A study of 20 healthy, normal-weight people found that changing their evening meal from 6PM to 10PM significantly increased their markers for becoming obese and developing diabetes (J Clin Endocrinol Metab, Aug 1, 2020;105(8):dgaa354). They had:
• higher blood sugar
• higher insulin
• higher cortisol levels
• reduced ability to remove and use fat from their cells
These are all major risk factors for obesity. Eating before going to bed did not affect their ability to fall asleep. Other studies have also shown that eating before going to bed promotes obesity and diabetes (Pharmacol Res, 2017;125(Pt B):132–141), and that eating most of your food in the morning and less in the evening helps people to lose weight (J Nutr, 2017;147(9):1722–1728).
Why Eating at Night is Unhealthful
Eating just before you go to bed causes high blood sugar levels and increased amounts of fat to be deposited in fat cells. Resting muscles draw almost no sugar from the bloodstream and what little they do remove from the bloodstream requires insulin (Sports Medicine, Feb 2, 2018;1-13), while contracting muscles pull large amounts of sugar from the bloodstream and don’t even need insulin to do so (Diabetes Research and Clinical Practice, Sept 2007;77(3):S87–S91).
• If you do not move around and contract your muscles after eating, you increase risk for high blood sugar levels.
• You burn the lowest amount of calories when you sleep. When you go to sleep after eating, you burn fewer calories from that food so more of it is stored as fat (Metabolism, 2009;58(7):920–926).
• Several studies show that blood sugar levels respond best to insulin during the day and worst at night (Nat Rev Endocrinol, 2019;15(2):75–89).
• Cortisol raises blood sugar levels by blocking the effects of insulin, and cortisol levels are higher when you are sleeping (Ann NY Acad Sci, 2017;1391(1):20–34).
Sequence of Abnormal Blood Tests as You Become Diabetic
Normal morning fasting blood sugar is <100 mg/dL. As you become diabetic, you develop (in order):
• high blood sugar one hour after meals >145mg/dL
• high fasting insulin level >25 mIU/L
• high triglycerides >150 mg/dL (insulin converts all extra sugar into a fat called triglycerides)
• low good HDL cholesterol, <40mg/dL (you use up the good HDL cholesterol carrying triglycerides from the bloodstream to your liver)
• fatty liver
When your liver fills up with fat, you are diabetic. A fatty liver does not accept the sugar driven by insulin from your bloodstream, so blood sugar remains high and you develop diabetes that causes heart attacks, strokes and death. Your doctor can determine that you have a fatty liver with a sonogram.
The least healthful time to eat is just before you go to bed, and the most healthful times to eat are before you exercise or within an hour after you finish exercising. Exercising after eating causes contracting muscles to pull sugar from the bloodstream, which helps to prevent high rises in blood sugar. Eating within an hour after exercising also prevents a high rise in blood sugar. Your muscles can extract sugar from the bloodstream maximally without needing insulin for about an hour after you finish exercising, but this ability is then gradually lost over about 17 hours or until you contract your muscles again (J Appl Physiol, 2005;8750-7587).