Doctor’s Tip: More about the link between circadian rhythms and health
The dictionary definition of circadian rhythm is “repetition of certain phenomena in living organisms at about the same time each day.” The dictionary definition of the closely related word chronobiology is “the scientific study of the effect of time on living systems.” In his new book “How Not to Diet,” Dr. Greger defines chronobiology as “the study of our bodies’ natural rhythms.”
Last week’s column discussed how eating more calories early in the day and fewer late in the day contributes to attaining and maintaining ideal body weight — an example of chronobiology. Today’s column will discuss other examples of chronobiology. Dr. Greger points out that “most of our genes exhibit daily fluctuations in expression, making the circadian rhythm the largest known regulatory system in our bodies.” At night, when we’re sleeping, “a whole array of internal housekeeping activities can be switched on, such as clearing accumulated waste products from the brain … and as dawn approaches, our bodies can shift back into activity mode.”
Here are some other examples — aside from time of eating and weight gain — of how circadian rhythms affect health:
• If chemotherapy is given at the proper time, it can be five times less toxic and twice as effective versus giving it at “the wrong time.”
• Recent studies show that high blood pressure is best controlled if blood pressure medication is taken at bedtime. Furthermore, fewer heart attacks and strokes occur, and risk of death is cut in half.
• Core body temperature is lowest at around 4 a.m. (97.6), down from an average of 98.6 during the day.
• Hormone production varies according to time of day.
• Digestive processes vary according to time of day, as anyone knows who had gone to bed on a full stomach.
• There are circadian swings in immune function.
• Glucose tolerance — the ability of our tissues and organs to use insulin — is high in the morning but declines later in the day. Insulin resistance (I.R.) is common in people with extra weight around the middle, and leads to pre-diabetes and diabetes. High triglycerides and low HDL (good cholesterol) are common signs of insulin resistance — which is the cause of at least 70 percent of cardiovascular disease. The gold standard for diagnosing I.R. is a 2-hour glucose tolerance test (GTT). Due to chronobiology, a person with a predisposition to I.R. can have a normal GTT in the morning but an abnormal one later in the day. In diabetics, a meal eaten at 1 p.m. can result in a 37% higher blood sugar response than the same meal eaten at 7 a.m.
• People who skip breakfast have a higher risk of developing diabetes.
• Cholesterol levels vary throughout the day. Skipping breakfast is associated with higher levels.
• Breakfast skippers have a higher rate of hardening of the arteries and heart attacks.
• Even the trillions of bacteria in our gut microbiome have their own, 24-hour circadian rhythms.
What happens if we disrupt our circadian rhythms? Jet lag is a good example. Night-shift works are another example — they have higher rates of obesity, diabetes, cardiovascular disease, and cancer (it isn’t called the “graveyard shift” for nothing). Bright light — especially LED light — before bedtime can disrupt sleep and circadian rhythms.
Greg Feinsinger, M.D. is a retired family physician who has a nonprofit: Prevention and Treatment of Disease Through Nutrition. He gives a free presentation at 7 p.m. the first Monday of the month at the Third Street Center in Carbondale; is available by appointment for free consultations (379-5718); and conducts a shop-with-a-doc session at 10 a.m. the first Saturday of the month at Carbondale City Market.
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