How sustained prepartum energy overfeeding programs metabolic dysfunction at calving in dairy cows remains mechanistically unresolved. Janovick, Boisclair and Drackley (2011) showed that Holstein cows overconsuming energy (about 150% of requirements) during the dry period were hyperglycemic and hyperinsulinemic before calving and accumulated more liver triacylglycerol, mobilized more body fat, and suffered more ketosis and displaced abomasum after calving, and they listed five unresolved issues. Two of the original issues were substantially resolved by follow-up work - the parity-versus-exposure-duration confound (matched-duration trial of Richards et al. 2020) and the hepatic biomarker relationships such as paraoxonase and albumin (Janovick et al. 2023) - while the mechanistic core remains open: (1) the physiological signals by which long-term overfeeding alters tissue responses at calving (insulin-sensitivity evidence is mixed; glucocorticoid, growth-hormone, and catecholamine action at calving is untested; the diet-independent parturition spike in hepatic pyruvate carboxylase is still unexplained), and (2) the causal direction and mediators linking periparturient nonspecific inflammation, insulin resistance, and hepatic triacylglycerol accumulation, which reviews from 2020 to 2026 still describe as missing or incompletely understood. The response of lower-body-condition primiparous cows to graded prepartum energy intake also remains without a dedicated trial.

Background

The transition (periparturient) period of the dairy cow - roughly the three weeks before through three weeks after parturition - is when most metabolic and infectious disease of the lactation occurs. At calving, the cow must suddenly support milk synthesis at a time when feed intake cannot meet energy demand, so body reserves are mobilized: adipose triacylglycerol (TAG, the storage form of fat) is hydrolyzed to nonesterified fatty acids (NEFA), which the liver partly oxidizes to ketone bodies such as beta-hydroxybutyrate (BHBA) and partly re-esterifies back into TAG that accumulates in hepatocytes. Excess hepatic TAG is the lesion called fatty liver (hepatic lipidosis), graded from tissue lipid content in biopsy samples, and it is associated with ketosis (a metabolic disease in which ketone bodies accumulate in blood and milk, typically with loss of appetite), displaced abomasum (a common digestive disorder in which the cow's fourth stomach compartment twists out of place), impaired immune function, poor fertility, and culling. Because the antecedents of this failure accumulate before calving, attention turned to the dry period (about 60 days of non-lactation before the next calving). Experiments in the 1990s-2000s at Wisconsin, Illinois, and elsewhere showed that allowing cows to over-consume energy during the dry period - which happens easily when energy-dense close-up diets are offered for ad libitum intake - worsens postpartum metabolic status relative to restricting or controlling intake.

Janovick, Boisclair, and Drackley (2011) compared three prepartum energy planes in primiparous (first-lactation) and multiparous Holsteins: a bulky, wheat-straw-diluted controlled-energy diet fed ad libitum at about 100% of National Research Council energy requirements (CON); and a single higher-energy diet fed either ad libitum, so that cows voluntarily overconsumed to about 150% of requirements (OVR), or restricted to 80% of requirements (RES). Overfed cows were hyperglycemic, hyperinsulinemic, and had higher circulating leptin (an adipose-derived hormone that signals energy reserves to the brain) prepartum, then had higher NEFA and BHBA, greater liver total lipid and TAG, and more ketosis and displaced abomasum postpartum; controlled and restricted intake gave strikingly similar metabolic profiles. The authors interpreted this as an 'overnutrition syndrome' resembling the insulin resistance of obese, diabetic nonruminants - insulin being the hormone that normally suppresses adipose lipolysis and promotes glucose uptake, with insulin resistance denoting a blunted tissue response to it.

The paper's discussion posed five unresolved issues: (1) the physiological signals and pathways by which long-term prepartum overfeeding alters tissue responses at calving, including sensitivity to insulin, glucocorticoids, growth hormone, and catecholamines, and the regulation of gluconeogenic enzymes - notably the cause of a sharp spike at parturition in hepatic pyruvate carboxylase (PC, the rate-controlling enzyme of the first committed step of gluconeogenesis, which in cattle is essentially confined to the liver); (2) whether the low-grade, 'nonspecific' inflammation seen around parturition (hepatic expression of proinflammatory cytokines such as tumor necrosis factor alpha and interleukin-1 beta, and acute-phase proteins - liver-derived blood proteins whose concentrations shift during inflammation) is causally related to hepatic TAG accumulation and metabolic dysfunction; (3) whether observed parity differences reflected parity itself or the unequal duration of experimental-diet exposure (multiparous cows were fed about 65 days, primiparous only 35-40); (4) whether primiparous cows of lower body condition score (BCS, the 1-5 subjective measure of fatness) would show the same metabolic profiles across energy planes; and (5) clarification of clinical biomarkers of hepatic dysfunction (for example the relationships of prepartum intake, hepatic lipid, and circulating albumin or paraoxonase, an enzyme carried on high-density lipoprotein that serves as a positive marker of liver function) for on-farm risk prediction.

