Continuing from my last post about metabolism in babies and children, today I will address what happens when a child goes through a period of inadequate food intake.
Viral or bacterial infections, prolonged hospital stay and chronic disease are some of the common causes of temporary malnourishment in Western children. Frank underfeeding is fairly rare though it still happens in certain extreme dietary lifestyles. While there are definitely differences between chronic malnutrition (the kind we associate with “the starving children in Africa”) and a week-long diarrhoeal illness in a typical Australian child, there are many similarities.
A few older studies (they weren’t too hampered by ethical considerations in those days) examined underfeeding in babies and children in some detail trying to establish how the body copes with the lack of energy and nutrients.
If you remember from the previous post, energy intake in children accounts for their BMR (basal metabolic rate), activity and growth.
Intake = BMR + Activity + Growth
As it turns out, the body puts a different priority on each of these components. Survival (BMR) comes first followed by Growth and then Activity. As a consequence, a malnourished child will first decrease activity levels.
If reducing energy expenditure does not compensate for inadequate intake then growth becomes affected. Within growth 2 parameters are easily measured: height (length in infants) and weight, both can be tracked along growth centiles on standard growth charts. A prolonged illness might result in the slowing of weight gain which will be reflected in drop down a centile. Weight loss comes both from fat loss and muscle loss. While the organs are relatively protected in minor illness, the muscle mass is not. Catabolism (breakdown) of muscle tissues has been reported even after measles immunisations and asymptomatic Q fever.
A profoundly malnourished child will also slow their height velocity possibly resulting in stunted growth. All of the above will happen way before the metabolic rate of individual organs is affected.
Reduction in activity in a malnourished infant may be subtle to the outside observer. The child does not necessarily lie motionless and exhausted in her cot but they might be less vigorous and inquisitive. An interesting study was conducted back in 1979 in a Mexican village. The investigators followed two groups of children: one was given supplementation of vitamins, minerals, strained foods and milk (in fact the diets of their mothers were supplemented in pregnancy as well), the other group was not. Supplemented babies were more active, cried less, spent more time out of their cribs. In one of the experiments 2 year olds were taken out onto a 3 x 3 quadrangle and their movements recorded for 10 minutes. Investigators recorded the movements of well-fed and malnourished infants. Some toys were placed in one end, their caregiver and two observers at opposite ends.
This is a tracing of the movements of a control child (no supplementation):
This is a tracing made by a supplemented child:
Whoa, somebody went a little crazy! Obviously the results could be related to both quality and quantity of the food given, better development in better fed infants, an uncertain deficiency in controls or some other factor. Nevertheless it is clear that malnutrition can affect the physical activity of children. One of the first questions a parent is asked by a pediatrician or a general practitioner is: “Is your baby happy and active?” This graph makes it clear why.
Anorexia, or loss of appetite, is something that most parents witness in a sick child. Spontaneous decrease in food intake is well described in scientific literature. What can be simpler: a sick child doesn’t feel like eating and loses weight as a result? However, while anorexia accounts for some of the weight loss, it does not explain all of it.
As I have mentioned before, children tend to lose muscle mass in addition to fat loss during illness. Muscle wasting, the two words that terrify any self-respected gym junkie, is a costly exercise in a child. Body composition is notoriously hard to measure in infants but we know that adipose tissue in 2 year olds can measure between 20-25%. So why don’t the children use their soft cushioning for energy when sick? As it turns out being ill makes it harder for the body to access fat stores. The signals from body’s own hormones and the activation of the immune system causes a switch to protein catabolism and utilisation of amino acids stored in the muscle for gluconeogenesis (creating glucose in the liver for release into the bloodstream).
Another mechanism by which acute disease contributes to the loss of muscle mass is the paradoxical increase in BMR, or hypermetabolism. Mechanisms of this are not entirely clear but it has been recorded across many studies. One of the main suspects is fever. The often quoted figure is the 13% raise in BMR for every 1 degree Celsius above 37° (It comes from an old study by Dubois, 1938, unfortunately I do not have a full text). However, these mechanisms cannot compensate for muscle wasting and decreased organ metabolic rate in chronic active disease and profound malnutrition. Protein can also be lost directly from the gastrointestinal tract via the process called protein-losing enteropathy due to a variety of infectious, inflammatory or congenital causes.
It may be helpful to think of energy status in malnutrition and disease as a balance between energy conservation and energy loss. The adaptive mechanisms that the body tries to use fail in the face of a serious and/or prolonged illness resulting in negative energy balance.
So let’s apply some of these factors to a hypothetical situation: an otherwise healthy 18 month old catches a nasty gastrointestinal bug, like Campylobacter, at his playgroup.
The first sign that something is wrong might be a slight decrease in activity. Vomiting and diarrhoea, energy-draining processes by themselves, prevent nutrient absorption. Appetite decreases and the actual food intake may stop altogether. Negative energy balance kicks off a conservation response and the child’s activity reduces even more. If the episode is complicated by high fever you can expect BMR to rise slightly. Reduced activity will only be able to compensate the energy deficit up to a point. Laying down new tissues for growth has already come to a halt. By this stage the body may start to break down muscle for glucose and fat stores for energy. The longer this infection hangs around the more muscle is wasted. BMR starts to reduce due to the loss of fat free mass. After a week or two of being unwell you have on your hands a lethargic possibly dehydrated child, with reduced fat and muscle mass and a reduced appetite.
If the adverse stimulus (=infection in this case) is not removed the road to chronic malnutrition with subsequent growth stunting and reduced organ metabolic rate continues. The resolution of infection sends a powerful recovery signal and catch up growth begins.
However, catch up growth pattern is not the same as the normal growth. After taking a step back, the body has to leap 2 steps forward to get back on track. And it seems to have some trouble doing that. In my next post I’ll discuss the catch up growth and how it’s linked to obesity.
If you have children of your own I apologise if this post was a little unsettling. When your baby is unwell the last thing you want is a little paranoid voice in your head whispering about the inevitable lean body mass loss etc. I will discuss my ideas about the optimal nutritional strategies for recovery a little later, I promise.
Scrimshaw NS, Energy cost of communicable diseases in infancy and childhood, Proceedings of an I/D/E/C/G Workshop held in Cambridge, USA November 14 to 17, 1989
Wiskin AE et al Energy expenditure, nutrition and growth Arch Dis Child 2011;96:567-572
Veldhuis JD et al Endocrine Control of Body Composition in Infancy, Childhood, and Puberty Endocrine Reviews 2005;26(1):114-146
Beisel WR Magnitude of the host nutritional responses to infection, American Journal of Clinical Nutrition 1977;30(8):1236-1247