Blood Tests for Weight Loss: Key Indicators

Your doctor may prescribe a comprehensive blood test to understand the cause of unexplained weight loss. Weight loss is considered significant when it exceeds 5% of body weight over six months or 10% over a year, without any deliberate change in lifestyle. This blood test assesses several specific biological markers. Without causing undue alarm, these tests help to explore various possibilities and, together with your doctor, draw up a suitable plan of action. The tests assess your general nutritional status, the proper functioning of vital organs, hormonal balance, and the possible presence of inflammatory processes. The aim is to distinguish between normal weight loss and weight loss of pathological origin requiring medical management. The tests generally include a complete blood count (CBC), liver function tests, an assessment of kidney function, a lipid and glucose profile, as well as measurements of certain essential vitamins and minerals. Depending on the clinical context, the doctor may also prescribe a hormonal profile.

Good to Know: A blood test in the event of weight loss is not only used to identify a medical condition, but it also helps to detect any nutritional deficiencies, which can sometimes be easily corrected by adjusting your eating habits. In adults, a third of cases of weight loss are linked to nutritional deficiencies or functional disorders rather than serious medical conditions.

A blood test helps guide the medical diagnosis of unexplained weight loss. It is prescribed by a doctor based on several factors, including the patient’s age, the rate of weight loss, and the presence of any associated medical conditions. Understanding the mechanisms at play enables an appropriate response and helps prevent the risks associated with this weight loss.

Nutritional Deficiencies

This is one of the main causes identified by blood tests in cases of unexplained weight loss.

Iron Deficiency Is Common and Causes Iron-Deficiency Anaemia, which is usually associated with severe fatigue. Measuring ferritin, serum iron, and transferrin levels helps to assess the body’s iron stores. When iron is not the cause, normal values generally range between 20 and 400 nanograms per millilitre (ng/ml) of blood. This deficiency is monitored particularly closely in women, due to blood loss associated with menstruation.

Vitamin B12 May Also Be Implicated in Weight Loss Accompanied by Neurological Disorders. It can cause a specific type of anaemia, known as megaloblastic anaemia, characterised by the production of abnormally large red blood cells that are unable to function properly. As this vitamin is found mainly in animal-based foods, people following a strict vegan diet are at greater risk of deficiency.

Deficiencies in Folate (Vitamin B9) Can Also Disrupt the Absorption of Nutrients in the Gut. Measuring erythrocyte folate levels provides an accurate picture of the body’s reserves and helps to address a cause which, on its own, may explain unexplained weight loss. In fact, we have a comprehensive article on what to do in the event of low folate levels.

Vitamin D, Known for Its Role in Bone Health and Immunity, Also Influences Energy Management and Appetite. Insufficient levels can contribute to weight loss, particularly in people with limited exposure to sunlight.

Thyroid Disorders

Hyperthyroidism is a common cause of weight loss, despite an unchanged or even increased appetite. It speeds up the basal metabolic rate, leading to excessive energy expenditure, even during routine activities. A TSH test is one of the first tests prescribed by a doctor and may, if necessary, be supplemented by tests for the T3 and T4 hormones, in order to refine the assessment of thyroid function. Paradoxically, certain forms of hypothyroidism can also cause weight loss, particularly in older people. This is due to their effect on appetite and the accompanying digestive problems.

Metabolic Disorders

As mentioned above, a metabolic disorder may also be responsible for unexplained weight loss. This is the case, for example, with type 1 diabetes or certain severe forms of type 2 diabetes. As the body is no longer able to utilise glucose properly, it draws on its fat and protein reserves. Measuring fasting blood glucose and glycated haemoglobin allows this cause to be identified quickly. Liver disorders are another possible cause. They can disrupt numerous metabolic functions and lead to weight loss. A comprehensive liver function test (transaminases, bilirubin, albumin, total protein) is often prescribed to assess liver function and its capacity for protein synthesis. Finally, kidney function may also be a factor. Chronic kidney disease can lead to weight loss due to the build-up of toxins (known as uraemic toxins), which reduce appetite. Measuring creatinine levels and calculating glomerular filtration rate (GFR) help to assess kidney function.

