Timing your calories right can help prevent weight gain and diabetes

Folk wisdom from both East and West – codified by American nutritionist Adelle Davis¹ in the 1960s – urges us to

On the other hand, many proponents of time-restricted eating² suggest limiting one’s food consumption to an eight-hour ‘eating window’, typically between 12 pm and 8 pm. And there’s even a small but vocal faction who advocate ‘OMAD’ – One Meal A Day. Both of these protocols almost inevitably result in the majority of calories being eaten at night. So who’s got the right idea – the breakfast maximalists, or the breakfast skippers?

According to a meticulous study on eating timing and metabolic efficiency, carried out by German researchers, eating the bulk of one’s calories earlier in the day is definitely the best plan, if you want to maximise insulin sensitivity and decrease your risk of developing diabetes. The fly in the ointment is that meal time preference has strong genetic influences, making it rather difficult to shift. So is it worth making that effort, if you’re a night owl who tends to eat most of your calories later in the day? Let’s dig into the study, to find out.

Chrononutrition: The new frontier of nutrition science

Chronobiology – the study of the influence of the circadian rhythm³ on biological function – is a burgeoning field of research. Among its major sub-fields is chrononutrition – the relationship between the timing of our meals, and what our bodies do with the food we eat:

“Our body reacts differently to the same food consumed at different times of the day, showing diurnal variation in glucose tolerance, postprandial [after-meal] hormone secretion, thermogenesis [after-meal ‘calorie burn’], metabolite levels, and other metabolic processes.”

Later eating timing in relation to an individual internal clock is associated with lower insulin sensitivity and affected by genetic factors

In other words, the very same meal has vastly different metabolic effects, depending on whether you eat it as breakfast or as dinner (‘supper’ or ‘tea’, for our English friends). Regardless of its size or composition, that meal, when eaten in the morning, will require less insulin secretion, and produce a smaller ‘bump’ in blood glucose level and a greater after-meal calorie burn, resulting in less of it ending up in fat stores than if you were to eat it at night.

Another field of study within chronobiology is chronotype – the tendency of individuals to be ‘morning larks’ (early chronotype) who naturally rise early and go to bed early; ‘night owls’ (late chronotype) who gravitate toward sleeping in and going to bed late, and intermediate chronotypes who are somewhere between these extremes. Chronotype is strongly heritable, meaning that our genes exert a powerful influence on our preference to either rise early and fade with the sunset, or to come alive at night.

The confluence between chrononutrition and chronotype makes twins an interesting group to study. Why? Because there are two types of twins: monozygotic twins – commonly called ‘identical twins’ – who result from a single sperm fertilising a single egg, which subsequently splits in two; and dizygotic twins – also known as fraternal or nonidentical twins – who develop from two separate eggs fertilised by two separate sperm cells. Monozygotic twins have exactly the same genes, whereas dizygotic twins are only as genetically similar to each other as are any two siblings, sharing roughly 50 per cent of their DNA⁴. So if you want to tease out the effects of genes vs environment (including upbringing) on various behaviours and health outcomes, a combination of monozygotic and dizygotic twins, with each twin pair having been raised together, is just what the scientist ordered.

The German researchers recruited 46 twin pairs without diabetes, of which 32 were monozygotic and 12 were dizygotic. All participants took an oral glucose tolerance test to assess their insulin sensitivity; kept detailed written food records for five days, specifying what, how much, and when they ate; and also noted when they woke up, and when they felt ready to fall asleep.

From these data, the researchers first sorted participants into early, intermediate and late chronotypes. Then, the clock times at which participants had recorded their food intake were adjusted for chronotype, to derive circadian times of their onset of first eating occasion, cessation of latest eating occasion, and caloric midpoint (the time at which they had consumed half of their daily calorie intake).

This adjustment from clock times of meals to circadian times was made to account for the fact that one’s individual biological circadian rhythm plays a major role in chrononutrition. For example, if a morning lark who’s up’n’at’em at 5 am and fast asleep by 9 pm, were to eat breakfast at 9 am and dinner at 7 pm, these meals would be late according to their personal circadian phase. But these same meal times would be early for a night owl who rolls out of bed at 8.59 am and goes to sleep at 1 am.

Considered as a whole, participants consumed on average 24 per cent of their daily calories in the first meal, and 34 per cent in the last meal, with the caloric midpoint of intake at around 3:50 pm (clock time).

But later chronotypes consumed more of their calories in their last meal, and hence had a later circadian time of the caloric midpoint – that is, their calorie intake was shifted into the latter part of their day, even after accounting for the fact that they got up and went to bed later than the early chronotypes.

