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When you should eat – according to science

food in the shape of a clock

When I was about 14, I decided that getting a six pack was really important. Certainly more important than GCSE French. Ever since then, I’ve had an eye on how much I’ve eaten. A teacher at school said that you just needed to burn off what you put in, and as a sporty, quantitative-minded teenager, that was appealing. If I burned off tonnes of calories running around, I could eat what I wanted, when I wanted, and I’d be in great shape.

Putting aside the fact that my goals were clearly misaligned, this story contains some important misconceptions about nutrition. For a start, the teacher’s maxim didn’t say anything about what to eat. I’ve since tried to make up for that by eating veg, making sure I get enough protein, and avoiding having tonnes of sugar. But the advice also missed out a further important factor: it matters when we eat.

The problem with diet advice

Diet advice is notoriously difficult to get a grip of. For starters, there’s just so much of it. And it seems to change every few years, if not sooner. Fat is bad. Then it’s good again. You need to eat for your blood type. Or that’s a load of bullshit. Do superfoods really exist, or are they just a way of marketing spirulina capsules?

The same goes for when you should eat. Eating little and often was once in vogue. Now chugging down as much as you can in one meal seems to be popular, at least amongst the tech bros. So I decided to look at the research on meal timings and try to keep an open mind about what’s best, rather than copying the diet of the latest internet guru with rock hard abs.

Why it matters when you eat

First, I want to note some caveats. Eating affects so many things that there isn’t a one-size-fits-all diet for everyone. When you eat depends on convenience, enjoyment, social and family life, your working patterns and your finances. You can also be trying to achieve different things with your eating habits, like maximum longevity, fighting disease, or optimal nutrition for a particular sport. However, I’m only going to talk about how meal timing affects how much weight you tend to gain or lose. In other words, when you should eat to maintain a low level of body fat.

I’m not suggesting everyone should be as shallow as my 14-year-old self in trying to get a six pack, by the way. But being overweight or obese – if that weight is fat rather than lean muscle mass – is linked to increased risk of diabetes, heart disease, some cancers, and stroke – not to mention depression and low self-esteem1. So even if you’ve got a healthier attitude towards how your body looks, keeping excess fat off is no bad thing.

Eating later means storing more energy as fat

There are lots of studies on when to eat, but a 2019 review of the evidence suggests that having a higher energy intake later in the day puts you at significantly greater risk of obesity than having higher energy intake in the morning.

To give you an idea of how these studies tend to work, one of the trials reviewed was of 420 obese people in Spain. Those who ate lunch after 3pm lost less weight than those who had an earlier lunch, in spite of having similar age, energy intake and expenditure, sleep duration, and distribution of macronutrients like protein and carbohydrate.

Even if you eat the same amount and types of food, eating it later in the day means you’re more likely to store it as fat.

How does that work?

Our bodies have a different response to food depending on when we eat it. The authors of the review conducted their own trial of 32 subjects, giving one half an early lunch and one half a late lunch. The late lunch group burnt less energy when resting, and broke down carbohydrates slower. If we don’t break down food to use as energy when we eat it, it’s stored as fat in the liver and ‘adipose tissue’ (i.e. what we see on our body as ‘fat’).

If we think about this from an evolutionary perspective, it makes sense. Any energy that we don’t immediately need is stored as fat for future use. Later in the day, it’s less likely we’ll need to use food as energy, because it’s more likely we’ll be going to sleep. In other words, your body makes predictions about how active you’re going to be for the rest of the day based on how late it is in your circadian day. It then uses this prediction to store more or less food as fat.

But your body clock matters more than the actual time

However, there is further nuance. A 2017 study of 110 young people found that it was not the ‘clock hour’ of meal timings that determined the fat storage, but the ‘circadian timing’ i.e. where that fitted into the person’s own day/night rhythm. Whilst the sun goes up and comes down at a certain point, we each have our own ‘body clock’ that doesn’t sync exactly with light and dark. So how much of our food is stored as fat depends not on the actual time of day, but how ‘late’ our bodies perceive it to be.

So generally speaking, if we want to turn less of the food we eat into fat, it’s better to shift that eating further back in the day. A decent rule of thumb is to finish eating before the sun goes down, although that’s only a proxy for the real thing that matters, which is our body clocks.

It doesn’t matter too much whether you eat breakfast

Another question that often comes up is whether eating breakfast helps maintain a healthy weight. The same scientific review suggests that while eating breakfast is often associated with lower levels of obesity, it is not clear why. It is possible that those who don’t eat breakfast compensate by consuming more calories later in the day, and thus gain fat.

What about an early breakfast?

There is even some evidence to suggest that having a very early breakfast can cause you to gain fat. That could be because people haven’t always got rid of all the melatonin in their bodies by the early morning. Melatonin is the hormone that induces sleep, so having some left in your system could trick your body into thinking you aren’t going to be active.

A study of rats suggests that the timing of your first meal i.e. breakfast can play a role in regulating the circadian (day/night) rhythms of your peripheral organs such as the liver. Eating early might thus cause those organs to move into ‘night mode’ earlier, and store food as fat from an earlier point in the day.

In conclusion, it can pay to have breakfast if it prevents you gorging on food later in the day, but it’s probably not strictly necessary for maintaining healthy weight. And eating breakfast before the sun comes up might actually cause you to put on more fat.

Reducing the number of meals might make a difference – but only if you eat less

The research on the number of meals is more limited than the above two questions. However, this 2007 randomised controlled trial comparing a group who ate one meal with a group that ate three meals found that those eating one meal had:

a significant increase in hunger; a significant modification of body composition, including reductions in fat mass; significant increases in blood pressure and in total, LDL-, and HDL-cholesterol concentrations; and a significant decrease in concentrations of cortisol.

The results are mixed here. Reductions in fat mass and the stress hormone cortisol are positive effects, and the increase in ‘good’ HDL-cholesterol is probably beneficial. But increased hunger, ‘bad’ LDL-cholesterol and blood pressure are not.

Another 2007 study found increased glucose sensitivity (i.e. a ‘spike’ in blood sugar following a meal, which we’d rather not have) in those having one meal rather than three. The authors suggested that the purported benefits of so-called ‘intermittent fasting‘ might be due more to consuming fewer calories overall than in restricting the number of meals.

Having more meals probably isn’t better either

A 2010 study found that those who had six meals over the course of a 12-hour window had higher glucose levels over the course of the day than those who had only three meals. Since elevated glucose levels are associated with conditions like low glucose tolerance (which can be a precursor to diabetes), it’s generally better if glucose levels stay within a lower range.

As you might expect, the glucose levels were less extreme if the meals were high in protein, relative to carbohydrate. This study was only of eight subjects however, so there is more research to be done.

Overall, it seems that three decent-sized meals – without tonnes of sugary snacks in-between – is probably a reasonable approach. However, if you find it hard to restrict the amount of food you eat, then reducing the number of meals you have can be an effective strategy for reducing total calories.

Conclusion

The science of when to eat is not exact, and will vary from person to person. But it’s clear that when we eat does play a role in how food is broken down and thus whether we use it for energy or it’s stored as fat. That is important for our long-term health, as well as just trying to get beach body ready.

My own takeaways from the science, that I will be applying to my own life are as follows:

  • Try to make breakfast and lunch bigger than dinner
  • Try to eat within daylight hours, and especially not close to bedtime
  • Don’t worry too much about the number of meals, but try not to have many carbohydrate-heavy snacks between meals.

Footnotes

  1. See the NHS page on obesity