Chronic Inflammation

This is a silent underlying process that disrupts the body’s functioning. Chronic inflammation consumes a great deal of energy and can lead to gradual weight loss. In such cases, C-reactive protein and erythrocyte sedimentation rate are measured. Consistently high levels of these markers may indicate a chronic inflammatory condition requiring specialist medical care. These inflammatory conditions also disrupt intestinal absorption and increase the body’s nutritional requirements.

Unexplained weight loss is often accompanied by other symptoms that can guide the diagnosis and justify a full blood test.

• Severe Fatigue: This is commonly caused by an iron deficiency. Iron is essential for transporting oxygen around the body. Insufficient oxygenation leads to a reduced appetite and impaired physical performance. A vicious circle then sets in: fatigue reduces appetite, which in turn exacerbates the deficiency. This fatigue, which is not relieved by rest, develops gradually and disrupts daily life

• Muscle Loss: A loss of muscle mass is often a sign that weight loss has a pathological cause. Unlike voluntary weight loss, where weight loss is associated with an increase in muscle mass, here there is a reduction in physical strength and increasing difficulty in carrying out everyday activities

• Neurological Disorders: certain deficiencies, particularly in B vitamins, cause typical symptoms such as tingling in the extremities, memory problems, difficulty concentrating, or marked mood swings

• Hair Loss: This is one of the most visible and frequently observed signs of unexplained weight loss. Hair loss may be diffuse, affecting the entire head of hair or localised. It is often linked to a deficiency in iron, protein, or B vitamins. The hair becomes fine, brittle, and loses its shine. Read our advice on stimulating growth and reducing hair loss

• Brittle Nails: This condition often accompanies hair loss. Nails become more fragile, soft and brittle. Ridges may appear, as well as white spots

• Appetite Disorders: This symptom varies depending on the underlying cause. Some people experience a gradual loss of appetite with early satiety, while others retain a normal or increased appetite, yet continue to lose weight despite an adequate food intake

• Digestive Problems: Symptoms such as nausea, bloating, and changes in bowel habit can be both the cause and the consequence of nutritional deficiencies, creating a vicious circle that is difficult to break

If you notice unexplained weight loss, it is essential to see your doctor promptly. Significant weight loss can sometimes be difficult to reverse. A blood test is only the first step in the process of identifying the cause of the problem.

Unfortunately, there is no miracle cure, but certain lifestyle changes, particularly to your diet, can help limit unexplained weight loss:

1. Adjust Your Diet: It is important to adopt your diet to be suited to your circumstances. For example, older people sometimes tend to consume less protein, due to a lack of appetite or financial constraints. However, there are affordable alternatives, such as legumes (lentils, chickpeas, beans, etc.), which are good sources of protein

2. Take Your Time When Eating: Chewing slowly and thoroughly aids the pre-digestion of food. It also stimulates the production of digestive enzymes, which in turn improves the absorption of nutrients in the gut

3. Include Fermented Foods: Kefir, sauerkraut, miso, and yoghurt are natural sources of probiotics. They help support gut health and promote better nutrient absorption

4. Opt for Targeted Supplements: If you follow a specific diet, such as a vegetarian diet, or have digestive problems, dietary supplements may be helpful. For example, as part of a vegetarian diet, it is essential to prevent deficiencies in B vitamins

5. Manage Your Stress and Sleep: These two risk factors exacerbate weight loss. It is therefore vital to put in place strategies to improve sleep quality and manage your emotions

6. Have Regular Check-Ups: Depending on your health status (age, pregnancy, chronic conditions, etc.), your doctor may recommend regular tests. Having these carried out promptly helps to detect emerging deficiencies and take immediate action

When having a blood test for unexplained weight loss, it is important not to take any dietary supplements, as these could affect the results. If in doubt, do not hesitate to ask your doctor or pharmacist for advice.

Unexplained weight loss is not something to be taken lightly and requires a gradual, methodical approach. Start by consulting your doctor to identify any deficiencies and address the underlying cause. You will then need to focus on regaining a healthy weight, which may take time, especially if the deficiencies were significant. Don’t hesitate to speak to a nutritionist, who can help you draw up meal plans tailored to your situation.

Guillaume is a pharmacy technician specialising in dermocosmetics. With more than ten years’ experience in the pharmacy sector, he now supports health and cosmetics brands with writing blog articles. Having studied naturopathy, he takes part in podcasts to explain health and cosmetics in accessible terms.