Is intermittent fasting the best approach for fat loss?

Take-aways

1. Intermittent fasting does not appear to benefit hunger, fullness, prospective calorie intake or weight loss compared to a continuous energy restriction approach.

   1. Adopt whichever approach better suits your lifestyle, but be aware that intermittent fasting may be less sustainable and require greater lifestyle sacrifices.

2. Intermittent fasting doesn’t appear to benefit overall health, as measured by markers of diabetes or blood lipids any more than simply being in a calorie deficit. Fasting itself doesn’t seem to benefit your health; only the resulting deficit.

3. Many of the purported benefits of intermittent fasting for health and fat loss were derived from animal models; generally, these promising mechanisms have failed to translate to improved human outcomes.

For a few years in the 2000s/2010s, intermittent fasting was ubiquitous. All you needed to do was create a fasting/feeding window, stick to it, and you were guaranteed fat loss.

While the consensus on fat loss has slowly shifted as more evidence has been published, intermittent fasting has seen a renaissance for longevity/overall health. However, does intermittent fasting actually have merit for either of these goals?

Here’s the most recent evidence on intermittent fasting for both fat loss and overall health.

First, we have a meta-analysis looking at hunger management and fat loss.

The authors found no evidence that fasting meaningfully impacted hunger, fullness, desire to eat or prospective food consumption. In other words, essentially no positive effect of fasting on hunger. This was true irrespective of the strategy adopted, broadly speaking (e.g. alternate-day fasting, 5:2 dieting, restricted time feeding). You can see their findings in the graphs below.

Hunger when performing IF (intermittent fasting) or CER (continuous energy restriction - aka no fasting but the same deficit).

Fullness when performing IF (intermittent fasting) or CER (continuous energy restriction - aka no fasting but the same deficit).

Desire to eat when performing IF (intermittent fasting) or CER (continuous energy restriction - aka no fasting but the same deficit).

Prospective food consumption when performing IF (intermittent fasting) or CER (continuous energy restriction - aka no fasting but the same deficit).

As regards fat loss, the same systematic review found no evidence that intermittent fasting impacted weight loss differently from isocaloric continuous energy restriction (aka a diet with the same calorie deficit but no restrictions on when you eat). So, whether we’re talking about hunger or weight loss, fasting doesn’t seem beneficial (or detrimental!).

Weight loss differences when performing IF (intermittent fasting) or CER (continuous energy restriction - aka no fasting but the same deficit).

For health, IF also doesn’t seem to make a big difference. The most recent systematic review comparing IF to isocaloric, non-time-restricted diets found no major benefits for markers of diabetes or blood lipids.

Differences in diabetes markers: (a) glucose (b) HBA1C (c) insulin (d) HOMA-IR.

Differences in blood lipids: (a) triglycerides (b) total cholesterol (c) HDL-cholesterol and (d) LDL-cholesterol.

Why?

As regards both fat loss and health/longevity, intermittent fasting has amassed much of its hype from mechanistic studies, predominantly in animal models. While there are a number of mechanisms that would benefit both fat loss (e.g. preferential oxidation of adipose mass during severe energy restriction, lowering of leptin) and health (e.g. reduced oxidative stress and chronic inflammation), these benefits do not seem to (1) generalize to humans perfectly and/or (2) affect the outcomes we are interested in too much.

At present, intermittent fasting appears no better than traditional energy restriction at promoting physical health or fat loss in humans. However, for the right person, in the right circumstances, it can complement their lifestyle very well and facilitate weight loss. Conversely, though, it can also be difficult to sustain and adhere to for a lot of people. Compared to a diet that has the same deficit but no fasting patterns, it is a restrictive approach that will limit the extent to which you can socialize and adapt to different circumstances. Regardless, it does not appear to have major inherent benefits over other isocaloric approaches.