Insulin resistance is one of those topics that gets thrown around in fitness circles with a lot of fear and not much clarity. If your muscles can’t efficiently use insulin to absorb glucose and amino acids, your ability to build and retain lean tissue takes a serious hit. But before you overhaul your diet or panic about blood sugar, you need to understand who’s actually at risk and what actually moves the needle.

Here is what the science says about insulin resistance, muscle growth, and what you can realistically do about it.

What Is Insulin Resistance and Why Does It Matter for Muscle?

In the context of lifting and body composition, we are mostly talking about peripheral insulin resistance — when your muscle cells and fat cells become less responsive to insulin’s signal to transport glucose from the bloodstream into the cell.

Insulin is a transport facilitator. When it works well, it shuttles glucose and amino acids into your muscles where they fuel training, recovery, and growth. When resistance develops, the same amount of insulin moves less glucose through. Your pancreas compensates by pumping out more insulin, but the efficiency is shot.

Here is where it gets ugly for lifters specifically:

  • Muscle cells become resistant before fat cells. This means your muscles are starving for glucose and amino acids while your fat cells are still happily absorbing nutrients. You get less muscular and more fat simultaneously.
  • Insulin is directly anabolic. It activates growth pathways in muscle cells and is a powerful anti-catabolic signal. When insulin locks into its receptors, it tells the muscle cell to stop breaking down its own structure. Lose that signal, and you lose muscle faster while building it slower.
  • Amino acid transport suffers. Insulin mediates the transport of several amino acids into muscle cells. Reduced insulin sensitivity means reduced delivery of the raw materials your muscles need to grow.

In short, insulin resistance creates a metabolic environment that is the exact opposite of what you want for hypertrophy: poor nutrient delivery to muscle, enhanced fat storage, and suppressed anabolic signaling.

Should Bodybuilders Worry About Insulin Resistance?

For the majority of lifters, the answer is no. If you check the following boxes, insulin resistance is almost certainly not something you need to lose sleep over:

  • Under 40 years old
  • Training hard and regularly (4-6 sessions per week)
  • Not obese (below ~25% body fat for males, ~35% for females)
  • Maintaining a reasonably lean physique year-round
  • No family history of severe metabolic disease

If you are active, muscular, and not chronically overfed, it is extremely difficult to develop meaningful insulin resistance. Your training itself clears glucose from the bloodstream and temporarily improves insulin sensitivity for hours after each session. Stack six hard training sessions per week together, and you are running a powerful anti-diabetic protocol without even thinking about it.

Even if you run an aggressive bulk and gain 30-40 pounds over the course of a year, most of it fat, a few weeks into your first cut will restore normal insulin sensitivity. For the average natural lifter under 40 who trains consistently, this is a non-issue.

Who Is Actually at Risk: Older Lifters and Enhanced Athletes

The conversation changes meaningfully for two groups: lifters over 40 and athletes using growth hormone (GH).

Age naturally reduces insulin sensitivity. The older you get past 40, the more attention you need to pay to metabolic markers. And growth hormone, while incredibly effective for staying lean and building tissue, directly causes acute short-term insulin resistance. Combine advanced age with GH use, and you have a real risk that needs active management.

If you fall into either category — or especially both — here are the evidence-based guardrails:

  • Never exceed 20% body fat (males) or 30% (females). Just don’t. The risk is not worth the marginal mass gain.
  • Cap weight gain at half a pound per week during massing phases. Over a 12-week block, that is six pounds maximum. Slow, controlled bulking is your insurance policy.
  • Stay very active. A minimum of 8,000 steps per day on average, with 10,000-12,000 being the real target. Daily activity is one of the strongest predictors of insulin sensitivity.
  • Train hard with high volumes. Hard training stimulates glucose uptake, clears the bloodstream, and reverses insulin resistance temporarily. The more training volume you can productively recover from, the better your metabolic profile.
  • Mini-cut as needed to stay below 15% body fat (males) or 25% (females). Do not let body fat creep for months unchecked.

For GH users specifically, there are additional considerations:

  • Use a long-acting insulin (like Lantus) alongside growth hormone, at minimum during massing phases.
  • Consider workout-window insulin (like Humalog), especially as you get deeper into a mass phase at higher body weights.
  • Supplement with metformin — an insulin sensitizer that also shows longevity benefits across multiple animal studies. Discuss dosing with your doctor.

The key insight is this: growth hormone is a powerful tool, but it demands a disciplined lifestyle to use safely. If you are eating mostly junk food, bulking recklessly to 280 pounds, and taking GH on top, you are doubling your probability of serious metabolic consequences. GH makes it easier to stay lean — use it for that purpose. Bulk slowly, stay lean, stay active.

Does Low Carb on Rest Days Improve Insulin Sensitivity?

This is one of the most popular pieces of broscience in bodybuilding circles, and it is largely meaningless.

The logic sounds reasonable on the surface: fewer carbs on off days means less insulin required, which should improve sensitivity. But there is a critical problem with this reasoning.

If you drop carbs on rest days, you need to keep calories adequate for recovery by increasing fat intake. Here is the inconvenient truth: in animal models, insulin resistance is most reliably induced by overfeeding saturated fat, not carbohydrates. An excess of total calories exposed to the system over time is the primary driver of insulin resistance. It is not specifically about carbs.

