Micronutrients during weight loss: what runs short when you eat less

Calories, protein and fibre get most of the attention in weight loss. Vitamins and minerals get very little, because deficiency develops slowly and the early signs are vague: tiredness, hair shedding, poor recovery. That is exactly why it is worth a look.

Why the risk rises

The 2025 joint advisory on nutrition with GLP-1 therapy cites observed calorie reductions of 16 to 39 percent on these drugs. It says that this large, rapid reduction can lead to inadequate vitamin and mineral intake, especially at intakes below about 1,200 kcal a day for women and 1,800 kcal a day for men. The same applies to any diet at those levels. Nausea, food aversions and skipped meals make it more likely.

The nutrients of concern

The advisory names iron, calcium, magnesium, zinc and vitamins A, D, E, K, B1 (thiamine), B12 and C. The table gives the Daily Values used on US labels, which are reference amounts for a 2,000 calorie diet rather than personal targets, and where each nutrient is found.

The advisory lists general signs of deficiency worth raising with a clinician: fatigue beyond what you would expect, excessive hair loss, flaky or itchy skin, muscle weakness, poor wound healing and unusual bruising. None of them is specific, which is why a blood test beats guessing.

Who is most at risk

• Older adults. The 2025 to 2030 Dietary Guidelines note that some older adults need fewer calories but equal or greater amounts of protein, B12, vitamin D and calcium, and that fortified foods or supplements may be needed under medical supervision.
• Vegetarians and vegans. The guidelines list wider shortfalls and suggest periodic checks, especially of iron, B12, vitamin D, calcium and iodine.
• People eating very little, for any reason, including on GLP-1 receptor agonists.
• People who menstruate heavily, for iron.
• After bariatric surgery, which has its own lifelong supplement plan set by the surgical team.

Bone

Bone deserves its own line. The joint advisory notes that weight loss that is both large (14 percent or more) and fast (over three to four months) is associated with significant bone loss, more so in older people and women. In a Danish trial of 195 adults with obesity, published in JAMA Network Open in 2024, a year of liraglutide alone reduced bone density at the hip and spine compared with exercise alone, while liraglutide combined with exercise kept bone density similar to placebo despite the largest weight loss. Calcium, vitamin D, protein and resistance training are the levers. See exercise during weight loss.

Food first, then supplements

The advisory's food advice is to favour nutrient-dense, minimally processed foods: fruit, vegetables, whole grains, legumes, lean protein, nuts and seeds, with fewer refined carbohydrates and sugary drinks. When intake stays low, it says supplements such as vitamin D, calcium, B12 or a multivitamin and mineral tablet can be considered proactively, at appropriate doses and tailored to the person. A standard multivitamin is the simplest version of that; high-dose single nutrients are better started after a test shows a need, because some (vitamin A, D and iron, for example) cause harm in excess.

Reading a supplement label

Supplement labels list each nutrient as an amount and as a percentage of the Daily Value. A basic multivitamin usually sits near 100 percent for most vitamins and lower for calcium and magnesium, which are bulky. More is not better past the Daily Value for most nutrients: vitamins A and D, iron and zinc in particular can cause harm at high doses over time, and iron supplements are a leading cause of accidental poisoning in young children, so they need to be stored safely. If you take several products, add up the totals; the overlap is often larger than people expect.

Protein counts too

Protein is a nutrient that runs short in the same way. When intake falls, the 2025 joint advisory's suggested 1.2 to 1.6 g/kg (or 80 to 120 g a day) is hard to hit without planning. A shortfall shows up as weakness and slow recovery rather than as a lab value. See the protein guide.

What to test

There is no single standard panel, and the right tests depend on your history. Common ones a clinician may consider during significant weight loss are a full blood count and ferritin for iron, vitamin B12, vitamin D, and kidney function and electrolytes, especially if there has been vomiting. Our guide to reading a lab report explains the reference ranges.

Tracking it

Deficiencies show up as a slow trend, so they are easiest to see over time. Peptide AI's bloodwork tracking keeps ferritin, B12, vitamin D and the rest on one timeline next to the food log and, on a GLP-1, the dose log, so a falling ferritin can be read against what you were eating at the time. The supplement scanner logs what is in each product you take.