Digestive absorption and available nutrient delivery
Digestive absorption describes the fraction of an ingested nutrient that moves from the intestinal lumen into the body for metabolism, storage, or tissue use. A digestive absorption estimator helps compare ingested amount, absorbed amount, unabsorbed loss, absorption efficiency, and available energy after absorption. This is useful because food intake alone does not always equal nutrient delivery.
Core formulas
When absorption efficiency is known, absorbed mass is calculated from ingested mass and the absorption percentage:
\[
\begin{aligned}
m_{\text{absorbed}} &= m_{\text{ingested}} \cdot \frac{\eta}{100} \\
m_{\text{lost}} &= m_{\text{ingested}} - m_{\text{absorbed}}
\end{aligned}
\]
When fecal or unabsorbed loss is known, absorption efficiency can be calculated directly:
\[
\begin{aligned}
m_{\text{absorbed}} &= m_{\text{ingested}} - m_{\text{lost}} \\
\eta &= \frac{m_{\text{absorbed}}}{m_{\text{ingested}}} \cdot 100
\end{aligned}
\]
Available absorbed energy is estimated by multiplying absorbed grams by the selected energy factor:
\[
\begin{aligned}
E_{\text{absorbed}} &= m_{\text{absorbed}} \cdot F_{\text{kcal/g}} \\
\Delta E_{\text{not absorbed}} &= E_{\text{ingested}} - E_{\text{absorbed}}
\end{aligned}
\]
Here, \(m_{\text{ingested}}\) is nutrient intake in grams, \(m_{\text{absorbed}}\) is absorbed mass, \(m_{\text{lost}}\) is unabsorbed or fecal loss, \(\eta\) is absorption efficiency, and \(F_{\text{kcal/g}}\) is the energy factor for the nutrient.
How to interpret results
A high absorption efficiency means most of the ingested nutrient is available for uptake and energy delivery. A lower value means more nutrient remains unabsorbed, which may represent reduced digestion, rapid transit, fat malabsorption, or a general malabsorption teaching case. Available energy is usually reported in kcal, while absorbed and lost nutrient amounts are reported in grams.
- Do not enter fecal loss greater than ingested amount.
- Use percentage mode only when absorption efficiency is already known.
- Use fecal-loss mode when intake and unabsorbed loss are known.
- Choose an appropriate kcal/g factor for custom nutrients.
Example: if 100 g of carbohydrate is ingested with 95% absorption, then 95 g is absorbed and 5 g is lost. With 4 kcal/g, absorbed energy is 380 kcal.
The digestive absorption estimator is best used for physiology teaching, nutrient delivery comparisons, and meal-level absorption scenarios. It should not be treated as a clinical diagnosis tool; abnormal absorption patterns require additional context such as symptoms, stool studies, digestive enzyme function, intestinal transit, and medical evaluation.
