01Resolve the names and species

GIP is used for glucose-dependent insulinotropic polypeptide and for the older name gastric inhibitory polypeptide. The IUPHAR/BPS human ligand entry identifies GIP(1–42) and links it to the GIP receptor.IUPHAR/BPS — Human gastric inhibitory polypeptide / GIP (opens in a new tab)

That entry also records sequence differences between human and rodent GIP. A species label therefore concerns the ligand sequence as well as the organism or cells used in the experiment.IUPHAR/BPS — Human gastric inhibitory polypeptide / GIP (opens in a new tab)

A study using a human receptor in a laboratory cell line is not a human clinical trial. Equally, the words mouse experiment do not tell you which peptide sequence was supplied. These details must be read separately.

02Truncation changes the functional question

Deacon and colleagues studied conversion of GIP(1–42) to GIP(3–42) through DPP-IV cleavage. The latter lacks the first two residues of the native sequence.Deacon and colleagues — GIP(3–42) at physiological concentrations (opens in a new tab)

Their experiments found weak antagonism by GIP(3–42) in cell and isolated-pancreas settings at sufficiently high concentrations. In the anaesthetised-pig comparison designed around physiological concentrations, adding the metabolite did not change the measured GIP responses.Deacon and colleagues — GIP(3–42) at physiological concentrations (opens in a new tab)

That is not a contradiction requiring one result to be discarded. The studies asked whether an effect appeared under different exposure and system conditions. An effect requiring a large excess in vitro need not dominate at physiological concentrations.

03The glucose state changes the endocrine response

Christensen and colleagues studied ten healthy men in a randomised, double-blind comparison of GIP and saline under low, normal and high glucose conditions.Christensen and colleagues — Glucose-dependent insulin and glucagon responses to GIP (opens in a new tab)

At high glucose, GIP strongly increased insulin secretion without a distinct glucagon response relative to saline. At normal or low glucose, it increased glucagon responses while having little or no insulin-secretory effect.Christensen and colleagues — Glucose-dependent insulin and glucagon responses to GIP (opens in a new tab)

The important comparison is within the defined glucose state. Combining the high-glucose insulin result and low-glucose glucagon result into one unconditional statement would describe a response pattern the experiment did not test.

Also retain the timing: in the low-glucose condition, the reported early glucagon response differed even though later peak levels were similar. An early response and a peak value can support different descriptions.Christensen and colleagues — Glucose-dependent insulin and glucagon responses to GIP (opens in a new tab)

04Build an evidence map that keeps the dependencies

For each paper, begin with the ligand sequence and any truncation or modification. Add glucose conditions, receptor species and the endpoint actually measured. This creates a usable comparison before a mechanism is proposed.

If two reports disagree, inspect those fields first. Different glucose levels, sampling windows or ligand forms can explain why apparently opposing summaries are not testing the same proposition.

A native GIP study also does not isolate the GIP contribution of a compound acting at several receptors. That contribution requires a comparison designed to separate the pathways.

The resulting reference can explain why GIP is biologically interesting without promising weight loss, improved performance or suitability of an unspecified preparation. Those are different questions requiring direct evidence.