GLP-1: The Gut Hormone Behind the Incretin Effect, Food and the Medicines

GLP-1, short for glucagon-like peptide-1, is a hormone made by endocrine cells in the lining of the intestine and released when food arrives. It prompts the pancreas to release insulin, holds back glucagon, slows the emptying of the stomach and tells the brain that you have eaten. In healthy adults, gut hormones of this kind accounted for 72.8 per cent of the insulin response to a glucose drink in one classic experiment. The natural hormone is broken down very rapidly, which is the gap the GLP-1 medicines were designed to close. This entry sits in NNWA's handbook of hormones.

What is GLP-1, and what does it do?

It is a 30-amino-acid peptide produced by the L-cells of the intestinal lining and released in response to a meal. A major review of its physiology lists its main actions: it stimulates insulin secretion, it inhibits glucagon secretion, and together these limit the rise of glucose after eating. It also slows the movement and secretions of the gut, and it appears to be a physiological regulator of appetite and food intake.

Its push on insulin depends on glucose, so it acts when blood sugar is raised and eases off as sugar falls. The two hormones it works through have their own pages: the handbook entry on glucagon and the entry on insulin and insulin resistance.

What is the incretin effect?

It is the extra insulin released when glucose is swallowed, compared with the same rise in blood glucose produced through a vein. The difference is credited to gut hormones, the incretins, chiefly GLP-1 and a second hormone called GIP. In the 1986 experiment that measured it, incretins contributed 72.8 per cent of the insulin response in eight healthy adults and 36.0 per cent in fourteen people with type 2 diabetes. A consensus statement by South Asian endocrinologists agrees: incretins may account for up to 70 per cent of insulin secretion after a meal, and the effect is severely reduced or absent in type 2 diabetes.

How long does the body's own GLP-1 last?

Hardly any time at all. The review of GLP-1 physiology describes the hormone as extremely rapidly metabolised and inactivated by an enzyme, dipeptidyl peptidase-4 or DPP-4, even before it has left the gut. The medicines are built differently: ICMR's 2018 guideline on type 2 diabetes explains that the GLP-1 receptor analogues resist breakdown by DPP-4 and so have longer half-lives than the native hormone. The consensus statement gives liraglutide a half-life of 13 hours.

Which foods raise GLP-1?

Protein has the best evidence, and the effect is short. A meta-analysis of randomised trials in healthy adults pooled 49 publications on single meals and 19 on longer feeding. After a single protein-rich meal GLP-1 rose, hunger fell and fullness increased, but the change in GLP-1 reached significance only at 35 g of protein or more. Over the longer term protein did not raise GLP-1, and the authors called those trials inconclusive.

Whey taken shortly before a meal has been tested in people with type 2 diabetes. In an Australian study of eight patients, 55 g of whey before a potato meal slowed stomach emptying and produced the highest GLP-1. In a 2026 study from Mumbai, ten patients took 30 g of whey in water 30 minutes before breakfast; GLP-1 peaked at about 75 pmol/L against about 45 pmol/L with water, and the glucose rise after breakfast and lunch was about 12 per cent lower. These are small one-day experiments, and a meta-analysis of five such trials in 134 people found lower glucose after the meal but no significant difference in GLP-1.

Fibre is plausible and less certain. A systematic review of 26 trials of prebiotic fibres in 831 people found better fullness and lower glucose and insulin after meals, while results for GLP-1 itself were contradictory. The entry on dietary fibre covers the wider case for it. Dal, rajma and chana bring protein and fibre together.

Fuller lists are in the protein table for Indian foods and the fibre table; the protein calculator gives a daily figure, which is more useful than a per-meal hormone target.

ICMR-NIN's guidance for India: protein and fibre, not a GLP-1 target

No guideline sets a requirement or a target level for GLP-1, because the body makes it. What ICMR-NIN's 2020 report sets are the intakes of the two nutrients linked with it: a protein allowance of 0.83 g per kg of body weight a day for adults, which comes to 54.0 g for the reference man of 65 kg and 45.7 g for the reference woman of 55 kg, and a fibre intake of about 30 g per 2,000 kcal that the committee considered safe. ICMR-NIN's Dietary Guidelines add that good quality protein should come from combinations of foods, and advise against protein supplements for building muscle.

The Indian picture: diabetes, pregnancy and the incretin system

The conditions in which GLP-1 matters are common in India. ICMR-INDIAB, a survey of 113,043 adults, found diabetes in 11.4 per cent, prediabetes in 15.3 per cent, generalised obesity in 28.6 per cent and abdominal obesity in 39.5 per cent. Indian researchers have also looked at the incretin system directly. A study of 48 pregnant women at 24 to 28 weeks found lower fasting GLP-1 in those with gestational diabetes (17.3 against 22.2 in the study's units).

