Weight loss and your blood values: why it stalls and what to measure

If you are not losing weight despite a strict diet and enough training, the brake often sits in a blood value you cannot feel. In people with insulin resistance, research saw that how you handle carbohydrates can strongly steer the result (Solomon, 2012). The four systems that most often work against your weight are your insulin, your thyroid, your stress hormones and your satiety hormones.

I notice athletes who cannot lose weight almost always train harder and eat less first. Sometimes that works. But just as often you push your metabolism further into a corner and lose muscle instead of fat.

This guide shows which values steer your metabolism, links your pattern to the right marker and helps you lose weight without sacrificing your muscle.

Which blood values decide whether you lose weight?

Weight loss is not only about calories, but also about the hormones that steer your fat storage and hunger. The four values that say the most are fasting insulin with HOMA-IR, your thyroid via TSH, your cortisol and your satiety hormone leptin. The table below links your pattern to the value to check.

Below I work through the main systems, starting with the brake most athletes overlook.

Why am I not losing weight despite clean eating and training?

If you do everything right and the scale still stands still, that often points to a hormonal brake you cannot see from your lifestyle. Think of insulin resistance, a thyroid that works slightly too slowly, or a chronically raised cortisol. You do not feel these values directly, but they steer your fat storage.

Strict dieting sometimes makes it harder, not easier. A large calorie deficit can slow your thyroid and raise your cortisol, and then your body starts to burn more sparingly.

That is exactly why a blood test adds value here. You swap guessing for measuring.

One caveat first: a single value is a snapshot. You always read it next to your weight trend, your nutrition and your training load.

Insulin and HOMA-IR: the most underrated brake on fat loss

Fasting insulin is the earliest marker of a metabolism that stalls, and it often runs years ahead of your glucose. HOMA-IR combines your fasting insulin and glucose into a single number for your insulin sensitivity. If that sensitivity is low, your body stores fat more easily and breaks it down harder.

Research in non-obese women showed that the degree of insulin resistance was linked to how well different diets worked (Solomon, 2012). Your insulin status therefore partly decides which approach fits you.

For athletes this matters, because you can have a low fat mass and still be less insulin sensitive.

For the details, read fasting insulin and HOMA-IR. Always ask for your fasting insulin alongside glucose, because without that number you miss the early phase.

HbA1c and glucose: your blood sugar over the long term

Where fasting insulin catches the early phase, HbA1c shows your average blood sugar over the last two to three months. A slowly rising HbA1c within the normal range is an early signal that your metabolism is changing. Together with your glucose it draws your metabolic direction.

Energy dips after a meal are a common pattern. They sometimes point to a blood sugar that spikes too hard and then drops too fast.

For measuring, timing counts. You measure glucose fasted, while HbA1c you can draw at any moment because it is an average.

Dig into HbA1c in non-diabetics. A rising trend you discuss with your GP, even if you are still within the range.

Thyroid: why a slow engine holds back your weight loss

Your thyroid sets the pace of your metabolism, and a slow thyroid makes losing weight harder. TSH is the first signal, free T4 and free T3 add the rest. If your thyroid works slowly, you burn less at rest and hold on to weight more easily.

The nuance matters. A strict diet can slow your thyroid temporarily, so a deviating value during a diet is not automatically a disorder.

For an athlete, the combination with your complaints and your weight trend counts most.

You can read more in thyroid and weight. A deviating TSH you discuss with your GP, who weighs the whole picture.

Cortisol and stress: why belly fat stays put

Cortisol is your main stress hormone, and a chronically raised level can drive fat storage around your belly. Research shows that people who respond to stress with a stronger cortisol rise appear more prone to weight gain (Vicennati, 2016). For athletes that counts double, because hard training is itself a stressor.

A busy period, poor sleep and pushing through hard sessions stack up. Then your belly fat stays while you lose it everywhere else.

You measure cortisol at a fixed moment, ideally in the morning, because it has a strong daily rhythm.

If you recognise belly fat after stressful blocks, look at your recovery and your sleep too. How training and hormones connect you can read in fasted training and hormones.

Leptin: the satiety hormone that steers your hunger

Leptin is the hormone that tells your brain you have eaten enough, and it plays a key role in how your body guards your weight. At a higher body fat percentage your body can become less sensitive to leptin, so the satiety signal comes in weaker (Obradovic, 2021). Then you keep feeling hungry while you eat enough.

This explains why losing weight sometimes fights back. As your weight drops, your leptin drops, and your body reacts as if a shortage is coming.

Constant hunger during a diet is therefore not only willpower.

Dig into leptin and satiety. You measure leptin fasted, and you always read the result next to your body fat percentage.

GLP-1 and muscle preservation: losing weight without losing your muscle

Losing weight almost always costs some muscle mass, and as an athlete that is exactly what you want to limit. Anyone using a GLP-1 drug often loses weight fast, and then the risk of muscle loss is extra high. Enough protein and strength training are your best protection here.

A meta-analysis showed that older adults kept more muscle mass and lost more fat during weight loss with a higher protein intake (Kim, 2016). That principle also holds for athletes who lose weight fast.

