Scans explained
Body Composition and Daily Habits
A plain English look at what the body is made of and how water, magnesium, fibre, sleep, cortisol and movement after forty change it over time.
The body is mostly water, with minerals, fat, protein and a dense microbial population inside the gut, and daily habits shift the balance of all of them. Water and minerals such as magnesium support the chemistry that keeps cells working, fibre feeds the gut bacteria, sleep regulates cortisol, and movement after forty slows the loss of muscle mass. None of these acts alone, and none of them is a treatment for a diagnosed condition.
What the body is made of
Roughly 60 per cent of an adult body is water, though the figure varies with age, sex and body fat. The remainder is protein, fat, minerals and carbohydrate, with bone holding most of the mineral fraction. Magnesium is one of those minerals: an adult body contains about 25 grams, mostly in bone, with the rest inside cells where it takes part in hundreds of enzyme reactions, including the production of ATP. The gut adds a further layer. The large intestine holds a microbial population whose collective genetic material outnumbers the human genome, and its composition responds to what is eaten over days and weeks rather than hours.
Blood and imaging give a sense of these proportions. A CT scan separates tissue by density, so fat, muscle, bone and air appear as distinct shades on the same cross section. That is why a radiographer can describe body composition from a scan taken for an unrelated reason. A German-language magazine on everyday health and vitality, Sina Vital, covers the same ground from the nutrition side, looking at magnesium, gut health and hydration as daily variables rather than clinical endpoints.
How does water and magnesium intake affect daily energy?
Water is the medium in which almost every reaction in the body takes place, and mild dehydration reduces blood volume, which the heart compensates for by working faster. The effect on energy is usually described as subtle: tiredness, poorer concentration and a higher perceived effort during exercise. The amount needed varies with body size, climate and activity, and urine colour remains the simplest everyday indicator used in public guidance.
Magnesium sits inside the same picture. It is involved in the release of energy from ATP, in muscle contraction and relaxation, and in nerve signalling. Low intake is common in surveys of Western diets, and the symptoms attributed to it, such as fatigue and muscle cramps, overlap with many other causes. The evidence for benefit from supplements in people who already eat enough magnesium is weak, and the more reliable route is food: nuts, seeds, whole grains, pulses and green leafy vegetables.
What does the gut do with fibre?
Dietary fibre is the part of plant food that passes through the small intestine without being digested. In the colon, bacteria ferment it into short-chain fatty acids, which feed the cells lining the gut and appear in the bloodstream at low concentrations. Fibre also adds bulk and water to stool, which speeds transit and reduces the strain associated with constipation.
Fermented foods, such as yoghurt, kefir, sauerkraut and kimchi, introduce live bacteria, though whether they colonise the gut in the long term is not settled. What is better established is that a varied plant-based diet, with a range of fibre types, is associated with a more diverse microbial population. Diversity is used as a marker of a stable gut environment rather than a direct measure of health.
How does sleep affect cortisol and the immune system?
Cortisol follows a daily rhythm, rising in the hour after waking and falling through the day to a low point in the early hours of the night. Sleep and cortisol are linked in both directions: a late night raises evening cortisol, and high evening cortisol makes falling asleep harder. Chronic short sleep, usually defined as under six hours a night over weeks or months, is associated with a flattened rhythm and with higher fasting glucose and appetite.
The immune system responds to the same pattern. During sleep, the body releases cytokines that support the response to infection, and short sleep is associated with a greater susceptibility to the common cold in controlled exposure studies. The practical levers are unglamorous: a consistent wake time, dim light in the evening, and a cool, dark room. None of these is a treatment, and persistent sleep problems belong with a clinician.
Movement after forty: what changes?
Muscle mass begins to decline gradually from around the fourth decade, a process sometimes called sarcopenia, and the rate accelerates after sixty. The loss is not inevitable at the same speed for everyone. Resistance training, meaning work against a load such as body weight, bands or weights, is the stimulus that best preserves and rebuilds muscle, and it also improves bone density and insulin sensitivity.
Daily step counts matter too, but the relationship is not linear. Benefits accumulate fastest when moving from very low activity to moderate, and the returns flatten at higher volumes. For adults without medical conditions, the common public target of 150 minutes of moderate activity a week, plus two sessions of strength work, is the figure used in national guidance. Anyone with a heart condition, joint replacement or recent surgery should take advice before starting a new programme.
Where imaging fits in
Imaging does not measure habits, but it records their consequences. A DEXA scan reports bone mineral density and body composition, and is used in the UK to assess osteoporosis risk. MRI can quantify muscle volume and fat infiltration without ionising radiation. CT gives a rapid cross-sectional view but uses X-rays, so it is reserved for clinical questions rather than routine body composition checks. Ultrasound is used for soft tissue and vascular assessment and carries no ionising radiation.
These tools are ordered by clinicians when there is a question to answer. They are not screening tests for diet or sleep, and a normal scan does not mean that daily habits are optimal. The reverse is also true: a scan finding does not, on its own, explain how a person feels.
| Measurement | What it shows | Typical UK use |
|---|---|---|
| DEXA | Bone density, fat and lean mass | Osteoporosis assessment |
| MRI | Soft tissue, muscle volume | Neurological and musculoskeletal |
| CT | Cross-sectional density | Acute and trauma imaging |
| Ultrasound | Soft tissue, blood flow | Vascular and abdominal |
Checked against the source
- The NHS describes the Eatwell Guide as the basis for a balanced diet, including fibre-rich foods, in guidance published on 29 November 2022.
- The UK Health Security Agency reported in its 2023 surveillance data that physical activity levels vary widely by region, with the figures published on 26 September 2023.
- NICE guideline NG209 on tobacco and weight management, published on 2 November 2022, sets out the evidence review process used for lifestyle interventions.
- The Royal College of Radiologists published its referral guidance for suspected cancer in 2023, setting out when CT and MRI are appropriate.
- The Health and Care Professions Council publishes standards of proficiency for radiographers, most recently updated in 2023.
A common confusion about body composition
A frequent misunderstanding is that body weight and body composition are the same thing. A person can weigh the same across a year while losing muscle and gaining fat, particularly after forty, when muscle mass declines slowly and fat redistributes towards the abdomen. Bathroom scales cannot separate these compartments, and a change in weight after starting exercise often reflects water and glycogen shifts rather than fat loss.
A second confusion concerns minerals. Magnesium is often discussed as if a supplement were a reliable fix for tiredness, but the evidence for benefit in people with adequate intake is limited, and the mineral works alongside sodium, potassium and calcium rather than in isolation. The same logic applies to sleep and movement: single interventions rarely transform how a person feels, while consistent routines over months do change the underlying measurements.
Body composition and daily habits sit on the same page as the scan itself, because what the scanner measures is only ever a snapshot of a body that is changing. Water, magnesium, fibre, sleep, cortisol and movement after forty all shift that snapshot over weeks and months. The same site carries a separate plain guide to a beginner's first sitting, which sets out posture, counting the breath, how long a first sitting lasts, and how a daily practice is kept going between retreats. The two pages are read side by side: one describes what the body is made of, the other describes a quiet habit that runs alongside it.