Nutrition labels
<p >Many governments <a href="http://en.wikipedia.org/wiki/Mandatory_labelling" title="Mandatory labelling" >require food manufacturers to label</a> the energy content of their products, to help consumers control their energy intake.<sup id="cite_ref-eu_4-0" class="reference" ><a href="http://en.wikipedia.org/wiki/Food_energy#cite_note-eu-4" >[5]</a></sup> </p>
<p>
In the European Union, manufacturers of packaged food must label the nutritional energy of their products in both kilocalories and kilojoules, when required. </p>
<p>
In the United States, the equivalent mandatory labels display only "Calories",<sup id="cite_ref-us-21cfr101_5-0" class="reference" ><a href="http://en.wikipedia.org/wiki/Food_energy#cite_note-us-21cfr101-5" >[6]</a></sup> often as a substitute for the name of the quantity being measured, food energy; an additional kilojoules figure is optional and is rarely used. The energy content of food is usually given on labels for 100 g, for a typical serving size (according to the manufacturer), and/or for the entire pack contents. </p>
<p>
A paper published in 2011 by the American Journal of Public Health proposed to also display the time needed to burn the food energy contained in the product by running.<sup id="cite_ref-6" class="reference" ><a href="http://en.wikipedia.org/wiki/Food_energy#cite_note-6" >[7]</a></sup> This would indicate how much exercise would be necessary if the food would bring the person's energy intake above what is necessary for that day.</p>
<p>
The amount of food energy associated with a particular food could be measured by completely burning the dried
food in a <a href="http://en.wikipedia.org/wiki/Calorimeter" title="Calorimeter" >bomb calorimeter</a>, a method known as direct <a href="http://en.wikipedia.org/wiki/Calorimetry" title="Calorimetry" >calorimetry</a>.<sup id="cite_ref-7" class="reference" ><a href="http://en.wikipedia.org/wiki/Food_energy#cite_note-7" >[8]</a></sup> However, the values given on food labels are not determined in this way. The reason for this is that direct calorimetry also burns the indigestible <a href="http://en.wikipedia.org/wiki/Dietary_fiber" title="Dietary fiber" >dietary fiber</a>, and so does not allow for fecal losses (i.e. the fact that not all food eaten is actually absorbed by the body); thus direct calorimetry would give systematic overestimates of the amount of fuel that actually enters the blood through digestion. </p>
<p>
What are used instead are standardized chemical tests or an analysis of the recipe using reference tables for common ingredients<sup id="cite_ref-8" class="reference" ><a href="http://en.wikipedia.org/wiki/Food_energy#cite_note-8" >[9]</a></sup> to estimate the product's digestible constituents (<a href="http://en.wikipedia.org/wiki/Protein" title="Protein" >protein</a>, <a href="http://en.wikipedia.org/wiki/Carbohydrate" title="Carbohydrate" >carbohydrate</a>, <a href="http://en.wikipedia.org/wiki/Fat" title="Fat" >fat</a>, etc.). These results are then converted into an equivalent energy value based on the following standardized table of energy densities.<sup id="cite_ref-uk_9-0" class="reference" ><a href="http://en.wikipedia.org/wiki/Food_energy#cite_note-uk-9" >[10]</a></sup><sup id="cite_ref-10" class="reference" ><a href="http://en.wikipedia.org/wiki/Food_energy#cite_note-10" >[11]</a></sup>
</p>
<table class="wikitable" border="1" ><tbody>
<tr>
<th rowspan="2" >Food component</th>
<th colspan="2" ><a href="http://en.wikipedia.org/wiki/Specific_energy#Energy_density_of_food" title="Specific energy" >Energy density</a></th>
</tr>
<tr>
<th >kJ/g</th>
<th >kcal/g</th>
</tr>
<tr>
<td ><a href="http://en.wikipedia.org/wiki/Fat" title="Fat" >Fat</a></td>
<td align="center" >37</td>
<td align="center" >9</td>
</tr>
<tr>
<td ><a href="http://en.wikipedia.org/wiki/Ethanol" title="Ethanol" >Ethanol</a> (drinking alcohol)</td>
<td align="center" >29</td>
<td align="center" >7</td>
</tr>
<tr>
<td ><a href="http://en.wikipedia.org/wiki/Protein_(nutrient)" title="Protein (nutrient)" >Proteins</a></td>
<td align="center" >17</td>
<td align="center" >4</td>
</tr>
<tr>
<td ><a href="http://en.wikipedia.org/wiki/Carbohydrate" title="Carbohydrate" >Carbohydrates</a></td>
<td align="center" >17</td>
<td align="center" >4</td>
</tr>
<tr>
<td ><a href="http://en.wikipedia.org/wiki/Organic_acid" title="Organic acid" >Organic acids</a></td>
<td align="center" >13</td>
<td align="center" >3</td>
</tr>
<tr>
<td ><a href="http://en.wikipedia.org/wiki/Polyol" title="Polyol" >Polyols</a> (<a href="http://en.wikipedia.org/wiki/Sugar_alcohol" title="Sugar alcohol" >sugar alcohols</a>, <a href="http://en.wikipedia.org/wiki/Sweeteners" title="Sweeteners" class="mw-redirect" >sweeteners</a>)</td>
<td align="center" >10</td>
<td align="center" >2.4</td>
</tr>
<tr>
<td ><a href="http://en.wikipedia.org/wiki/Dietary_fiber" title="Dietary fiber" >Fiber</a></td>
<td align="center" >8</td>
<td align="center" >2</td>
</tr>
</tbody></table>
<p >All the other nutrients in food are noncaloric and are thus not counted.</p>