The second-meal effect: why dinner helps decide your blood sugar in the morning

Lentils at breakfast lower blood sugar at lunch, whole barley kernels in the evening lower it at breakfast. How this works, and what role gut bacteria play.

How solid is the research? Promising

The effect has been shown repeatedly in several small studies for over 40 years. Two mechanisms are well founded: slowly absorbed carbohydrates and the fermentation of fibre in the large intestine. How large the benefit is in everyday life and in the long run is less clear.

Healthy state

Blood sugar after a meal does not only depend on what is on the plate but also on what you ate before. Researchers call this the second-meal effect. As early as 1982, seven healthy people were given lentils or wholemeal bread with the same amount of carbohydrate for breakfast. After the lentils, the area under the blood sugar curve was 71 percent smaller at breakfast, and 38 percent smaller at the identical lunch four hours later.

The first mechanism is speed. How fast sugar reaches the blood is decided already in the mouth and stomach: well-chewed but whole grains, intact legumes, fat, protein and viscous fibres such as beta-glucan from oats and barley slow stomach emptying and the breakdown of starch in the small intestine. Carbohydrates that enter the blood slowly keep insulin release going for longer and lower free fatty acids in the blood. At the next meal, muscles and liver then respond more sensitively to insulin. This effect lasts a few hours, so from breakfast to lunch.

The second mechanism works overnight, and this is where bacteria come in. Fibre and resistant starch (starch the small intestine cannot break down, for example in whole grains, legumes and cooled potatoes) reach the colon undigested. There, bacteria need hours to ferment them, which fits a night-time pause well. In Swedish studies, healthy people ate bread made of whole barley or rye kernels in the evening. The next morning, blood sugar after a standard white-bread breakfast was lower than after a white-bread dinner. The more hydrogen in the breath, i.e. the stronger the fermentation in the colon, the lower the blood sugar.

From this fibre, bacteria make Short-chain fatty acids Acids such as acetate, propionate and butyrate that gut bacteria make from fibre. They nourish the gut wall and act throughout the body. More in the chapter “Butyrate and other short-chain fatty acids”. These stimulate the hormones GLP-1 A hormone from the gut wall that controls insulin release after eating, slows the stomach and makes you feel full. Weight-loss injections mimic it. More in the chapter “GLP-1 from the gut: what bacteria do and what “natural Ozempic” cannot do” and PYY in the gut wall (more in the GLP-1 chapter), lower free fatty acids in the blood and make you feel fuller. In the studies, higher GLP-1 and fewer free fatty acids went together with the better morning value; inflammation markers such as IL-6 were lower. Whether a low glycaemic index is enough is not fully settled: a Canadian study from 1988 found the effect after a dinner with a low glycaemic index. The Swedish studies saw it over the long night pause only with fermentable fibre.

The internal clock plays a part too. In the morning the body usually responds more sensitively to insulin than in the evening, and at night you lie still for hours without muscles taking up sugar. A dinner with fast-digesting carbohydrates therefore hits the least favourable time; a fibre-rich dinner can cushion this. This is physiology; the contribution of time of day was not tested separately in these studies.

With legumes both work together: they supply slow starch and fermentable fibre. In a study of 25 young men, lentils and chickpeas at lunch lowered blood sugar after a pizza in the late afternoon; yellow peas did not. The effect also depends on the variety.

Fibre from the evening is fermented overnight in the colon. The fatty acids stimulate GLP-1, and blood sugar rises less after breakfast.

Disrupted state: causes and effects

The effect also works in the other direction. In people with type 2 diabetes, blood sugar after lunch rose much more when they had skipped breakfast. In a study of 22 people, the blood sugar area after lunch was 37 percent higher, while the insulin response and GLP-1 were lower. This carried on until dinner. Whether the same holds for healthy people is less clear, and studies on intermittent fasting partly come to other results.

A very low-carbohydrate first meal can also worsen the second. In the 1982 lentil study, a breakfast with only a quarter of the carbohydrate lowered blood sugar in the morning, but at lunch it rose more than after the normal breakfast. If you eat little in the morning, you often get the bigger spike at lunch.

If the food for the bacteria is missing, the overnight effect is missing. White flour is fully digested in the small intestine and hardly arrives in the colon. Anyone who eats mostly highly processed foods gives up a mechanism that supports metabolism the next day. Very finely ground wholemeal flour also often behaves more like white flour, because the starch is digested quickly without the grain structure.