And if you drop both carbs and total calories on rest days? Now you are sabotaging recovery and muscle growth. You cannot recover from hard training and support hypertrophy while eating next to nothing on your off days.

The real lever is total caloric exposure over weeks and months. Keep your surplus mild, your body fat under control, and your off-day versus on-day calorie cycling becomes irrelevant noise. A 280-pound lifter with a gut hanging over his belt doing “low carb off days” is addressing a symptom while ignoring the disease.

Getting lean is the single most reliable way to restore insulin sensitivity. It is the most predictable thing you will ever see in blood work. Drop body fat to 10% as a male or 17-18% as a female, hold it for a few weeks, and your insulin sensitivity will improve dramatically.

Does Cardio After Lifting Help With Insulin Resistance?

You will see lifters doing 15-20 minutes of HIIT or treadmill work after their lifting session, claiming it is for “insulin sensitivity.” This is another case of majoring in the minors.

Two short cardio sessions per week are a tiny drop in the bucket compared to your overall lifestyle factors. If you are:

  • Training hard 4-6 days per week
  • Getting 8,000-12,000 steps daily
  • Maintaining a reasonable body fat percentage

You have already checked every major box for insulin sensitivity. A 15-minute treadmill session after bench press and bicep curls does not meaningfully move the needle if the big-picture factors are handled.

And if the big-picture factors are not handled — if you are sedentary outside the gym, overfat, and eating poorly — no amount of post-workout cardio is going to save you. Insulin sensitivity is a lifestyle outcome, not something you can session your way into.

Do Supplements Help With Insulin Resistance?

The supplement industry loves to sell products targeting blood sugar management and insulin sensitivity. The reality is blunt: over-the-counter supplements and herbs do not work well enough to matter.

Some supplements have shown very marginal effects on insulin resistance in studies, but the magnitude is small, and most supplements show no detectable effect at all. If you are genuinely concerned about insulin resistance, the tools that actually work are:

  • Fat loss (the single most powerful intervention)
  • Consistent hard training with adequate volume
  • High daily activity (steps, movement)
  • Prescribed medications like metformin, if warranted
  • Proper management of exogenous hormones if applicable

No capsule or powder replaces those fundamentals. Save your money.

How Do You Test for Insulin Resistance?

This is the actionable part that most lifters skip. You do not need to guess where you stand on the insulin sensitivity spectrum — you can measure it.

The test you want is the glycosylated hemoglobin test, commonly called the A1C test. It measures how much glucose has been attaching to your hemoglobin molecules over the past three months. This gives you a rolling average of your blood glucose levels, not just a single-day snapshot.

Here is how to interpret your results:

  • Below 5.0: Excellent. This is the ideal range for health and performance. The lower toward 4.0, the better.
  • 5.0-5.2: Good. Normal range, no cause for concern.
  • 5.3-5.4: Pre-diabetic territory. Time to take action — prioritize a fat loss phase, increase activity, and consult your doctor.
  • Above 5.5+: Significant concern. Aggressive lifestyle changes and medical consultation are warranted.
  • 7.0+: Diabetic range. Medical intervention is essential.

The A1C test is inexpensive, covered by most insurance, and can be added to any routine blood panel. If you are serious about long-term performance and health, get it checked at least once a year. If you are over 40 or using performance-enhancing drugs, every six months is smarter.

The good news: insulin resistance is fully reversible when caught in the normal or pre-diabetic range. Change your body composition and activity level, and your blood glucose dynamics will improve. It is when things progress to full type 2 diabetes — A1C scores of 7, 8, 9 or higher — that irreversible damage begins to accumulate. Nerve damage, vision problems, impaired healing, and systemic degradation are all downstream consequences of chronic high blood sugar.

Do not let it get that far. Get your blood work done, know your number, and adjust accordingly.

Frequently Asked Questions

Can you build muscle if you are insulin resistant?

You can, but it is significantly harder. Insulin resistance impairs glucose and amino acid delivery to muscle cells, reduces anabolic signaling, and increases catabolism. Your body will preferentially store nutrients as fat rather than using them for muscle repair and growth. The most effective intervention is reducing body fat, which dramatically improves insulin sensitivity and creates a better hormonal environment for hypertrophy.

What is a good A1C level for bodybuilders?

For optimal muscle-building conditions and overall health, aim for an A1C below 5.0. Anything in the 5.0-5.2 range is considered normal and healthy. Once you creep above 5.3, it is time to prioritize a cut and increase your daily activity. The A1C test is cheap, widely available, and gives you a three-month average of blood glucose — making it the best single metric for monitoring insulin sensitivity over time.

The Bottom Line

For most lifters who train hard and stay reasonably lean, insulin resistance is not a practical concern. The combination of regular resistance training, adequate daily movement, and controlled body fat is a powerful defense against metabolic dysfunction.

The lifters who need to pay attention are those over 40, those using growth hormone, or those who chronically bulk to high body fat levels without periodizing their diet. For these groups, the strategy is straightforward: bulk slowly, stay lean, stay active, get blood work done, and manage exogenous hormones responsibly.

If you are tracking your training and want a structured way to manage your sets, weights, and progression across training blocks, an app like Splitt can help you stay consistent — which is ultimately the best thing you can do for both muscle growth and metabolic health.