“Starve a fever and feed a cold”: feeding and anorexia may be adaptive behavioral modulators of autonomic and T helper balance

"Feed a Cold, Starve a Fever?" A Review of Nutritional Strategies in the Setting of Bacterial Versus Viral Infections

PURPOSE OF REVIEW

Some data, mostly originally derived from animal studies, suggest that low glucose intake is protective in bacterial sepsis but detrimental in overwhelming viral infections. This has been interpreted into a broad belief that different forms of sepsis may potentially require different nutritional management strategies. There are a few mechanistic differences between the host interactions with virus and bacteria which can explain why there may be opposing responses to macronutrient and micronutrient during the infected state. Here, we aim to review relevant evidence on the mechanisms and pathophysiology of nutritional management strategies in various infectious syndromes and summarize their clinical implications.

RECENT FINDINGS

Newer literature - in the context of the SARS-CoV-19 pandemic - offers some insight to viral infections. There is still limited clinically applicable data during infection that clearly delineate the role of nutrition during an active viral vs bacterial infections. Based on contrasting findings in different models of viruses and bacteria, the macronutrient and micronutrient needs may depend more on specific infectious organisms that may not be generalizable as bacterial versus viral. Overall, the metabolic effects of sepsis are context dependent, and various host-specific (e.g., age, baseline nutritional status, immune status, comorbidities) and illness variables (phase, duration, and severity of illness) play a significant role in determining the outcome besides pathogen-specific (virus or bacterial or fungi and combined infections) factors. Microbe therapy (probiotics and prebiotics) seems to have therapeutic potential in both viral and bacterial infected states, and this seems like a promising area for further practical research.

Fever: Could A Cardinal Sign of COVID-19 Infection Reduce Mortality?

With mortality rising from the COVID-19 pandemic, we may be overlooking a key aspect of the immunological response. Fever is a cardinal sign of this rampant infection; however, little attention has been paid towards how a fever may work in our favor in overcoming this disease. Three key aspects of patient care - fever, fluid, and food - can be harmonized to overcome COVID-19 infection. Both animal and human studies have demonstrated that fever suppression during viral infections, either through low ambient temperatures or antipyretic use, may increase morbidity and prolong the illness. As fever rises, so do antidiuretic hormone levels, leading to solute-free water retention - making conservative fluid management essential. Finally, fever inhibits gastrointestinal function as energy is reallocated to the immunological response, underscoring the need to work in concert with these physiological changes. An opportunity awaits to investigate this natural barrier to infection, let us not pass it by.

Sickness behavior, its mechanisms and significance

Recent studies have begun to clarify the pathogenesis of sickness behavior. Cytokines released by macrophages, dendritic cells and mast cells act on the brain to trigger behavioral changes in infected animals. The major cytokines, interleukin-1, tumor necrosis factor alpha, and others, all act on the hypothalamus to provoke alterations in the normal homeostatic condition. These include elevated body temperature, increased sleep, and loss of appetite as well as major alterations in lipid and protein metabolism leading to significant weight loss. Some of these changes are clearly directed towards enhancing the normal immune responses. The benefits of others such as appetite loss are unclear. It is also important to recognize that other animals may recognize sickness behavior as a sign of weakness and mark the victim out for targeting by predators. As a result, some prey species may work very hard to mask their sickness, a response that serves to complicate veterinary diagnosis.

Pathophysiology of cancer cachexia: much more than host-tumour interaction?

Cancer cachexia is a syndrome of progressive nutritional depletion which causes significant morbidity and mortality in cancer patients. One of the main pathogenetic mechanisms underlying cancer cachexia is a complex interaction between the host and the tumour. Tumour cells interact with host cells within the tumour mass resulting in the production of catabolic mediators which degrade host tissue. In addition, the host may mount an aberrant metabolic response to the tumour. However, in recent years, it has also been understood that patient factors, including age and levels of physical activity, and the specific mechanics of protein metabolism in cancer patients may also have a significant impact. In this review article, we not only summarise previous knowledge surrounding host-tumour interaction, but we also discuss these broader concepts in the pathogenesis of cancer cachexia. Clinicians should consider such concepts in the design of an effective multimodal therapy for cachexia.

Growth failure after recurrent fever in young guinea pigs

Infection causes fever and suppression of appetite, a combination of effects which threatens normal growth in infected children. We have used an animal model to study the effects on growth of recurrent simulated Gram-positive bacterial infection. After weaning, 10 guinea pig pups underwent surgery under general anaesthesia for the implantation of temperature-sensitive radiotelemeters and thereafter were assigned to receive intramuscular injections of either 50 microg/kg muramyl dipeptide (MDP), or sterile saline. During a 30-day period corresponding to their rapid growth phase, the pups were given eight injections. MDP resulted in fevers of about 1.5 degrees C on each occasion, but no significant change in body temperature occurred after saline injections. Food intake was suppressed during each febrile episode such that 24-h intake was significantly lower on days of injections of MDP, compared to days between MDP injections in the same animals, and compared to that of animals injected with saline. The rate of weight gain of the MDP-injected guinea pigs was significantly lower than that of the control group and failed even to achieve a rate similar to the saline-injected group in their more adult-like growth phase. Plasma zinc concentration was significantly lower in MDP-compared to saline-injected animals sampled 8 days after the last injection. Our results show that recurrent fever during the growth phase of young guinea pigs results in irreversible growth failure, and that reduced food intake on days when the animals were febrile was at least partly responsible for this reduced rate of growth.

A model for predicting feed intake of growing animals during exposure to pathogens

A general model is proposed for predicting the effects of subclinical pathogen challenges of different doses and virulence on the relative feed intake (RFI) of animals. The RFI is defined as the feed intake (FI, kg/d) of the animal challenged by a pathogen divided by its FI in the same state had it not been challenged. Actual FI can be predicted from the RFI and the animal's state. The RFI was assumed to be affected only when animals were naïve to a particular pathogen (i.e., had not previously experienced it) and when the challenge dose was above a predetermined threshold. The model is for the period from recognition of a pathogen through acquisition and subsequent expression of immunity. The way in which RFI changes with time is described by 5 main parameters and is based on data for RFI during different pathogen challenges of a range of hosts. Lag time (L, d) is the delay from a pathogen challenge until any effects on RFI are seen. Reduction time (R, d) describes the time it takes for the lowest value of RFI (lambda) to be achieved. The duration time (D, d) describes the time that lambda is maintained for, and rho (RFI/d) describes the rate of recovery of RFI until RFI = 1. There is no compensatory intake, and RFI is always < or = 1. The effects of host resistance on the values of the model parameters are proposed. Attempts were made to parameterize the model; when data were scarce, initial parameter values were derived on conceptual grounds. Predictions of the effects of pathogen dose, virulence, and host resistance are described and discussed. When comparing the responses in RFI for different genotypes, it is crucial to define the pathogen challenge (in terms of dose and virulence) and the degree of resistance of different hosts. Possible interactions between dose, virulence, and resistance were explored. Feed intake of healthy and challenged animals, at a time, may be different once the challenged animal has recovered (RFI = 1). The issue of reductions in FI during pathogen challenges is important for nutritionist and animal breeders. The large variation that has been observed for reductions in FI during pathogen challenges may be a viable point of selection. The points highlighted will aid selection strategies by quantifying the effects of pathogen dose and virulence, and time, on the FI of challenged animals. The proposed model may be integrated with other models of growth to predict animal performance during exposure to pathogens.