Why you can’t lose weight (even with clean eating and exercise)

Sheet 20 · Rev A · 12 October 2026 · 6 minute read

From chapter one of Fat Loss Cheat Code: Nutrition Edition, now on Amazon.

Quick answer: if you’re eating clean and working out and the scale still won’t move, it’s not always a willpower problem. Three things inside your body can quietly slow fat loss: a slow thyroid, hormones that shift after 40, and nutrient gaps like low vitamin D. A full blood panel is how you find out.

Why can’t you lose the weight? You’re eating clean, you’re working out, you’re getting your steps in, but the scale still won’t budge. What are you doing wrong?

Most people blame themselves. They figure they’re not disciplined enough, they’re not training hard enough, or they just haven’t found the perfect diet yet. So they cut more, they run more, and they buy every new fat burner that promises to fix it. But what if the problem isn’t your effort at all? What if it’s something inside your body that you can’t see from the outside?

Here’s the plan. First, the master gland that decides how much fat you burn. Second, the hormones that change the rules, especially after 40. And third, the vitamin that about four in ten American adults are low in, and most probably don’t even know it.

Is it really just willpower?

Not always. In my book, Fat Loss Cheat Code, which is all about how to outsmart your metabolism, I compare your metabolism to a high-performance car. You can fill the tank with premium gas, but if the spark plugs are worn out or one of the sensors is off, the engine still won’t run the way it should.

Your body works the same way. Hormones, vitamin and mineral levels, stress and sleep all decide what your body does with the food you eat. When they’re working for you, your body can get at its stored fat. When they’re working against you, losing fat gets harder, even when you’re eating less.

That’s why one of the common mistakes in the book is blaming your willpower. When progress stalls and you automatically blame yourself, you can miss what’s going on inside.

Is a slow thyroid stopping your weight loss?

It might be. Your thyroid is a little butterfly-shaped gland in your neck, and it works like the thermostat in your house. It sets how fast your cells burn energy. When it slows down, it’s like someone turned the dial from 72 down to 65. The house still runs, it just burns a lot less fuel. You can be doing everything right on your end and still not see it on the scale.

Some common warning signs:

• Feeling tired all the time
• Always being cold when everyone else is fine
• Dry skin
• Constipation
• Struggling to lose fat even when you’re eating clean

And here’s what’s crazy. A national study in the Journal of Clinical Endocrinology and Metabolism tested the blood of over 17,000 Americans. About one in twenty had a low thyroid. Most of those cases were mild, and a big part of the people with lab signs of thyroid trouble had no idea.

The fix: ask your doctor for a full thyroid panel, not just one number. I break down which tests to ask for, and the foods your thyroid needs, in how to lose weight with a slow metabolism.

Why is it harder to lose weight after 40?

Because your hormones change the rules. Think of your hormones like the GPS that decides where your body delivers fat. For years, it runs the same routes, and the deliveries get spread around. Then you hit 40, the GPS installs an update, and suddenly every delivery gets routed straight to your belly. Same driver, same food, brand-new storage location.

For women, it’s estrogen. As it drops around menopause, your body can start storing more fat around the belly. For men, it’s testosterone. It helps you hold on to muscle, recover from your workouts, keep your energy up and handle carbs better. When it runs low, fat gets stubborn and muscle gets harder to keep.

A long-running study called SWAN followed over 1,200 women through menopause with body scans. Before the change started, they were gaining about 1% fat a year. Once the transition kicked in, that rate almost doubled. At the same time, their muscle went from slowly growing to slowly shrinking.

And here’s the part nobody tells you. Their weight on the scale kept the same trend. The fat went up while the muscle went down, so the scale said everything was fine. That’s why the scale alone can fool you. More on what to do about it in how to lose weight after 40.

Can you eat enough and still be low on nutrients?

Yes. You can eat the right number of calories, hit your protein, and still be missing the small stuff: the vitamins and minerals your body uses to actually make energy. Those are the spark plugs. Premium gas doesn’t help if the spark plugs are worn out.

The most common one might surprise you. A study in Nutrition Research looked at the blood of nearly 4,500 American adults. 42% were low in vitamin D, and it was more common in people carrying extra weight.

Vitamin D works more like a hormone than a vitamin. It’s tied to your testosterone, your muscles, your mood and how well your body handles insulin. And it’s not the only one. Magnesium is involved in over 300 reactions in your body, including how you make energy and handle blood sugar. Iron and B12 help carry oxygen and make energy, and when they’re low, you feel it in your workouts.

Where to find them:

• Vitamin D: salmon, trout, sardines, egg yolks
• Magnesium: pumpkin seeds, chia seeds, spinach, almonds, black beans, dark chocolate
• Iron: red meat, oysters, lentils, spinach
• Vitamin B12: clams, beef, tuna, nutritional yeast, dairy, eggs

What blood tests should you ask for?

Four things:

• A full thyroid panel: TSH, free T4 and free T3
• Your sex hormones: estrogen and progesterone for women, testosterone for men
• Your fasting glucose and insulin
• Your vitamins and minerals: vitamin D, magnesium, iron and B12

The goal isn’t a perfect score on every number. It’s awareness: finding out if something inside is working against you. I’m a nutrition consultant, not a medical doctor, so let your doctor read the results with you. The full list, with what each test tells you, is in blood tests for weight loss.

And if your fasting insulin comes back high, that’s its own fix. Start with how to lower fasting insulin.

What if your blood work comes back normal?

Then you’ve ruled out the engine, and that’s worth knowing too. The next places to look are the calories you don’t see and a body that’s adapted to dieting. I walk through both in not losing weight in a calorie deficit.

The one thing to do this week

When you’re doing the work and the scale won’t move, it’s not always a willpower problem. Your thyroid might have the heat turned down, your hormones might have changed the rules, or you might be running on worn-out spark plugs.

So this week, if you feel like you can’t make any progress even though you’re doing everything right, call your doctor and book a full blood panel. One blood test, four things to ask for, and no more guessing. Check the engine first.

Keep reading

• How to lose weight with a slow metabolism (check your thyroid first)
• How to lose weight after 40: why fat moves to your belly
• Blood tests for weight loss: the 4 things to ask your doctor for

Sources: Hollowell et al., Journal of Clinical Endocrinology & Metabolism, 2002 (17,353 people, NHANES III); Greendale et al., JCI Insight, 2019 (SWAN, 1,246 women); Forrest and Stuhldreher, Nutrition Research, 2011 (4,495 US adults, NHANES 2005-2006).

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