Intermittent fasting protocols have become increasingly popular among people seeking weight loss or easier weight management, improved gut function and better sleep, among numerous other health benefits touted by proponents of the practice.
The most popular form of intermittent fasting is time-restricted eating (TRE), in which food intake is limited to a defined window of time, generally 6-8 hours per day.
However, there is considerable variation in the time windowselected for TRE, with some TRE enthusiasts practising delayed TRE (e.g. skippingbreakfast and eating only between 12 noon and 6-8 pm) and others favouring earlyTRE (skipping dinner, eating a large breakfast, and ceasing eating by earlyafternoon).
Who’s right? According to a studyrecently published in the journal Obesity, if you want to lose weightwithout being racked by hunger pains, the right move is to skip dinner ratherthan breakfast.
Considerable evidence has already accumulated that points in this direction. Back in 2013, Israeli researchers tested the effects of allocating the same 1400 calorie per day weight loss diet in 2 different ways: a large (700 calorie) breakfast, medium-sized (500 calorie) lunch and a small (200 calorie) dinner; or a small (200 calorie) breakfast, medium-sized (500 calorie) lunch and large (700 calorie) dinner.
The researchers randomised women with overweight or obesityto follow one of the two meal patterns for 12 weeks. Those allocated to thelarge breakfast/small dinner pattern lost on average 5.1 kg more than the smallbreakfast/large dinner group, reduced their BMI by 10% vs 5% in the largedinner group, and also dropped their triglycerides by 33.6% while the large dinner eaters saw their triglyceridesrise by 14.6%.
Several other interventional and cohort studies since then havereported similar findings: shifting food intake to earlier during the daytimeincreases weight loss, even if the total number of calories consumed per day isthe same.
You see, our digestive and metabolic processes follow a circadianrhythm – that is, they fluctuate depending on the time of day, in a predictableand regular fashion.
We digest food more efficiently in the morning than we do in the afternoon or evening.
However, we expend more energy in metabolising and storing the calories we consume (known as the thermic effect of food, or TEF) in the morning than later in the day. Eating earlier in the daytime aligns with our circadian rhythm, supporting weight loss and easier weight maintenance.
The question remains: what is the mechanism by which eTRE supports weight loss? Earlier research – mostly on rats – suggested that skipping dinner elevates energy expenditure; the few studies conducted on humans contained significant design flaws.
However, other studies conducted on humans suggested that reduction in appetite was the primary mechanism behind the weight loss observed in those who ‘eat breakfast like a king, lunch like a prince, and dinner like a pauper’.
To tease out the answer to this question, the researchers behindthe study just published in Obesity recruited 11 men and women withoverweight or obesity (BMI 25-35) but without evident medical problems, and randomlyallocated them to either the control eating pattern (three meals per day, eatenbetween 8 am and 8 pm) for 4 days, or an early time-restricted eating (eTRE)pattern (three meals per day, eaten between 8 am and 2 pm) for 4 days.
On days 3 and 4, identical meals were provided to participants,who ate them under supervision at the allocated eating times (8 am, 2 pm and 8pm for the control diet, and 8 am, 11 am and 2 pm for the eTRE diet) to ensurecompliance.
Participants spent day 4 in a respiratory chamber to measuretheir energy expenditure (metabolic rate), thermic effect of food (the uptickin metabolism that occurs after we eat) and how much fat, carbohydrate andprotein they were burning.
Levels of hormones involved in appetite regulation were alsotested before and after spending the day in the metabolic chamber.
Finally, participants self-rated their hunger, desire toeat, capacity to eat, fullness, stomach fullness, energy levels, awakeness, andperceived body temperature at regular intervals throughout the day.
After a wash-out period of roughly one month, participantscompleted 4 days of the opposite eating pattern, along with another day in themetabolic chamber, repeat blood tests for hunger hormones, and self-ratings.
When all the data were collected and analysed, the researchersfound little difference in the energy expenditure of participants when theywere assigned to the control vs eTRE eating patterns.
There was a small increase in protein oxidation during theeTRE period, as the body converts some amino acids to glucose when we haven’teaten for a number of hours.
Metabolic flexibility (the ability to switch between burningdifferent substrates – fat, carbohydrate and protein – as fuels) increasedduring the eTRE period.
The key finding from the study was that eTRE substantially decreased levels of the ‘hunger hormone’ ghrelin, and increased the satiety (fullness-promoting hormone) PYY in the mid-evening.
In line with these hormonal changes, participants ratedtheir hunger, desire to eat and capacity to eat as lower when following the eTREeating pattern, and their sensation of fullness and stomach fullness as higher (exceptat bedtime).
Interestingly, perceived body temperature was also higherduring the eTRE phase – good news for the many people (especially women!) whoconstantly feel cold.
The take-home points from this study are:
- The higher the percentage of your daily calorieintake that you consume in the earlier part of the day, the easier it will befor you to lose weight and keep it off without feeling hungry.
- The earlier in the day that you finish eating,the better. Even if you can’t manage the 6-hour eating window (8 am – 2 pm)used in this study, try to structure your mealtimes so that you don’t eat afterdark, on at least most days of the week.
And if you’re one of the many people who just doesn’t feel hungry in the morning, be aware that hunger is very much a matter of habit, and is therefore trainable. If you stop eating in the late afternoon or early evening, you’ll soon find you have much more of an appetite in the morning.
