Insulin vs. Metformin: What to Choose and for Whom
«I was offered insulin, and my acquaintance was offered metformin. Why is that?» is one of the most common questions from people who encounter a glucose metabolism disorder for the first time. The choice between these drugs is made by the physician, and it depends on the type of diabetes, glucose levels, kidney condition, pregnancy and many other factors. The editors explain the logic of this choice so that it is easier for you to understand the specialist's recommendations.
What the Choice Depends On
Insulin and metformin are not a «stronger» and a «weaker» variant of the same treatment. Insulin replaces or supplements a hormone the body lacks, while metformin affects glucose production by the liver and tissue sensitivity. Therefore the key question for the physician is why a person has elevated glucose: due to a lack of insulin, due to insulin resistance, or due to both mechanisms.
The second factor is how pronounced the hyperglycemia is and whether there are symptoms of decompensation: thirst, frequent urination, weight loss, ketosis. With such signs a rapid reduction of glucose is needed, and insulin may be necessary even in a person with type 2 diabetes.
The third block is comorbid conditions: kidney and liver function, heart failure, pregnancy, a tendency to hypoglycemia, occupation (for example, drivers), and the person's ability to perform self-monitoring.
Finally, cost, availability, convenience and patient preferences are taken into account. Modern ADA and EASD guidelines emphasize a personalized approach and shared decision-making by physician and patient.
Type 1 Diabetes: Insulin Without Alternative
In type 1 diabetes the immune system destroys the beta cells of the pancreas, and one's own insulin is almost not produced. In this case insulin is a vital replacement therapy, without which diabetic ketoacidosis develops, which is a fatal condition without treatment.
Metformin cannot replace insulin in type 1 diabetes, because without insulin its mechanism simply has nothing to build on. Sometimes it is considered as an adjunct in people with type 1 diabetes and overweight, but this is an endocrinologist's decision and is not a standard of treatment.
Modern insulin therapy for type 1 diabetes usually includes long-acting basal insulin and bolus injections before meals, or an insulin pump. Continuous glucose monitoring systems, which increase the safety of treatment, are used increasingly often.
People with type 1 diabetes can and should exercise, but they need an individual strategy for adjusting insulin and nutrition around training, developed together with an endocrinologist.
Type 2 Diabetes: From Metformin to Insulin
In type 2 diabetes insulin resistance usually plays the main role at the start, while one's own insulin is still produced. Therefore initial therapy, as a rule, combines lifestyle change with oral or injectable non-insulin drugs. Metformin was for a long time the standard first drug and remains so for many patients thanks to its effectiveness, safety and low cost.
The latest editions of the ADA/EASD guidelines have added an important nuance: for people with established cardiovascular disease, heart failure or chronic kidney disease, drugs with proven organ protection — GLP-1 receptor agonists or SGLT2 inhibitors — are recommended regardless of whether the person is taking metformin.
Insulin in type 2 diabetes is prescribed when other agents do not allow the target values to be reached, with very high glucose or glycated hemoglobin levels, with symptoms of catabolism (weight loss, ketosis), during hospitalizations, surgeries and acute illnesses.
Metformin and insulin are often combined: metformin is continued and basal insulin is added, which can reduce the required insulin dose and weight gain.
Special Situations: Prediabetes, Pregnancy, Kidneys
Prediabetes.The basis of prevention is lifestyle change: in the Diabetes Prevention Program study it reduced the risk of developing diabetes by about 58%, and metformin by 31% compared with placebo (Knowler et al., 2002). ADA standards allow consideration of metformin for certain high-risk groups, for example people with severe obesity or with a history of gestational diabetes. Insulin is not used in prediabetes.
Pregnancy.In gestational diabetes, if diet is insufficient, insulin has long been considered the standard of pharmacotherapy, since it does not cross the placenta in significant amounts. Metformin crosses the placenta; in some countries and situations it is used, but the decision is made by the physician taking into account current data on long-term consequences for the child.
