What not eating enough protein actually looks like

Search this question and you will find a list: tiredness, brittle nails, thinning hair, slow healing, getting ill often. The lists are not fabricated. They are also close to useless for answering the question you asked, and it is worth explaining why before offering anything better.

Two different things share one name

Clinical protein deficiency is a medical condition. It produces unmistakable signs, and it is genuinely rare in countries where food is plentiful, because protein is present in almost everything and the floor for avoiding deficiency is low. If you are eating enough food in general, you are very unlikely to be here.

Eating less than a useful target is a different situation entirely, and it is extremely common. It is the gap between the 0.8 g/kg floor that prevents deficiency and the 1.0 to 1.2 g/kg or more that the expert groups recommend for older adults and for people in a deficit.²

Nearly everyone asking this question is asking about the second one. Almost every symptom list answers the first.

The shortfall is quiet, and that is the actual problem

Here is the uncomfortable shape of it: falling short of a good target does not feel like anything, for a long time.

What it costs is muscle, lost slowly. Not in a week, and not in a way that announces itself. The consensus definition of age-related muscle loss puts strength first as the thing to measure precisely because mass changes are hard to see and hard to feel until they are substantial.³

And the instrument most people would reach for is the wrong one. If you lose a kilogram of muscle and gain a kilogram of fat over a year, the bathroom scale reads exactly the same number both times. That is the whole difficulty with the scale in midlife, and it is why "I would have noticed" is not a safe assumption.

Why the symptom lists fail

Take the four that appear on every list and ask what else causes them.

A sign that appears in twenty conditions cannot tell you which one you have. Worse, it fails in both directions: it will convince somebody eating plenty of protein that they are deficient, and reassure somebody eating 45 g a day that they are fine because they feel alright.

The answer that does work

The reliable method is dull and takes a week.

1. Work out your target from body weight and situation. How the daily number is derived has the rates, or the calculator does the arithmetic and tells you which recommendation it used.
2. Log an ordinary week. Not a good week. Include the evening you ate at nine and the day you skipped lunch.
3. Compare the two.

That is it. It replaces a guess about your body with a measurement of your diet, and it is the reason tracking one number exists as a habit separate from tracking everything.

Most people find the same thing when they do this: they were closer to the floor than they thought, and the gap lives at breakfast.

When it is not a diet question

There is a line here worth stating clearly, because an article like this one can be read as an invitation to self-diagnose.

Persistent tiredness, hair loss, unexplained weakness or weight you did not intend to lose are things to take to a doctor, not to a protein calculator. They have causes that matter and that nutrition content cannot distinguish between. The same goes for anyone whose protein intake is already managed as part of their medical care: your target is set by your clinician, and it is not the number on this site.

This page is reference material about ordinary eating. It is worth being clear about the difference.