Subsequent work has settled issues (3) and (5) in outline: Richards et al. (2020) fed cows and heifers identical diets for the same 60-day prepartum exposure and found primiparous animals responded much like multiparous ones, and Janovick, Trevisi, Bertoni, Dann, and Drackley (2023) quantified the requested serum biomarker relationships on the original trial's serum, finding paraoxonase negatively and ceruloplasmin positively associated with liver lipid accumulation and poorer composite liver-function indices in overfed cows. The mechanistic core, however, is unresolved. Intravenous glucose-tolerance tests (IVGTT, the standard challenge in which glucose is injected intravenously and the clearance of glucose and the insulin response are tracked) have given mixed evidence that dry-period overfeeding impairs insulin sensitivity at all: Mann et al. (2016) found no effect of overfeeding on IVGTT parameters or on adipose insulin-signaling proteins, Salin et al. (2017) found no whole-body insulin-sensitivity difference, whereas Jaakson et al. (2018) found that overconditioned (though not necessarily overfed) cows developed prepartum adipose insulin resistance with reduced glucose transporter 4 (the insulin-responsive carrier that moves glucose into muscle and adipose tissue). The PC spike at parturition was re-observed by Janovick, Dann, Loor, and Drackley (2022) in all energy groups without an explanation of its signal. And the inflammation-fatty liver question remains correlational: an intramammary endotoxin (lipopolysaccharide, LPS) challenge interacted with prepartum overfeeding to sustain liver TAG (Graugnard et al. 2013), interleukin-1 beta expression in overfed cows peaked with the liver lipid peak (Janovick et al. 2022), but reviews through 2026 still state that a clear causal link is missing and the field is actively divided over direction - one school proposing that lipid mobilization drives inflammatory signaling, another that immune activation drives the reduction in feed intake and hence the lipid mobilization. The problem below is this surviving mechanistic core, together with the unanswered lower-BCS primiparous sub-question.

Problem Statement

Determine the mechanisms by which the plane of prepartum energy intake programs metabolic health at calving in dairy cows, as left open by Janovick, Boisclair, and Drackley (2011) and unresolved by the 2011-2026 literature, specifically: (1) identify the physiological signals and pathways by which sustained prepartum energy overconsumption (ad libitum intake supplying roughly 150% of stated energy requirements through the dry period) alters tissue responses at calving - including tissue sensitivity to insulin, glucocorticoids, growth hormone, and catecholamines, and the regulation of hepatic gluconeogenic enzymes, in particular the signal responsible for the sharp, diet-independent spike in hepatic pyruvate carboxylase expression at parturition; (2) establish the temporal and causal relationships among prepartum energy intake, the low-grade nonspecific inflammation of the periparturient period (hepatic proinflammatory cytokine expression such as interleukin-1 beta and tumor necrosis factor alpha, and circulating acute-phase proteins), insulin resistance, and hepatic triacylglycerol accumulation - that is, whether inflammation is a cause, a consequence, or a parallel accompaniment of fatty liver development and through which mediators; and (3) as a remaining empirical sub-question from the same source, determine whether primiparous cows of lower body condition score respond to graded prepartum energy intake with the same metabolic profiles as the relatively higher-condition primiparous cows originally studied. A resolution must address the mechanistic core in sub-questions (1) and (2); sub-question (3) is a separate empirical sub-goal rather than a component of that mechanistic account. The overarching objective is a mechanistic account of how prepartum energy intake programs transition-period metabolic dysfunction, sufficient to identify which animals are at risk and which physiological nodes - endocrine, inflammatory, or gluconeogenic - can be targeted to prevent fatty liver and ketosis. The parity-versus-duration confound and the clinical-biomarker relationships from the original five-part formulation were substantially resolved by later work and are excluded from the open core.

References

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