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Why are calories eaten in the morning apparently less fattening than calories eaten in the evening? One reason is that more calories are burned off in the morning due to diet-induced thermogenesis. That’s the amount of energy the body takes to digest and process a meal, given off in part as waste heat. If you take people and give them the exact same meal in the morning, afternoon, and night, their body uses up about 25 percent more calories to process it in the afternoon than night, and about 50 percent more calories to digest it in the morning. That leaves fewer net calories in the morning to be stored as fat.

Let’s put some actual numbers to it. A group of Italian researchers randomized 20 people to eat the same standardized meal at 8am or at 8pm, and then a week later had them all come back in to do the opposite. So, each person had a chance to eat the same meal for breakfast and for dinner. After each meal, the subjects were place in a “calorimeter” contraption to precisely measure how many calories they were burning over the next three hours. The researchers calculated that the meal given in the morning took about 300 calories to digest, whereas the same meal given at night used up only about 200 calories to process. The meal was about 1,200 calories, but given in the morning, it ended up only providing about 900 calories, compared to more like 1,000 calories at night. Same meal, same food, same amount of food, but effectively 100 fewer calories. So, a calorie is not just a calorie. It depends when we eat them.

Why do we burn more calories eating a morning meal—is it behavioral or biological? If you started working the graveyard shift, sleeping during the day and working all night, which meal would net you fewer calories? Would it be the “breakfast” you had at night before you went to work, or the “supper” you had in the morning before you went to bed? In other words, is it something about eating before you go to sleep that causes your body to hold on to more calories, or is it built into our circadian rhythm, where we store more calories at night regardless of what we’re doing? You don’t know, until you put it to the test.

Harvard researchers randomized people to identical meals at 8am versus 8pm while under simulated night shifts or day shifts. And regardless of activity level or sleeping cycle, the calories burned processing the morning meals was 50 percent higher than in the evening. So, the difference is explained by chronobiology; it’s just part of our circadian rhythms to burn more meal calories in the morning. But why? What exactly is going on?

How does it make sense for our body to waste calories in the morning when we have the whole day ahead of us?

Our body isn’t so much wasting calories as investing them. When we eat in the morning, our body bulks up our muscles with glycogen, which is the primary energy reserve our body uses to fuel our muscles. But this takes energy. In the evening, our body expects to be sleeping for much of the next 12 hours, so rather than storing blood sugar as extra glycogen in our muscles, it preferentially uses it as an energy source, which may end up meaning we burn less of our backup fuel (body fat). In the morning, however, our body expects to be running around all day, so instead of just burning off breakfast, our body continues to dip into its fat stores while we use breakfast calories to stuff our muscles full of the energy reserves we need to move around over the course of the day. That’s where the “inefficiency” may come from. The reason it costs more calories to process a morning meal is because instead of just burning glucose (blood sugar) directly, our bodies are instead using up energy to string glucose molecules together into chains of glycogen in our muscles, which are then just going to be broken back down into glucose later in the day. That extra assembly/disassembly step takes energy—energy that your body takes out of your meal, leaving you with fewer calories.

So, in the morning, our muscles are especially sensitive to insulin, rapidly pulling blood sugar out of our bloodstream to build up glycogen reserves. At night, though, our muscles become relatively insulin resistant. Our muscles resist the signal to take in extra blood sugar. So, does that mean you get a higher blood sugar and insulin spike in the evening compared to eating the exact same meal in the morning? Yes. In that 100-calorie difference study, for example, blood sugars rose twice as high after the 8pm meal compared to same meal in the morning. So, shifting the bulk of our calorie intake towards the morning would appear to have a dual benefit—more weight loss, and better blood sugar control.

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