Constant tiredness on a diet: when to check for nutrient gaps and thyroid function
If you eat less but feel tired all the time and your weight stays the same, the problem is sometimes too few nutrients rather than too many calories. Low iron leaves you drained, and an underactive thyroid slows your metabolism. That does not mean I recommend tests for everyone. I suggest a test only when symptoms or medical history give a reason to check. Filling a gap is where weight loss starts, and a supplement on its own will not bring your weight down.
- ACADEMYSenior Academic Director, Korean Academy of Dermatology, Obesity and Plastic Surgery (KACS)
- PROFESSORAdjunct Professor, Korea University Medical Center
- FELLOWSHIPFellowship, University of Florida Shands Hospital
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- Ewha Womans University College of Medicine
- Residency, Korea University Medical Center
- Adjunct Professor, Korea University Medical Center
- Fellowship, University of Florida Shands Hospital
- Senior Academic Director, Korean Academy of Dermatology, Obesity and Plastic Surgery (KACS)
Contents5
- 01Why plenty of calories with too few nutrients keeps weight from coming off
- 02Five nutrients that tend to run low on a diet and foods that supply them
- 03Symptoms that call for nutrient and thyroid tests and when a test is not needed
- 04Why weight does not drop straight away when you take supplements
- 05Frequently asked questions
Some people cut their portions and then find their body feels heavy every afternoon while the scale has not moved for weeks. That is why the first chapter of the book I co-wrote, Eat to Succeed (a Korean-language book, 2026), is about exactly this. In it we explain a body that stays hungry after a full meal, stays tired after enough sleep and gains weight on little food as a body short of nutrients.
Why plenty of calories with too few nutrients keeps weight from coming off
A croissant for breakfast brings 300 kcal and plenty of sugar, and it fills you up. But it has almost no vitamins, minerals or fibre and little protein. The book puts it this way: "Calories are in excess, but nutrients are lacking." Even after eating, the body keeps sending hunger signals because what it actually needs has not arrived.
When one nutrient runs short, others follow. The book notes that without enough vitamin D calcium is not absorbed, and without enough magnesium vitamin D is not activated. Too little omega-3 makes inflammation more likely, and inflammation drives insulin resistance. So the conclusion of the first chapter is to fill in missing vitamins, minerals, protein, fibre and sleep before cutting carbohydrates or fat across the board.
Five nutrients that tend to run low on a diet and foods that supply them
The book advises choosing foods with high nutrient density rather than going by calorie numbers. Here are the five nutrients the book covers one by one.
| Nutrient | What the book says happens when it runs low | Foods to include |
|---|---|---|
| Vitamin D | Leptin, the fullness hormone, works less well, so you keep reaching for food when full · less muscle is built and basal metabolic rate drops | 15-20 minutes of sun on arms and legs a day · salmon, mackerel, sardines · egg yolk · mushrooms |
| Magnesium | Insulin does not work properly and blood sugar rises · less melatonin is made, which can make insomnia worse | Almonds, cashews, walnuts · pumpkin and sunflower seeds · spinach, kale · brown rice, oats |
| Omega-3 | Inflammation sets in more easily · leptin resistance, where the brain does not notice you are full | Salmon, mackerel, sardines, herring two or three times a week · flaxseed, chia seeds, walnuts |
| B vitamins | Sugar is not turned into energy and fat metabolism slows · low B6 reduces muscle building | Brown rice, oats, quinoa · spinach, broccoli · eggs · beans · fish and meat |
| Iron | Less oxygen reaches tissues, so you tire easily and do not want to move · less thyroid hormone is made and metabolism slows | Haem iron from meat and fish · eat with vitamin C · avoid tea and coffee for an hour before and after meals |
Iron runs low especially often in women of childbearing age, because menstruation means losing iron every month. The book notes that blood vitamin D tends to be low in people who get little sun, people with reduced kidney function, older people and people with obesity. Because it is fat-soluble, a lot of body fat can hold on to it so that less reaches the blood.
Symptoms that call for nutrient and thyroid tests and when a test is not needed
When tiredness drags on, people often ask whether they should have a blood test. Looking at the whole body does not mean recommending tests to everyone. I suggest a test only when symptoms or medical history give a reason to check. These are the symptoms from the book that bring a test to mind.
- When low iron is suspectedsevere tiredness · pale skin · cold hands and feet · poor concentration · shortness of breath · a craving to chew ice
- When an underactive thyroid is suspectedweight gain you cannot explain · ongoing tiredness · feeling the cold · constipation · dry skin and hair · low mood · slow heart rate · poor memory and concentration
- When vitamin D is likely to be lowvery little sun · reduced kidney function · older age · obesity
The thyroid controls basal metabolic rate, body temperature and energy production, so when it slows, in the book's words, everything goes into slow motion. If the cause is an autoimmune condition such as Hashimoto's thyroiditis, changing your diet alone will not fix it. It needs to be confirmed with a test and treated.
Often the reason for tiredness shows up in daily life first. If you eat one big meal a day, sleep too little or are going through a stressful stretch, that alone unsettles appetite hormones and cortisol, as the book explains. For these patients I start by talking about getting meals back on track and sleeping more.
Why weight does not drop straight away when you take supplements
When people hear they are short of a nutrient, it is easy to hope one supplement will sort out their weight too. But the evidence that vitamin D supplements alone reduce weight is weak. A review that gathered 18 randomised controlled trials of vitamin D without calorie restriction and pooled 12 of them found no difference from placebo in body weight, fat mass or body fat percentage.
The book calls filling in what is missing the start of sustainable weight loss. As the word start suggests, filling the gap is not the end of it. Weight moves when meals, exercise and sleep change together.
For people whose shortfall has been confirmed by a test, filling it can make a difference. In a trial of 198 women aged 18 to 53 who were not anaemic but had ferritin below 50 μg/L and complained of tiredness, 12 weeks of iron tablets or placebo reduced fatigue scores by 47.7% in the iron group and 28.8% in the placebo group. There was no difference in quality of life, depression or anxiety, and the study did not measure weight. The researchers suggested checking the effect with blood markers after six weeks of iron.
The amount and form of a supplement differ from person to person, which is why the book says to decide them with a doctor. Rather than taking several supplements at once without a test, I recommend filling your meals with the foods in the table first.
Frequently asked questions
QWhy am I so tired on a diet?
Often, eating less also means running short of nutrients such as iron, vitamin D, magnesium and B vitamins, or sleeping too little, or eating everything in one meal. If severe tiredness comes with pale skin, cold hands and feet and breathlessness, or you feel the cold and have constipation and dry skin, tests for iron and thyroid function may be needed.
QWill taking vitamin D make me lose weight?
A pooled analysis of 12 randomised controlled trials of vitamin D without calorie restriction found no difference from placebo in weight or body fat. If you are low it is right to top it up, but weight comes down when meals, exercise and sleep change together.
QDo I need a blood test while dieting?
Not everyone does. I suggest a test only when symptoms or medical history give a reason to check.
QHow should I take iron so it is absorbed well?
Haem iron from meat and fish is absorbed better than iron from plants. Vitamin C increases absorption, while tea and coffee get in the way, so it is better to avoid them for an hour before and after meals.
QDoes an underactive thyroid make you gain weight?
Thyroid hormone controls basal metabolic rate, body temperature and energy production, so when it is low you can get unexplained weight gain, ongoing tiredness and feel the cold. If the cause is something like Hashimoto's thyroiditis, diet alone will not fix it and testing and treatment are needed.
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