Kidney disease.Metformin is excreted by the kidneys unchanged, so with a reduction in the glomerular filtration rate the dose is limited, and in severe renal failure the drug is contraindicated because of the risk of lactic acidosis. Insulin can be used at any stage of kidney damage, but the need for it often decreases, and the risk of hypoglycemia increases.
Elderly people.For them it is especially important to avoid hypoglycemia, which can lead to falls, injuries and cardiovascular events. Therefore treatment regimens are simplified, and glycemic targets are sometimes made more lenient.
| Situation | Metformin | Insulin |
|---|---|---|
| Type 1 diabetes | Does not replace insulin | Mandatory |
| Type 2 diabetes, start | Often first line | With pronounced hyperglycemia or decompensation |
| Prediabetes | Possible for high-risk groups | Not used |
| Gestational diabetes | Used in a limited way | Traditional standard |
| Severe renal failure | Contraindicated | Possible, with dose adjustment |
Healthy People and Athletes
For people without diabetes and prediabetes, neither of these drugs is indicated. Insulin for non-medical purposes, in particular for gaining muscle mass, is one of the most dangerous practices in the sporting environment: severe hypoglycemia can develop quickly and lead to coma and death. In addition, insulin is prohibited by WADA at all times.
Metformin is sometimes taken by healthy people for the purpose of «cutting» or «slowing aging». Large studies on longevity in healthy people are still ongoing, while the data that metformin may blunt the gain of muscle mass and aerobic adaptations to training give those who train pause for thought.
For athletes with type 1 diabetes insulin is a legitimate treatment, but participation in competitions requires a Therapeutic Use Exemption (TUE). Athletes with type 2 diabetes who take metformin do not need an exemption.
- Without a diagnosis — neither insulin nor metformin is a «supplement for shape».
- With type 1 diabetes — insulin plus a training plan from an endocrinologist.
- With type 2 diabetes — therapy according to guidelines; sport is part of the treatment.
Editorial Conclusions
The choice between insulin and metformin is determined not by preferences, but by the nature of the disorder. In type 1 diabetes insulin is irreplaceable. In type 2 diabetes metformin often stands at the start, and insulin is added when the disease progresses or a rapid reduction of glucose is needed.
Special conditions — pregnancy, kidney disease, old age — substantially change the choice, so it is always individual.
Healthy people and athletes without a diagnosis do not need these drugs: the risks outweigh any imagined benefit.
To understand the matter more deeply, read our articles on the tests for detecting prediabetes, on hypoglycemia and how to recognize it, and on training with diabetes.
References
- Davies MJ, Aroda VR, Collins BS, et al. Management of hyperglycemia in type 2 diabetes, 2022. A consensus report by the American Diabetes Association (ADA) and the European Association for the Study of Diabetes (EASD). Diabetes Care. 2022;45(11):2753–2786.
- American Diabetes Association Professional Practice Committee. Standards of Care in Diabetes. Diabetes Care. Suppl 1; актуальна редакція.
- Knowler WC, Barrett-Connor E, Fowler SE, et al. Reduction in the incidence of type 2 diabetes with lifestyle intervention or metformin. N Engl J Med. 2002;346(6):393–403.
- UK Prospective Diabetes Study (UKPDS) Group. Effect of intensive blood-glucose control with metformin on complications in overweight patients with type 2 diabetes (UKPDS 34). Lancet. 1998;352(9131):854–865.
- Walton RG, Dungan CM, Long DE, et al. Metformin blunts muscle hypertrophy in response to progressive resistance exercise training in older adults: the MASTERS trial. Aging Cell. 2019;18(6):e13039.
- Evans PJ, Lynch RM. Insulin as a drug of abuse in body building. Br J Sports Med. 2003;37(4):356–357.
- World Anti-Doping Agency. Prohibited List. Montreal: WADA; актуальна редакція.
Andriy Melnyk
A strength-sports coach and author of programs for beginner and intermediate levels. Writes about training planning.
