factors (e.g., exercise, sleep, stress) are just some of the things that can alter your appetite and physiology and, in turn, your calorie balance.
So, calories certainly aren’t everything, but they do matter … a lot. With that in mind, we’ll look at intermittent fasting from the perspective of how it influences calorie balance.
Continuous calorie restriction has been studied for many decades, so it’s a good reference point for evaluating intermittent fasting. And meta-analyses, which are typically the strongest form of evidence, have been done on this comparison, giving us a good foundation of data to evaluate.
At the time of this writing, the most recent, largest, and most relevant meta-analysis (1) represents data from 18 different studies, indicating that when calorie intake is matched, continuous and intermittent calorie restriction led to similar weight loss.
In other words, according to this analysis, intermittent fasting is no better than typical continuous calorie restriction. However, no better means no worse, implying that intermittent fasting is an effective alternative. Furthermore, in some of the studies, for some metrics, intermittent fasting did lead to better results.
But we still need to consider that not all weight loss is equal – we want to maximize fat loss while minimizing muscle loss. The results above, however, still stand when the comparison is based on changes in fat and muscle mass.
This leads us to the underwhelming conclusion that Intermittent fasting is a viable alternative to traditional calorie restriction that only sometimes produces better results.
Now for the common sense …
The challenge in interpreting this data is that intermittent fasting can be implemented in so many ways, and so many factors influence weight loss (and compliance to dietary restriction), that it’s unrealistic to expect a black-or-white, yes-or-no answer. It’s just not that simple.
Fortunately, adding a bit of common sense and practical insight to the conflicting data can give us some better direction.
Intermittent fasting implies less time to eat, and having less time to eat makes it more difficult to overeat. But you don’t need someone with a PhD to tell you that! 🙃
Considering the limitations is what’s important and requires a bit more perspective. Intermittent fasting isn’t magic. If you fast every other day but eat two days’ worth of calories on the other days, you won’t make much progress.
The main goal is still restricting calories. Intermittent fasting is just a tool to make it easier – and, for some, it’s very effective!
But what you eat still matters, as does how you live. Your food choices can significantly influence your appetite, for better or worse, and your appetite obviously has a major influence on your calorie intake. Sleep, stress, and other lifestyle factors can also influence your appetite.
We do recommend intermittent fasting to our clients, typically in the form of time-restricted eating, but we do so as an extension of health-focused food choices and lifestyle habits, not as a replacement for them.
If you were hoping for intermittent fasting to be the elusive silver bullet for losing weight while eating whatever you want, you may still notice some benefit, but you’ll probably be disappointed by marginal results.
Potential for a metabolic advantage
In case you’re wondering about those studies showing intermittent fasting to be more effective than continuous calorie restriction, there is indeed some merit to this line of evidence that goes beyond having less time to eat.
Energy is like a budget, and your body is a very frugal accountant, as it should be. If this weren’t the case, the human race wouldn’t have survived the food scarcity that’s hard to imagine today, but was once a very real thing for many thousands of years. Therefore, it shouldn’t be hard to understand why chronic calorie restriction will likely slow your metabolic rate down. It’s your body’s way of making the most of the limited energy budget it’s being given.
Some studies show that intermittent fasting preserves metabolic rate better than continuous restriction, suggesting that periodically consuming more calories in a shorter window may lessen the restriction needed to maintain a net caloric deficit from week to week.
This implies an inherent advantage of intermittent fasting. But again, it will be influenced by food choices during the eating window, as well as other lifestyle factors.
In summary, while the science unfortunately doesn’t give a clear yes or no, it does provide enough foundation to support intermittent fasting as a viable option, especially when combined with a health-driven emphasis on food selection.
Benefits beyond weight loss (metabolic health)
Intermittent fasting can do something for you that’s much more important than weight loss. It can improve your metabolic health.
Metabolic health is a major predictor of risk for most chronic diseases, including diabetes, cardiovascular disease, cancer, and dementia. Plus, the better your metabolic health is, the more likely you’ll be to maintain a stable energy level and mood throughout the day, which is fundamental to enjoying life!
Metabolic health is basically how well your body metabolizes carbohydrate and fat. When compromised, the resulting high blood sugar levels cause damage throughout the body (some of which is irreversible) that can dramatically accelerate aging and disease risk.
Many lifestyle factors can affect metabolic health. The factors with the most potential for a negative influence are overeating, a high carbohydrate intake, limited physical activity, inadequate sleep, and high stress. These factors can be especially problematic when they exist in combination.
More than half of Americans are pre-diabetic or diabetic (many of whom don’t even know it), and more than three-quarters of Americans meet at least one of the diagnostic criteria for metabolic syndrome, which is a state of compromised metabolic health associated with high disease risk. So, don’t be too quick to dismiss the possibility that you have room for improvement!
Managing carbohydrate tolerance
Two very important ways that intermittent fasting can help your metabolic health are by helping you not overeat and, more specifically, by helping you not overeat carbohydrate.
Dietary carbohydrate that isn’t needed for immediate energy use can be stored in muscle and, to a lesser extent, in the liver. However, this storage capacity is limited, and metabolic health declines when dietary carbohydrate intake regularly exceeds it.
Stored carbohydrate, which is called glycogen, is more prominently used as fuel during fasting (and also during demanding physical activity). Based on this, the fasting period of intermittent fasting forces your body to burn through more glycogen. This subsequently increases reliance on fat metabolism, making it more efficient and, in turn, giving you more stable energy and more capacity to metabolize body fat and preserve muscle during weight loss.
