Nutrition, digestion and health evidence definitions

Digestion and absorption in the human gut

Digestion

The hydrolysis of large biological molecules into small soluble molecules that can be absorbed across cell membranes.

Physical digestion

The mechanical breakdown of food into smaller pieces, increasing the surface area available to digestive enzymes without hydrolysing anything.

Absorption

The movement of the products of digestion from the lumen of the gut, across the epithelium, into the blood or the lymph.

Endopeptidase

An enzyme that hydrolyses peptide bonds within a polypeptide chain, producing shorter chains and so more ends. Pepsin, trypsin and chymotrypsin.

Exopeptidase

An enzyme that hydrolyses the peptide bond at an end of a chain, removing a single amino acid. Carboxypeptidases work from the end carrying the free carboxyl group, aminopeptidases from the amine end.

Dipeptidase

An enzyme, bound to the membrane of the ileum epithelium, that hydrolyses a dipeptide into two amino acids.

Co-transport

The transport of two substances across a membrane through the same carrier protein at the same time, one moving down its concentration gradient and providing the means for the other to move against its own.

Indirect active transport

Transport that is driven by a gradient which was itself established using ATP elsewhere in the cell. Glucose uptake in the ileum is the standard example.

Diet, energy and how to read a health claim

Cohort study

A group of people free of the disease is recruited, their exposure is recorded, and they are followed forward in time to see who develops it. Gives incidence and relative risk directly, and gets the time order right by construction.

Case-control study

People who already have the disease are compared with matched people who do not, and both groups are asked about past exposure. Fast and efficient for rare diseases; yields an odds ratio, not a risk.

Randomised controlled trial

Participants are allocated to an intervention group or a control group, often given a placebo, and the outcomes are compared. Allocation is by chance, so the groups tend to be balanced on every variable, including the ones nobody thought to measure.

Relative risk

The risk in the exposed group divided by the risk in the unexposed group; 1 means no association.

Conflict of interest

A financial or personal interest that could influence how a study is designed, analysed or reported. Journals require authors to declare them.

Publication bias

The tendency for studies with positive or striking results to be published more often than studies that found nothing, so the published literature overstates effects.

Meta-analysis

A statistical combination of the results of several studies of the same question into one estimate, with larger and more precise studies given more weight.

DCPIP

Dichlorophenolindophenol, a blue dye that becomes colourless when it is reduced. Vitamin C reduces it, so the volume of a sample needed to decolourise a fixed volume of DCPIP measures the sample's vitamin C content.

End point

The point in a titration at which the reaction is just complete, seen as a change that does not reverse, such as the last of the blue of DCPIP disappearing.

Cholesterol, atheroma and what a risk factor means

Atheroma

A plaque in the wall of an artery, consisting of lipid, foam cells and cell debris beneath a fibrous cap, which narrows the lumen and stiffens the wall.

Thrombosis

The formation of a blood clot inside a blood vessel, most often on a ruptured atheroma, which may block the vessel completely.

Aneurysm

A localised bulge in an artery wall that has been weakened, which may rupture and cause severe internal bleeding.