Why does nutrition advice vary from person to person?

The numbers in nutrition guidelines are worked out for a population, not for a single person. TÜBER, Türkiye's national dietary guideline, writes this plainly: reference values are set taking into account the physiological or metabolic requirements related to age, sex and special conditions. How well a number fits you depends on which question that number answers.

Below we look at what a reference value means, how five different values do five different jobs, why energy stands apart, how activity level shifts the table and which questions to ask while reading your own requirement.

What a reference value means

Reference values form the scientific basis of nutrition advice and are set on the assumption that physiological requirements do not vary greatly between populations. In recent years, studies examining the relationship between a nutrient and disease risk have joined this work as well.

To produce a value specific to a population, that population's measured height and weight data and its food consumption data have to be known. For Türkiye, the energy and protein values have been adapted from the database of the national nutrition surveys.

Five values, five separate jobs

The guideline does not give a single number; it defines different values for different jobs. The amount recommended to be taken with the diet is the amount that meets the requirement of nearly everyone in the population, that is 97.5 percent of them, and it is used to build nutrition plans.

The average requirement, on the other hand, is the amount that meets the requirement of half the individuals in a population, and it is used not to build a plan but to estimate how often intake falls short.

The adequate intake amount is determined by looking at the average daily intakes of healthy individuals in cases where the first two values cannot be set. The tolerable upper level shows the upper limit judged not to cause adverse effects when taken continuously.

Same number, different meaning

This distinction matters in practice: the value used when building a plan is not the same as the value used when measuring a shortfall in a population. The guideline does not recommend that the recommended amounts be used for determining the prevalence of inadequate intake.

So the number you see in a table answers a different question depending on which column it stands in. A number has not been read until the column heading has been read.

Why energy stands apart

A “recommended amount” cannot be given for energy. The guideline states its reason: because physical activity levels and the variability of body measurements are large across populations, energy is stated not as a recommended amount but as the average energy requirement.

This means that the energy target is personal by its nature. The value in the table is not a target but a starting point.

How activity level shifts the table

The measured difference is large. In the table the guideline gives for adult women, the daily energy requirement for a woman aged 18-29 at median height and weight is given as 1786 kilocalories at the low active level and 2551 kilocalories at the very active level.

The same person, the same age, the same measurements; the only thing that changes is the activity level. The difference between them passes seven hundred kilocalories and stands side by side in a single table.

Age works in the same direction. In the same table the range shifts lower for median values in the 60-69 age group; and when the height and weight percentile changes, every row is recalculated.

Values given per kilogram

Some values are already written per person. For protein, the amount recommended to be taken with the diet is 0.83 grams per kilogram per day in adults; when the protein quality of Türkiye's average diet is taken into account, the equivalent rises to 1.04 grams per kilogram.

This form shows directly why the number varies from person to person. The same recommendation gives a different amount in grams at a different body weight.

While reading your own requirement

When you meet a number there are four questions to ask: which type of value is this, for which age and sex was it given, which activity level was assumed and which body measurements was it calculated for.

All four are written in the table. A number that is not written down cannot be used without knowing which question it answers.

The fifth question is on your side: which row in the table does your activity level today fall on. The same person can fall on different rows on a weekday and at the weekend.

There are values that look fixed too

Not every value shifts from person to person. In adults the adequate intake for fiber is 25 grams a day, and for water it is 2.5 liters in men and 2.0 liters in women; these values stay the same across wide age ranges.

Looking fixed does not mean that personal difference has disappeared. In the case of water, the guideline separately writes that the need changes with air temperature, body build, physical activity and the diet consumed.

In short: when you read a number, look first at which type of value it is, then at which age, sex and activity level it was given for; treat the result as a starting point, not a target. This content has been prepared to support general nutrition literacy; a personalized plan is prepared only by a licensed dietitian. We covered the definition of the portion in this guide, what a calorie measures in this guide, and whether portion recommendations fit everyone in this tip.