We know that what we eat and how much we eat has significant health consequences. But what about the timing of the meal?
I know there are many professionals who believe that a calorie is a calorie, no matter what time of the day - but science has started to prove otherwise.
There is a new term called ‘chrono-nutrition’, which refers to coordinating food intake with the body’s daily rhythms or your “body clock”.
Any disruption to this rhythm (think “night owls”, like shift workers) can throw the body’s rhythm out of balance and increase the risk of chronic disease.
For many decades, it has been thought that “your body doesn’t care about what time you eat, it’s how much you eat that matters”. However, alarming results from observational studies report that shift workers are at 40% higher risk of type-2 diabetes and cardiovascular disease, which may be related to their night time eating habits.
Chrono-nutrition is defined as the relationship between food and eating and the circadian clock system.
Let’s understand what the circadian clock is.
It has 2 main components: the master clock, and multiple peripheral clocks in the body.
The maser clock is in the hypothalamus (our brain) and it receives signals from the environment (day-light/sunshine) which tells the body if it’s day time or night time.
There are also multiple clocks around the peripheral organs of the body (the heart, the liver and gut) and both these clock systems work together to generate the circadian rhythm: the circadian rhythm is a cycle that happens once daily (~24 hours) in humans – and the processes that follow this circadian rhythm are the sleep/wake cycle, body temperature, and all other metabolic processes that need to happen to keep us alive.
For example: our body temperature drops slightly while we sleep (between ~12:00am – 6:00am or the early hours of the morning) and then increases again from approximately 8:00am onwards.
This circadian rhythm therefore dictates when is the most important time for us to eat, do work, and sleep. For humans, we are diurnal animals, meaning we are made to eat and work during the day, and sleep and fast during the night.
Why is it beneficial for organisms, animals and humans to do certain things during the day and others during the night?
Our body clocks ensure that certain processes are active during the day, and others during the night, so our energy can be used most efficiently. Our human-body clocks tell us that it is best to eat during the day and fast during the night. So if we happen to be people who do things ‘around the clock’, we’re actually inefficiently using our energy.
What happens when we do this? What happens when we do work against our body clock?
This is where chrono-nutrition comes into it – let’s understand how eating at night (habitually) can affect our metabolic health.
Contemporary lifestyles have resulted in us eating very late at night: A study done in 2015 asked 150 participants to record how often they were eating and at what time. What they found was that, people are eating for around 14.75 hours/day (between 7am to 12am) with up to 14 eating occasions per day!! This portrays that at a population, we are eating way too often, and against our natural body clock.
Why is this important to realise?
Eating late in the night has been associated with increased risks of obesity, metabolic syndrome, hyperglycaemia, type 2 diabetes and cardiovascular disease.
How does eating during the day differ from eating at night?
Researchers have previously tested how food is digested differently from day to night. Ten healthy, non-diabetic adults were given a meal either at 8:00am, or at 8:00pm and their blood glucose levels were tested before, right after, and 2 hours after the meal was consumed.
The evening meal resulted in double the increase in blood glucose level compared to the morning meal! Even though the meal was exactly the same.
Not only did blood glucose increase by double at night time, blood glucose levels stayed elevated for a longer time. They even found that one of the participants’ glucose levels got so high that they would have been within the criteria of being defined as a diabetic! (Reminder: the adults were health and non-diabetics)
Eating late at night causes increased blood glucose and insulin response which is related a decrease in insulin sensitivity and B-cell responsiveness.
What does this all mean?
Insulin sensitivity looks at how well our peripheral tissues (muscles and organs) respond to the signals of insulin – studies have reported that this is less at night, which means these peripheral tissues aren’t as efficient / good at responding to the insulin – so the insulin has to ‘nag’ the tissues to take up the glucose. This means two things: insulin is going to continue to rise (bad news) and so is the glucose (also bad news).
Increased glucose and insulin in the blood stream are both independent factors for type 2 diabetes and cardiovascular (heart) disease.
Our master clock tells us that night time is for resting – so eating at night time, causes a disruption between what the clock tells us and what we are actually doing.
This study then looked at what happens when we eat the same low glycaemic index (GI) meal at 8am, 8pm and midnight. Eating at night time (8pm) had a 69% higher glucose response than the morning meal. After the midnight meal, blood glucose levels didn’t return to baseline for 3 hours! Whereas after the morning meal, the blood glucose levels returned to baseline after 45 minutes.
Figure from Leung, GKW presenting for Nutrition Society of Australia
Even though a low GI meal works well in the morning (that is, doesn't skyrocket blood glucose levels), the glycaemic response was not replicated during the night.
There is good news!
A habitual high protein and low carbohydrate diet has been shown to improve glycaemic control. A study (by Davis 2018 of Monash Uni, results are not yet published) invited participants to come in and have the same high protein-low carbohydrate meal at both morning and night. A few weeks later, the same participants came in to have a ‘control’ meal both morning and night (this control meal was low GI with moderate protein).
As expected, the evening response for glucose was higher compared to the morning, no matter which meal it was (either the high protein/low carb, or the low GI with moderate protein). BUT the high-protein meal did produce a much lower glucose response compared to the control meal.
Figure from Leung, GKW presenting for Nutrition Society of Australia
What does this all mean?
This is a pretty new research area and is mostly directed at shift workers. However, the more we unravel, the more evident it is that we should try to eat during the day and avoid, where possible, eating late at night.
For shift workers, this document is very helpful in explaining shift work and it’s effect on our health, and some key recommendations to try to follow.
For the general public, especially those who are interested in cardiovascular disease prevention, the American Heart Association has developed “Top Ten Things to Know: Meal Timing and Frequency: Implications for CVD Prevention”
So, all in all, what can we do with all this information?
Eating a greater share of our total calorie intake earlier in the day can have positive effects on risk factors for heart disease and diabetes. Research shows that a higher-calorie breakfast and a lower-calorie dinner can help support weight loss and reduce the risk of metabolic syndrome. Ever heard of the saying: “breakfast of a king, lunch of a prince and dinner of a pauper”? It has never rang truer!
Consistent fasting overnight (i.e. when we’re sleeping) has been shown to have many positive effects on health.
What if we are shift workers and have no other choice?
Having a small (~5 hour) fasting window during the night – any time between 10pm and 6am, has been shown to be more beneficial than eating during those hours.
If eating must occur during the night, choosing a snack with healthy protein, rather than a carbohydrate-rich*** food, can help blood glucose control and reduce the severity of negative impacts that eating at night might have on our body. I’ve popped a couple of info-graphics below for you for some ideas!
***This does not mean carbohydrate foods are bad, we just have to consider the time we are eating them – especially for those who are eating against their body clock.
Finally, be on a regular sleeping schedule. Our bodies love routine! Although sleeping during daylight hours and working during night-time hours may impact metabolism, leading to a greater chance of putting on weight if you’re a shift worker. Regardless, however, try to get about eight hours of sleep per day – even better if it’s around the same time every day. Create a calm and dark space where you can wind down and settle into a restful sleep.
Hope I've taught you something new today! Just keep in mind this research is mainly based for those 'night owls' who are awake all night and sleep during the day. So if you are someone who doesn't have that lifestyle, I wouldn't stress about putting a timeline on when to stop eating. It is more about having a balanced intake of food (type/quantity) during the day so your body doesn't [regularly] feel the need to eat at night!
Joyce Haddad, Director of A Dietitian's Mission, is an Adelaide based Dietitian, Nutritionist and Master Personal Trainer with a passion for health and wellbeing. ADM aims to help the public make informed and realistic nutritional choices and ensure everyone has a healthy relationship with their body and with food.