Intermittent fasting, calorie restriction, and weight loss: Insights from recent data

At this time of year, many of us set ourselves resolutions to lose weight. This might be to boost our power-to-weight and therefore sharpen our performance, to stave off middle age’s creeping effects on the scales, or just to slim down after a little hard-earned festive overindulgence. The science of weight loss is an amazingly noisy field, and it is clear that there are a number of methods people have successfully used to drop pounds. In this blog, we cannot possibly cover the entirety of the science on how best to chip away at excess body fat mass. What we will cover, though, is a recent and excellent study published by researchers from the University of Bath in the UK that examined the effects of intermittent fasting on weight loss and body composition. Specifically, in the study the researchers sought to quantify the effects of intermittent fasting on weight loss and body composition that are independent of effects on energy balance – i.e. does intermittent fasting lead to effects on weight loss and body composition beyond those achieved by similar degrees of overall energy restriction using traditional daily dieting without fasting (1). Please note that intermittent fasting may have health-giving effects other than as a weight-loss strategy.

Quick recap: What do we mean by energy balance?

Energy balance may be defined as the difference between energy consumption through the diet, and energy expended through all metabolic processes – physical activity, the breakdown of food and drink, and the cost of the basic processes required to keep us alive, like the heart beating. Energy is measured in kilocalories, and a positive energy balance means that we are consuming more calories than we are expending, whereas a negative energy balance means that we are consuming less than we are expending.

What did they do?

Accordingly, the researchers recruited 36 lean, healthy participants, and allocated them randomly to one of three experimental groups for a three-week intervention period. The intervention period was preceded by a four-week lead-in period in which energy intake, expenditure, and body mass were meticulously recorded such that the daily calorie consumption required to achieve energy balance was identified. The first experimental group was intermittent fasting with negative energy balance condition, in which participants, on alternate days, consumed either no calories at all or 150% of daily energy balance requirements. This means that they were consuming 25% fewer calories than was calculated as being required to achieve energy balance over each two-day period. The second experimental group achieved the same level of negative energy balance in each two-day period but spread equally. They, therefore, consumed 75% of the calories required for energy balance each day and can be considered as a traditional dieting group. The third experimental group went through alternate-day fasting, but achieved energy balance over each two-day period, by consuming no calories at all followed by 200% of daily requirements.

By comparing these three groups, the researchers were able to disentangle the effects of alternate-day fasting on weight loss that are attributable to, and independent from, energy balance. For example, if the fasting group with a negative energy balance exhibited more profound changes than the traditional diet group, the researchers would be able to conclude that alternate-day fasting had effects on weight loss above and beyond those achieved by a similar degree of negative energy balance achieved through traditional means. Please note that this study design does not permit immediate application to free-living, real-world humans, given the tight experimental controls and relatively ‘extreme’ forms of intermittent fasting adopted, and was not intended for such application. This study was designed to improve our scientific understanding of weight loss processes in relation to energy balance.

What happened?

The researchers recorded a number of outcome measures before and after the intervention period, but here we will focus on the effects seen for overall body mass, and body composition (which was assessed using the gold-standard DXA scan).

As expected, the group adopting the traditional dieting approach (75% of the calories required to achieve energy balance per day) lost weight. This group lost almost ~2 kg, with almost all of that body mass loss coming from reductions in body fat mass. The intermittent fasting group that did achieve an overall negative energy balance also lost weight, and this overall weight loss was not significantly different vs. the traditional diet group. However, this intermittent fasting group lost significantly less body fat mass than the traditional diet group, as some of this weight loss appeared to come from fat-free mass (i.e. tissue other than body fat). Intermittent fasting without an overall negative energy balance did not result in significant overall body mass or fat mass loss.

Summary and practical applications

So, this study appeared to show that intermittent fasting does not have additive effects on weight loss above those achieved by dieting to achieve equal negative energy balance by daily energy restriction, and in fact may have had less helpful effects in terms of fat mass loss and fat-free mass retention. The study was conducted rigorously from a scientific and experimental perspective. However, as alluded to earlier, there are a number of considerations when trying to apply these results to free-living people in the real world (which was not really the purpose of the work – the study was about isolating the effects of fasting from energy balance).

For example, it could be argued that the style of intermittent fasting used in this study – that is, zero energy consumption on alternate days – is not what intermittent fasters do in the real world. This is a fair point. A more common approach, like 16:8, may have had different effects on body composition, as well as behaviours like physical activity. Similarly, this trial was conducted in lean individuals that may not be metabolically terribly incentivised towards shrinking fat mass (our understanding is that this trial is going to be repeated in an overweight cohort). We may also see different results if the individuals involved were simultaneously undertaking a structured exercise programme.

That said, if your goal is to lose weight through diet, this study does not tell us that intermittent fasting won’t work (the intermittent fasting with negative energy balance group did lose weight), it suggests that effects may not be above and beyond what is seen through traditional dieting approaches. As has been seen elsewhere, it seems to be that generating an overall negative energy balance is important. How best to achieve that may well in practice be specific to our individual preferences.

References

1. Templeman I, Smith HA, Chowdhury E, Chen YC, Carroll H, Johnson-Bonson D, Hengist A, Smith R, Creighton J, Clayton D, Varley I, Karagounis LG, Wilhelmsen A, Tsintzas K, Reeves S, Walhin JP, Gonzalez JT, Thompson D, Betts JA. A randomized controlled trial to isolate the effects of fasting and energy restriction on weight loss and metabolic health in lean adults. Sci Transl Med 13: 1–16, 2021. doi: 10.1126/scitranslmed.abd8034.