Most importantly, the more glycogen you burn, the more readily your muscle and liver will subsequently take in glucose from dietary carbohydrate, which means the more carbohydrate you can eat at the next meal without impairing your metabolic health.
In other words, fasting makes more space for dietary carbohydrate, increasing the threshold at which carbohydrate will overwhelm your metabolism and cause adverse effects.
Several recent reviews in the scientific literature have discussed this potential of intermittent fasting to improve metabolic health (4, 5), albeit with reminders of the limitations. As mentioned earlier, intermittent fasting isn’t magic. For best results, you’ll likely need to do more than just shorten your eating window.
Although we once again lack a clear black-and-white, yes-or-no answer, there’s a strong basis of support for intermittent fasting pushing metabolic health in the right direction, particularly when combined with other health-focused habits. And that makes intermittent fasting much more than just a weight-loss fad!
Who should and shouldn’t try intermittent fasting
Intermittent fasting could be a good fit for you if you’re looking to lose weight or if you have indications of compromised metabolic health, such as high triglycerides or cholesterol, high blood pressure, low HDL cholesterol, or a waist circumference at or above 31 inches for women and 37 inches for men.
Intermittent fasting could be an especially good fit if any of the above circumstances apply, in addition to having struggled in the past with continuous caloric restriction. Eating all of your calories for the day within a smaller timeframe might make progress easier.
But, again, intermittent fasting is not magic. While doing nothing more than simply limiting the time that you eat might be enough to see improvement, it’s more likely that you’ll need to pay at least a bit more attention to what and how much you eat. Either way, the better your food choices are during your eating window, the more benefit you’ll likely see.
If you have any medical conditions, are pregnant or lactating, are younger than 18, or have any other health-related concerns, consult your physician for further guidance on whether or not intermittent fasting is appropriate for you.
Beyond the above considerations, intermittent fasting might not be a good fit for you if you’re looking to maximize muscle size or strength, or if a slight decrement in strength could threaten your physical independence. This is because fasting will likely increase muscle breakdown, but this can be mostly offset with appropriate strength training and an emphasis on adequate protein intake.
Tips for success while intermittent fasting
We recommend time-restricted eating because it’s the safest and most practical method of intermittent fasting. We suggest consulting a physician for further guidance if you’re interested in longer fasting periods spanning a full day or more.
Adjust the duration of your fasting and eating windows
The longer you fast and the shorter your eating window, the more benefit you’ll likely achieve. But there’s a point of diminishing return, and even a point at which intermittent fasting becomes too challenging, or even counterproductive.
We recommend a fasting window of 16 to 18 hours, which implies an eating window of 6 to 8 hours.
This is enough time for three full meals, each spaced about three or more hours apart. Having three full meals gives you more opportunities to maximally stimulate muscle protein synthesis with dietary protein, which will help to support your muscle size and strength. And three hours between meals will allow for adequate digestion.
If a fasting window of 16 to 18 hours seems overwhelming, you could start with 12 to 15 hours and gradually extend the duration as you become more comfortable.
Find the best timing for your eating window:
Research has shown that it’s better to have your eating window earlier in the day (e.g., starting at 8 or 9 AM). This is likely because a higher intake of calories later in the day (as in closer to bedtime) can have adverse metabolic effects and even negatively affect sleep.
However, an important social component of eating should also be considered. If an earlier eating window means missing meals with family and friends, it’s a tradeoff that’s probably not worth making. Instead, just be mindful of what and how much you eat at your last meal, and try to keep it lighter.
Emphasize protein
Eating enough protein should be a priority regardless of intermittent fasting. Because fasting can increase muscle tissue breakdown, and dietary protein helps offset this, protein intake is even more critical during intermittent fasting.
Plus, protein is satiating, meaning that a higher protein intake will help you manage your appetite and make you less likely to overeat.
A good minimum daily protein intake target (for people with healthy kidneys) is 1.6 grams per kilogram of body weight, or 0.7 grams per pound. Yes, this is twice the commonly stated recommendation of 0.8 grams per kilogram, which is just a bare minimum and doesn’t account for factors that increase requirements, like physical activity or eating at a caloric deficit for weight loss.
If you don’t like math, a simpler target is a minimum of 30 grams of protein per meal, or closer to 40 grams if you’re older, larger, or getting a considerable amount of your protein from plant sources.
To minimize muscle loss the most effectively, break your daily protein intake into three fairly similar portions. This will help ensure you’re maximally stimulating muscle protein synthesis more frequently throughout the day.
Don’t neglect exercise:
Don’t fall into the trap of thinking that fasting will lessen the importance of exercise.
Contrary to what you might believe, exercise alone is not very effective for weight loss, but it has many important health benefits and is effective for helping to keep weight off after losing it. Plus, as previously mentioned, strength training will help offset muscle loss that could result from fasting.
In addition, exercise (particularly aerobic exercise) will add to the glycogen depletion achieved through fasting, as mentioned earlier. Therefore, intermittent fasting plus exercise will support your metabolic health to a greater extent than intermittent fasting alone.
Need more help?
If you’re interested in trying intermittent fasting but need more guidance, we can help!
We’ll coach you through the day-to-day challenges of intermittent fasting and answer all your questions along the way. More importantly, we’ll empower you with the knowledge needed to make food choices that’ll help you get more out of intermittent fasting while also supporting your long-term health and wellness.
Plus, we’ll go out of our way to personalize our recommendations to your preferences and circumstances … so that you’ll be more likely to develop sustainable habits that’ll keep you looking and feeling your best for many years to come!
Book a FREE consultation now to see how we can help!