The metabolic consequences of late eating

Having a later circadian time of caloric midpoint was associated with poorer insulin sensitivity and greater insulin resistance, meaning that participants who ate the bulk of their calories later in their circadian day had to secrete more insulin to deal with the post-meal rise in blood glucose level, and their blood glucose stayed elevated for longer, compared to those who stacked their caloric intake into the earlier part of their circadian day. The late eaters also had a higher fasting insulin level, body mass index (BMI), waist circumference, and waist-to-hip ratio – all indicators of poorer metabolic health. Disposition index, a measure of the relationship between insulin secretion capacity and insulin sensitivity and hence a predictor of conversion from insulin resistance to type 2 diabetes, was also associated with circadian time of caloric midpoint, suggesting that consuming most of one’s calories early in the day may be protective against diabetes.

Conversely, those who consumed a higher percentage of their daily calories in the first half of their circadian day had a lower fasting blood glucose level and better glucose tolerance.

The short-and-sweet summary of these findings is that if you want to reduce your risk of developing type 2 diabetes, you should eat more of your calories at breakfast and lunch and less at dinner, regardless of what time you eat your meals.

Meal time preference: Nature, nurture or current environment?

Turning to the genetic influence on chronotype and meal timing, the researchers found that monozygotic twins were more likely than dizygotic twins to share a chronotype, which confirmed previous research showing that chronotype is highly heritable – that is, more than 50 per cent of the variance between individuals in personal circadian rhythm, is attributable to genetic factors. Likewise, monozygotic twins were more likely to have similar meal timing patterns. The only exception was duration of eating window, which showed a stronger correlation in fraternal than identical twins.

The circadian time of the last meal of the day, however, was less influenced by genetic factors than the first meal, which led the researchers to suggest that trying to eat breakfast earlier than one is naturally inclined to do, would be harder than shifting dinner to an earlier time. (As a practical matter, however, many people’s evening meal time is dictated by external factors over which they have little to no control, such as what time they get home from work, and their children’s extracurricular activities.)

One other finding of note was that the length of the eating window showed no associations with BMI, waist circumference or glucose tolerance, confirming other studies which did not find any benefits of time-restricted eating for weight management or metabolic health.

The bottom line

Chrononutrition research is providing ever more evidence that while what we eat is of paramount importance to health, when we eat it is an important factor too. Data from various studies, including the German one discussed above, converges on a single message: Stacking your calories earlier in the day is an important strategy for avoiding weight gain and maintaining insulin sensitivity, which reduces your risk of developing cardiometabolic diseases including type 2 diabetes.

If you’re a lifelong breakfast-skipper who just can’t summon up an appetite in the morning, don’t despair! The first step to take is to shift the timing of your final meal of the day, so that you finish eating earlier – at least two hours before going to bed, and preferably three hours. Once you polish off dinner, floss and brush your teeth so that you’re not inclined to snack throughout the evening. If you practise this habit consistently, you’ll find yourself getting hungry earlier the following day, which will facilitate a gradual transition toward eating more of your calories in your circadian morning, regardless of your chronotype.

And for those who tried to implement time-restricted eating but had trouble delaying their daily eating start-time, the good news is that you can relax! There’s no magic in maintaining a shorter eating window…. and in fact, late time-restricted eating – that is, a shorter eating window in which calorie consumption is pushed into the second half of the day – appears to be counterproductive.

Finally, comparison of fraternal vs identical twins sheds light on the power of genetic influences to shape our chronotype and meal timing preferences. But in this case (as in most conditions and behaviours with high heritability), genes are not destiny. It is possible to shift your meal timing and distribution of calories to a healthier pattern, if you have the motivation to do it. And avoiding type 2 diabetes – a truly horrible condition that can lop as many as 14 years off your life expectancy – ought to be a good motivation to make these changes!

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1. Apropos of absolutely nothing in this post, Adelle Davis died of multiple myeloma at the age of 70, a sad irony for this heavy milk drinker who claimed in one of her books that “she had never known a single adult who drank a quart of milk a day to develop cancer”. Davis was reportedly a chain smoker, and also experimented with LSD in the late 1950s “when it was still legal”, as she stressed. ↩︎
2. Often mistakenly called ‘intermittent fasting’, a term that more properly refers to protocols such as alternate day fasting and the 5:2 diet. ↩︎
3. Circadian rhythm refers to our 24-hour multi-system ‘body clock’ that regulates behaviour and metabolism via a central clock in the brain – the suprachiasmatic nucleus, which receives time cues primarily via fluctuating light levels throughout the day, and peripheral clocks in organs such as the liver, pancreas and intestines, which respond to behavioural cues such as food intake and nutrient availability. ↩︎
4. Although, as the younger sibling of opposite-sex dizygotic twins, I can attest that twins who shared a womb, but not each other’s DNA, still have a spooky connection with each other. I recall many instances of my brother coming inside from the backyard, humming a tune that my sister had been singing indoors (but inaudibly, from outside), and of one of them having a gut feeling that the other was in some kind of trouble, which turned out to be prescient. ↩︎