Measuring helps you keep your course here.

If you want to lose weight fast without sacrificing your muscle mass, read GLP-1 weight loss without muscle loss. Always combine a drug with protein and strength, and discuss its use with your doctor.

What if your blood values are normal but you still are not losing weight?

A result within the reference range does not rule out a stalled weight. Reference values are broad population averages, and your optimal value can sit just inside or just outside them. A fasting insulin at the top of normal can already be too high for you.

According to Thuisarts and the NHG guidelines, a blood test is used in a targeted way for complaints, not as a loose check of everything at once. The RIVM also publishes population figures that help to put your own value in perspective.

My advice is simple. Follow the trend and watch your pattern.

A repeat measurement at a comparable moment says more than a single number. If your weight still stands still after eight to twelve weeks while your nutrition is sound, discuss with your GP whether further testing makes sense.

Which blood test do you choose when weight loss stalls?

When your weight stalls, you choose a test that covers your metabolic systems at once: your insulin and HOMA-IR, your blood sugar, your thyroid and, where possible, your satiety hormone. That way you do not have to guess which single value to draw first. A targeted metabolic test gives you an overview in one go.

For anyone who suspects insulin resistance plays a role, our insulin resistance HOMA-IR blood test is a logical starting point. It measures your fasting insulin and glucose and calculates your HOMA-IR.

If you want a broader look at your whole metabolic picture, our 360 Health blood test gives a complete overview.

If you prefer to test in a targeted way, pick the single marker that matches your pattern from the table at the top.

How often should you test your metabolic values?

For most athletes, once or twice a year is enough to track your metabolic baseline values. If you adjust your nutrition or training, a retest after eight to twelve weeks is useful to see the effect. That way you build a trend instead of loose numbers.

Around a weight-loss phase or a competition season an extra measurement can help. Your insulin and your thyroid move most then.

Preferably draw blood fasted at a fixed time of day. Comparable conditions make your measurements more reliable.

Common mistakes when weight loss stalls

Most mistakes sit not in the measurement, but in what you do with it. Anyone who takes their weight seriously often runs into the same traps.

The first mistake is dieting even harder the moment the scale stands still. Too large a deficit slows your thyroid and raises your cortisol, and then you lose muscle mass instead.

The second mistake is looking only at your weight and ignoring your body fat percentage. You can lose weight while mostly losing muscle, and as an athlete you want to avoid that.

The third mistake is retesting too early. Give an adjustment eight to twelve weeks before you draw again.

Frequently asked questions

The questions I get back most from athletes whose weight loss stalls.

Which blood values do you check when you are not losing weight? A useful starting panel covers your fasting insulin with HOMA-IR, your glucose, your HbA1c and your thyroid via TSH. Which exactly you choose depends on your pattern and your weight trend.

Can insulin resistance work against weight loss? A lower insulin sensitivity is linked to storing fat more easily and breaking it down harder. Measuring your fasting insulin and HOMA-IR shows whether this is at play for you.

Do I need to fast for this test? For glucose and fasting insulin yes, because a meal disturbs the result. Preferably draw in the morning at a fixed moment.

Why do I stay hungry during a diet? As your weight drops, your leptin drops, and your satiety signal gets weaker. That partly explains why hunger during a diet is not only willpower.

How do I prevent muscle loss when losing weight? Enough protein and strength training protect your muscle mass best during a deficit. That certainly holds if you lose weight fast, for example on a GLP-1 drug.

Can stress block my weight loss? Chronic stress and a raised cortisol can drive fat storage around your belly. Sleep and recovery therefore weigh just as heavily as your training.

Which doctor do I ask for a metabolic blood test? You can have a blood test done yourself without a referral. If you want the result weighed in treatment, your GP is the right point of contact, because they know your history.

References

1. Solomon TP, Haus JM, Kelly KR, et al. Insulin resistance predicts the effectiveness of different glycemic index diets on weight loss in non-obese women. Obesity Facts. 2012. PMID: 23108147.
2. Kim JE, O'Connor LE, Sands LP, et al. Effects of dietary protein intake on body composition changes after weight loss in older adults: a systematic review and meta-analysis. Nutrition Reviews. 2016. PMID: 26883880.
3. Obradovic M, Sudar-Milovanovic E, Soskic S, et al. Leptin and Obesity: Role and Clinical Implication. Frontiers in Endocrinology. 2021. PMID: 34084149.
4. Vicennati V, Garelli S, Rinaldi E, et al. Stress, cortisol, and obesity: a role for cortisol responsiveness in identifying individuals prone to obesity. Domestic Animal Endocrinology. 2016. PMID: 27345309.
5. Thuisarts.nl / NHG. Blood testing. Accessed 2026.
6. RIVM. Population figures and reference values. Accessed 2026.

Disclaimer

Every blood test result includes a professional assessment by a BIG-registered doctor. This article gives general information and is not a substitute for medical advice. A blood test is a tool to walk into the conversation with your GP better informed, not a diagnosis in itself. For treatment decisions, discuss your results with your GP.