A case-control study from north India compared 93 non-obese adults with recently diagnosed type 2 diabetes with 40 adults without it, and found significantly higher blood levels of DPP-4, the enzyme that breaks GLP-1 down, in the diabetes group. Both studies are small and cross-sectional, so they describe an association and cannot say what causes what.

GLP-1 medicines in India, described factually

Two families of prescription medicine act on this system. GLP-1 receptor agonists mimic the hormone and last far longer; DPP-4 inhibitors, the gliptins, are tablets that slow the breakdown of the body's own GLP-1. ICMR's 2018 guideline describes both, and newer agents have arrived since. The table names them without doses, which are for the prescriber alone. Which medicines exist and who qualifies is set out in NNWA's piece on weight loss medicines in India.

According to the United States National Library of Medicine, semaglutide injection is prescribed to control blood sugar in type 2 diabetes, to lower the risk of heart attack and stroke in certain adults, and to assist weight loss in certain people with obesity or overweight who have weight-related medical problems. It adds that the medicine controls these conditions and does not cure them. Eating on the medicines is covered in NNWA's Indian diet guide for semaglutide and tirzepatide and its piece on muscle loss on GLP-1 medicines.

What does the trial evidence show?

The medicines have large randomised trials behind them; the food effects do not. In SELECT, 17,604 adults with cardiovascular disease and a BMI of 27 or more but no diabetes were followed for a mean of 39.8 months: a heart attack, stroke or cardiovascular death occurred in 6.5 per cent on semaglutide and 8.0 per cent on placebo. The manufacturer funded that trial, and 16.6 per cent on the medicine stopped it because of adverse events, against 8.2 per cent on placebo.

Indian data are thinner but growing. An observational study followed 388 adults with type 2 diabetes on the semaglutide tablet; HbA1c fell by 1.78 percentage points and weight by 7.23 kg, with no comparison group. A randomised trial in 320 Indian adults reported in 2026 that an Indian-made generic semaglutide injection was not inferior to the original over 24 weeks. And the benefit depends on continuing: a year after STEP 1 participants stopped, they had regained two-thirds of the weight lost, as NNWA's piece on weight regain after stopping semaglutide explains.

A worked example: a 35 g protein lunch beside a weekly injection

This case is illustrative, not a real person. A 38-year-old vegetarian bank officer in Kolkata, 55 kg, without diabetes, has read that a high-protein lunch will raise her GLP-1 like the injection her cousin takes.

Common myths and mistakes about GLP-1

The opposite mistake is to treat the medicine as a replacement for eating well. STEP 1 gave semaglutide as an adjunct to lifestyle intervention, in both groups. A third is to stop or restart a prescribed medicine without the doctor, which the official patient information warns against. A fourth is to add whey before meals on the strength of small trials: anyone with diabetes or kidney disease should ask the treating doctor first.

Is there a blood test for GLP-1?

Not one that forms part of routine care. GLP-1 is measured in research, and the physiology review explains why it is awkward: because the hormone is broken down so fast, a laboratory must decide whether to measure the intact hormone or the total including its breakdown products, and the two answer different questions. A doctor managing diabetes relies on glucose tests and HbA1c, and may order others to check the body's response to a medicine. Reading them is the doctor's work.

Side effects, cautions and when to see a doctor

GLP-1 medicines are prescription medicines with known risks. ICMR's guideline names nausea and vomiting as the main side effects and notes that a few cases of acute pancreatitis have been reported. In STEP 1, nausea and diarrhoea were the most common adverse events, and 4.5 per cent of people on semaglutide stopped because of gut symptoms against 0.8 per cent on placebo. The official patient information for semaglutide carries a warning about thyroid tumours seen in rats, says it is not known whether the risk applies to humans, and asks patients to tell the doctor about any personal or family history of thyroid cancer.

The same information asks patients to tell the doctor about past pancreatitis, diabetic eye disease, severe stomach problems, gallbladder or kidney disease, pregnancy, breastfeeding or plans to conceive, and any planned surgery, dental surgery included. It also asks people with diabetes to check with the doctor before a large change in diet, exercise or weight, because blood sugar can shift. A coach never adjusts, starts or stops these medicines, and a fast or a new diet is discussed with the prescriber first.

Learning this with NNWA

NNWA, a private skill academy in Kolkata, runs its teaching online in English and in Hindi. Its Weight Management course and related courses prepare people to coach healthy clients on food and habits and to refer anyone on prescription treatment back to the doctor. No NNWA course is a degree or a qualification in dietetics.