Not everyone tolerates the fuel. Legumes and some fibres turn into gas when fermented. With irritable bowel, with bacterial overgrowth of the small intestine (fermentation then starts too early, in the small intestine) or with untreated Coeliac disease An autoimmune disease in which gluten damages the lining of the small intestine. It is diagnosed with a blood test and usually a biopsy while you are still eating gluten. More in the chapter “Lactose, fructose, gluten: making sense of intolerances” or an intolerance, the same dinner leads to bloating and pain instead of a better morning. Then small amounts, well-cooked varieties and slow increases matter, and sometimes a medical work-up.

And not everyone responds the same. In a large Israeli study with 800 people and continuous glucose monitoring, people responded very differently to the same meals, and the Microbiome, gut microbiota The community of all microbes in the gut, mainly bacteria, plus viruses, fungi and archaea. Most live in the colon. More in the chapter “Building microbial diversity” contributed to the prediction. Age, weight, lack of sleep, stress and exercise additionally shift insulin sensitivity. General rules are therefore a good start but not a law.

One more misunderstanding: the second-meal effect is an organising principle, not a cure. What has been shown is a lower blood sugar after the next meal in small, short studies, not less diabetes or less weight after years. The principle does fit what large analyses on fibre and whole grains show.

White flour in the evening does not reach the colon. There is nothing to ferment overnight, and blood sugar rises more in the morning.

What often helps

Lifestyle

Diet, exercise, sleep, social contact and dealing with stress.

Plan in legumes Lentils, chickpeas and beans lower blood sugar at the meal itself and at the next meal. If your gut is sensitive, start with small amounts and increase slowly; well-cooked, rinsed canned legumes are often better tolerated. Promising How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open. [1][2]
Whole grains instead of white flour in the evening Barley, whole rye kernels, whole oat kernels and cooled potatoes or rice in the evening provide food for the night. In the studies, this improved blood sugar after breakfast, increased fullness and lowered inflammatory markers. These are short studies with few people, but the direction is consistent. Promising How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open. [3][4]
With diabetes, do not skip breakfast In people with type 2 diabetes, a skipped breakfast led to higher blood sugar spikes at midday and in the evening. If you want to do intermittent fasting, discuss the eating window with your care team, especially if you take blood-sugar-lowering medication. Promising How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open. [5][6]
Keep grains whole and chew well Whole or coarsely cracked grains, rolled oats instead of fine oat powder, and bread with visible kernels slow the digestion of starch more than fine wholemeal flour. Thorough chewing helps as well. This is a physiological consideration and, to a degree, comes from the studies that used whole grains. Plausible How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open.
Do not make dinner the sugar moment A large dinner of fast carbohydrates and sweets meets the lower evening insulin sensitivity. More vegetables, legumes and whole grains in the evening, and not too late, is the gentler option. The basis is the idea of the second-meal effect, not a tested evening rule. Plausible How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open.

Natural supplements

Supplements do not replace a medical check-up. Discuss dosage and interactions with your doctor or pharmacist.

Resistant starch as a powder In a study of 10 healthy people, 30 g of resistant starch a day for four weeks improved insulin sensitivity. Powders made from potato or banana starch are one option, but foods are just as good and bring more with them. Increase slowly, otherwise bloating is likely. Plausible How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open. [7][4]

Medical treatments

Information, not a treatment recommendation. Decide together with a doctor. Approval and availability differ from country to country.

Measure your personal response when medically useful With diabetes or prediabetes, temporary continuous glucose monitoring can show how you personally react to sequences of meals. For healthy people, the benefit of such measurements is not proven, and the readings can easily lead to unnecessary restrictions. Plausible How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open. [8]
Have strong bloating after legumes checked If fibre repeatedly causes strong symptoms, a conversation with your doctor is worthwhile. Possible reasons are irritable bowel, bacterial overgrowth of the small intestine or an intolerance. The aim is not to avoid fibre permanently but to find the trigger. Plausible How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open.

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How we rate the evidence

Trust score 5 of 5:Established: Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies.
Trust score 4 of 5:Well supported: Several randomised studies in humans with largely consistent results.
Trust score 3 of 5:Promising: Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies.
Trust score 2 of 5:Plausible: Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms.
Trust score 1 of 5:Experimental: Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open.

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