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Does when you eat matter?

Most weight loss advice is based on the idea that a calorie is a calorie, and it doesn’t matter if it is consumed in the morning or at night. The message is clear - eat fewer calories and weight will be lost. However recent research has shown that our bodies may use calories more efficiently when consumed earlier rather than later in the day which could help weight management.

The ancient clock that rules our lives

Within each of us is a biological clock geared to a 24-hour day/night cycle. This biological or “circadian” clock determines why we feel tired at night and alert during the day, it also regulates the timing of the body’s processes including digestion, metabolism, and appetite.

How does our biological clock affect how we eat?

If we eat three meals at regular four/five hour intervals throughout the day the body’s biological clock runs smoothly and the hormones controlling appetite and eating do their job well. They help to regulate energy intake and expenditure and keep body weight stable.

What happens if the biological clock is disrupted?

Data from a UK national survey diet which tracked the health of more than 5,000 people for over 70 years, showed people who had an irregular meal routine had a higher risk of gaining weight, developing high blood pressure and other metabolic disease.

Shift working

Shift workers work when they are sleepy and sleep when they are not. This has a long-term, profound effect on their biological clocks and health. Years of night-shift work has been shown to increase the risk of heart disease, cancer, type 2 diabetes and obesity. 

What is the science behind this?

Disrupting biological rhythms causes changes to many important hormones that regulate appetite, energy expenditure and glucose regulation. These changes can increase appetite while decreasing energy expenditure leading to more calories eaten, but less used during the day.

Eating late at night

Eating more calories late at night has been linked to weight gain and obesity possibly because of lower appetite regulation in the evening. Late night eating may also reduce sleep, interrupt the biological clock and lead to feeling too tired the next day for physical activity.

Eating in the morning

Some research has shown eating more calories in the morning may lead to greater weight loss and better regulation of glucose and insulin levels. Sensitivity to insulin is greater during the morning than at night.

Studies have also shown that people who eat breakfast have higher levels of physical activity compared to those that do not.

Can eating within a limited period help manage weight?

Time restricted feeding (TRF) is when food is eaten within a consistent daily window e.g., eight hours. Research shows this appears to support weight loss mainly though reducing calorie intake. Restricting the period of eating also encourages the body to use its fat reserves as it spends longer fasting.

Eating during a restricted period may also reinforce natural biological rhythms and prevent late-night eating.

To sum up

It is a good idea to try to regulate the timing of meals. Spending time outside during daylight hours helps to reset the master biological clock in the brain so you feel hungry and tired at the right time. Eating breakfast soon after you wake up reinforces the message to your brain, liver and digestive system that it is morning, and the day has begun and is important for keeping our biological clocks running smoothly.

Your body clock, metabolism and digestion interact in complex ways - meaning it's not just what you eat, but when you eat that matters.