Anyone who has tried to lose weight is familiar with the old advice to manage calories-in vs. calories-out. The thinking is all calories are equal, so in order to lose weight, you just have to reduce your caloric intake below your caloric expenditure (or calories burned) and weight will begin to shed from the body. It seems so simple, and the above link is a recent clinical trial comparing a calorie-reduction diet vs. intermittent fasting, with the results suggesting the calorie-reduction diet is more effective at very short term weight loss than methods including intermittent fasting. But is that really true, and does this disprove intermittent fasting?
Not So Fast
To start, it’s important to not just take a snippet of information from a short article as “the answer” without reviewing the details of the study conducted, as it helps clarify objective, process and variable factors. Second, there are some important distinctions in this study, such as:
- Participants in this study were of normal BMI, that is, not obese
- The study was only conducted for 3 weeks
- Meals were not controlled, but were self reported by the participants
Additionally, one other aspect of this test that was interesting, and raises questions, is the participants who followed a 24 hour intermittent fasting (I/F) cycle (alternating days) combined with a 150% re-feeding, demonstrated a marked drop in their activity levels when compared to the participants who followed a 25% calorie reduction plan daily. So, activity levels were also lower with the I/F test group, different from the calorie reduction group.
Short Term Dieting
It’s very common for people who begin a diet via calorie restriction, especially those who are considered obese, to see a short term weight loss. As in the case of the study participants, reducing calories by 25% out of the gate is a significant reduction, the equivalent of consuming 500 fewer calories per day on a 2,000 calorie per day diet. While it’s unclear exactly how long it takes to see a change in the basal metabolic rate (BMR), it typically will take 30 days or more to materially change. Not coincidentally, those that diet via caloric restriction often see good weight loss results in the first month, followed by a slow-down in month #2, and an eventual plateau in the subsequent months. Why? The body is constantly seeking homeostasis, which is an equilibrium, or balance, in function. So, if you cut your calories by 25%, and keep that new level of caloric intake for 3 – 4 weeks, the body will read that new level of caloric intake as the “new norm”, and reduce (or slow down) the BMR to match the intake. Conversely, if you were to increase caloric intake by 25% for 3 – 4 weeks, the body will increase the BMR in an effort to find equilibrium. This is oversimplified because it does not take into consideration macronutrient mix and the resultant hormonal response that can be dramatically altered. But, the point is it takes more than a few weeks for the BMR to shift, and in the meantime, weight loss can be expected.
So, back to the study. If participants with normal BMI, ate 25% less food, without any material change in the mix of macronutrients (fats, protein, carbohydrates), and maintained a consistent activity level, and did this all for a period of only 3 weeks, why would we be surprised that they lost the most weight?
A Better Way
Below are two studies that take a look at the power of I/F while activating a mix of important control factors:
- Including obese participants (women, in both) vs. normal BMI participants
- Controlling the actual food intake vs. “self reporting”
- Running the experiment for 8 – 12 weeks vs. only 3 weeks
https://translational-medicine.biomedcentral.com/articles/10.1186/s12967-020-02687-0
https://pubmed.ncbi.nlm.nih.gov/30569640/
Both of these studies demonstrate the power of I/F, in both weight loss as well as metabolic markers that show a positive shift is underway. Ironically, the pubmed study also shows the power of a calorie reduction combined with I/F, again related to obese participants.
Bottom Line
First, taking a very short term approach to comparing calorie reduction vs. I/F, among healthy BMI people, without controlling the food consumption, and not keeping activity levels consistent, presents some credibility challenges to the main study, and I think it is disingenuous to suggest based on a 3 week scenario, one method is proven superior to another. It’s also much more relevant to assess weight loss response among those who have weight to lose vs. those that are at a normal weight.
Second, before embarking on a change, the smart way to approach incorporating an I/F dietary lifestyle is to first meet and discuss with a medical professional who is well versed in nutrition and the drivers of metabolic syndrome, and ensure there is not a medical reason why I/F should be avoided.
Third, I/F has shown to have powerful benefits on weight loss, especially with those who are obese, but it also has additional benefits related to gastrointestinal health (GI), stimulating autophagy, reducing insulin resistance & leptin resistance, managing inflammation & offering cognitive benefits. When combined with a low-carbohydrate diet, combined with healthy fats & proteins while eliminating sugar and processed food, I/F is more than just a short-term diet…it’s a way of life. Also, while diet is 70%+ of the battle to managing weight & metabolic health, being fit and healthy requires activity, exercise, movement, and mental health. It’s a holistic package built for long-term health and wellness.
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