Intermittent Fasting May Match Calorie Restriction Without Mental Effort

Researchers have a measure for how much effort people put into avoiding overeating. Known as dietary restraint, it reflects how often someone dieting deliberately monitors what they consume, limits themselves at meals or scrutinises every food decision.

Across repeated studies, this score rises when people reduce their calorie intake – and higher scores are generally linked with greater weight loss.

An Australian study lasting 18 months examined whether this mental effort was genuinely required.

More than 200 adults followed one of three diets for a year and a half, one of which was intermittent fasting.

Participants who fasted lost as much weight as those carefully restricting calories – even though their dietary restraint score did not change.

Comparing intermittent fasting and calorie restriction

The 18-month trial was headed by Professor Leonie Heilbronn of Adelaide University, in collaboration with the South Australian Health and Medical Research Institute.

Every participant was living with obesity and wanted to lose weight.

The researchers allocated volunteers to three groups. One followed calorie restriction, reducing their usual food intake to roughly 70 per cent of normal.

A second group used intermittent fasting, while the third received only broad healthy-eating guidance.

On three days every week, the intermittent fasting group adhered to a rigid timetable.

They consumed just a small breakfast – around 30 per cent of their daily energy intake – between 8am and noon, before drinking only water for the next 20 hours. During the remainder of the week, they ate normally.

Different diets, identical weight loss

At the six-month point, the results on the scales were clear. Both the fasting and calorie-restriction groups had lost about 6.8 kg. Those given standard advice lost approximately 1.8 kg.

This level of weight loss is consistent with findings from other researchers.

Previous research indicates that fasting and continual calorie restriction produce similar losses over time, despite feeling very different in everyday life. One trial found the same equivalence.

Judged by weight alone, the two approaches were level. Their distinction lay beneath the figures – in participants’ relationship with food and the effort required to eat less.

Dietary restraint was the mental difference

During the trial, participants completed questions about their eating habits, including whether they held back while eating and the degree of control they felt around food.

Scientists describe this deliberate effort as dietary restraint. It normally increases when people follow a conventional diet.

Among the calorie-restriction group, conscious effort increased – and helped account for their weight reduction. Improved control explained around 15 per cent of the weight they lost.

The fasting group lost an equivalent amount, yet their weight loss did not depend on that deliberate control. Their restraint scores changed very little, while the weight still came off.

Before this study, no research had demonstrated that two diets could achieve equal weight loss through such different pathways.

Exactly why fasting succeeds without this mental effort has not been fully established. The fixed eating window may perform much of the work itself, reducing the available time for eating rather than requiring people to consciously resist.

Researchers say the precise mechanism remains unresolved.

Mood improved with both diets

Going without food for 20 hours might seem likely to cause misery, yet volunteers did not report that experience.

People on both diets said their mood improved, reporting fewer signs of depression and greater well-being.

The fasting days did not lower participants’ mood either. Even when they missed lunch and dinner, they said they felt well – with no increase in gloom or sudden drop in mood.

Weight loss often improves mood, so this aspect partly matched scientists’ expectations.

A wider review has connected fasting routines with more consistent energy and improved daily functioning for some people. The new result concerned effort more than mood.

Why intermittent fasting may help

For people who have attempted calorie counting without success, the attraction is straightforward.

Fasting gives a clear instruction – eat within this window and miss the rest – instead of demanding that every choice be calculated throughout the day. That simplicity may be its value.

“Psychological and behavioral effects have a major influence on people’s abilities to adhere to diets,” said Heilbronn.

She said fasting could support weight loss in ways that rely less heavily on conscious restraint.

People do not all face the same difficulties. Some establish and maintain the habit of eating less; others are never able to do so.

For the latter group, Heilbronn suggests, fasting may provide an option that traditional dieting has not offered.

What this could change

This trial provides a clearer understanding of how two widely used diets operate.

They produce the same result on the scales, but take different psychological paths to reach it – one based on conscious control and the other not.

Doctors and dietitians may be able to use this distinction. Instead of prescribing the same calorie-counting programme to everyone, they could direct people unable to maintain it towards fasting – fitting the diet to the individual.

The next task is to identify who is suited to each approach before they begin.

Some early evidence suggests intermittent fasting may reduce the uncontrolled eating that derails dieters, potentially helping professionals pair people with the appropriate plan.

For the millions who have abandoned calorie counting, that offers a promising alternative.