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We’ve known for more than a half century now that our “glucose tolerance” declines as the day goes on––meaning the ability of your body to keep blood sugars under control. If you hook yourself up to an IV, and just drip sugar water into your vein at a steady pace throughout the day, at about 8pm your blood sugars start to go up, even though you haven’t eaten anything and the infusion rate didn’t change. The same amount of sugar is going into your system every minute, but your ability to handle it deteriorates in the evening, but bounces right back in the morning. A meal eaten at 8pm can cause twice the blood sugar response as an identical meal eaten at 8am. It’s as if you ate twice as much!

Your body just isn’t expecting you to be eating when it’s dark outside. Our species may have only discovered how to use fire about a quarter million years ago. We just weren’t built for 24-hour diners.

One of the tests for diabetes is called the glucose tolerance test, to see how fast your body can clear sugar from your bloodstream. You swig down a cup of water with about four and a half tablespoons of regular corn syrup mixed in, and then have your blood sugar measured two hours later. By that point, your blood sugar should be under 140 (mg/dL). Between 140 and 199 is considered prediabetes, and 200 and up is a sign of full-blown diabetes.

The circadian rhythm of glucose tolerance is so powerful that a person can test normal in the morning, but as a prediabetic later in the day. Prediabetics who average 163 at 7am test out as frank diabetics by 7pm, at over 200.

Choosing lower-glycemic foods may help promote weight loss, but timing is critical. Due to this circadian pattern in glucose tolerance, a low-glycemic food at night can cause a higher blood sugar spike than a high-glycemic food eaten in the morning. We’re so metabolically crippled at night: researchers found that eating a bowl of All Bran at 8pm caused as high a blood sugar spike as eating Rice Krispies at 8am. High-glycemic foods at night would seem to represent the worst of both worlds. So, if you’re going to eat refined grains and sugary junk, it might be less detrimental in the morning.

The drop in glucose tolerance over the day could therefore help explain the weight-loss benefits of front-loading calories towards the beginning of the day. Even just an earlier versus later lunch may make a difference. People randomized to eat a large lunch at 4:30pm suffered a 46 percent greater blood sugar response compared to an identical meal eaten just a few hours earlier at 1pm. And a meal at 7am can cause 37 percent lower blood sugars than an identical meal at 1pm. Now there doesn’t seem to be any difference between a meal at 8pm and the same meal at midnight—they both seem to be too late. But eating that late, at midnight, or even 11pm can so disrupt your circadian rhythm that it can mess up your metabolism the next morning––resulting in significantly higher blood sugars after breakfast, compared to eating the same supper at 6pm the evening before.

So, these revelations of chronobiology bring the breakfast debate full circle. Breakfast-skipping not only generally fails to cause weight loss, but worsens overall daily blood sugar control in both diabetic and non-diabetic individuals. See how the breakfast-skippers have higher blood sugars even while they’re sleeping 20 hours later? This may help explain why those who skip breakfast appear to be at higher risk of developing type 2 diabetes in the first place.

Breakfast skippers also tend to have higher rates of heart disease and atherosclerosis in general. Is this just because breakfast-skipping tends to cluster with other unhealthy choices, such as smoking and sicklier eating habits overall? The link between breakfast skipping and heart disease—even premature death in general—seems to survive attempts to control for these confounding factors. But you don’t really know, until you put it to the test.

Does skipping breakfast lead to higher cholesterol, for example? Yes, a significant rise in LDL (bad) cholesterol in those randomized to skip breakfast––about 10 points higher within just two weeks. The Israeli 700/500/200 study found that the triglycerides of the king-prince-pauper group got significantly better—a 60-point drop—while those of the pauper-prince-king group got significantly worse (a 26-point rise). So, consuming more calories in the morning relative to the evening may actually have a triple benefit: more weight loss, better blood sugar control, and lower heart disease risk. So, if you’re going to skip any meal, whether practicing intermittent fasting or time-restricted feeding (where you try to fit all your food into a certain daily time window), it would be safer and more effective perhaps to skip dinner rather than breakfast.

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