In my last post I talked about the inconsistencies and often the sheer stupidity of some of the nutritional advice we hear on a daily basis. One of the readers made a comment “I don’t understand the point she is trying to make”. Well, actually I’m not trying to send a big message here. My blog is where I write about what I see, comment on it (yes, sarcasm is my way of coping with some of the rubbish) and leave you to make up your own mind. Occasionally I throw a bit of science so you can see where I’m coming from. I do not spend hours doing an exhaustive literature search in Medline (I don’t have that luxury) but I’ve accumulated my fair share of information in the last 8 years of studying science and medicine and especially in the last year of learning about nutrition in particular. If you see a study that supports some of the things here feel free to send it my way. Ditto if you feel it directly contradicts it.
Anyway let’s get into it. Today we are talking about metabolism in children. This post will be an overview of the basics and probably quite dry and boring. I’ll try to throw in some graphs and cute baby pictures for those who like that kind of thing.
I love the topic of child nutrition for several reasons. First, I have one of those little people at home so I’m naturally interested in what to feed her. Second, I believe that what you put in your body in infancy and childhood (and possibly even before, in the womb) is going to directly affect your health and size in the future. Third (and favourite), when you talk about child and particularly baby nutrition you take away the so-called “gluttonous sloth” theory of obesity and disease. It’s ludicrous to accuse a 1 year old of a lack of willpower and laziness, so we are left with the bare bones of nutrition. And more than ever how that little body self-regulates its own nutritional status.
Metabolism (n) – the chemical processes occurring within a living cell of organism that are necessary for maintenance of life.
The Free Dictionary by Farlex
In other words, metabolism is the way your body processes the nutrients you receive with food. It’s tempting to look at food as pure energy but as we know calorie is not just a calorie. Food provides both the energy and the building blocks: 2 components required by each cell to perform its own unique function. In an adult this means you need to take in enough nutrition to provide energy for metabolic processes necessary for pure survival and also the materials for the continuous repair, regeneration, special circumstances (illness, pregnancy, muscle growth).
In children, one more component is added to the picture = growth. The food they eat has to supply enough energy for basic metabolic processes and activity, building blocks for daily repair and regeneration AND additional intake to allow for growth.
Intake = Basal Metabolic Rate + Activity + Growth
Growth does not just come from extra energy but from extra nutrients required to create/synthesise new tissues. This has a few implications which I want to discuss in more detail later.
What is physical growth? Seems a silly question as this is a phenomenon most of us are very familiar with. We also have an intuitive understanding that different tissues grow at different rates, which accounts for a changed body composition. The body shape transition from a chubby baby to a stocky toddler to a lean pre-teen to the sex-specific changes in adolescence and finally into adulthood is multi-dimensional with unique composition corresponding to each stage.
An average newborn with a weight of 3.0kg (6.6 bs) increases her own weight by over 300% to around 10kgs (22lbs) by the end of her first year. Her brain weight goes from 450-500g to over 1000g in the same period of time, a weight remarkably close to 1400g in an average adult.
If you look at babies’ body composition their perceived chubbiness turns out largely illusionary. The body fat measurements of a newborn average a very svelte 12-15%. The remainder is allocated a name of FFM = fat free mass which consists of organs, muscles, skeletal structure and extracellular fluid. In the table below you can see how body composition changes in the first 18 months: muscle mass and fat mass increase, extracellular fluid decreases. Organ weight increases in proportion to body weight in the first year of life therefore the relative percentage of brain, liver, heart and kidney remain similar.
Basal Metabolic Rate (BMR) in babies is determined mainly by the metabolically active tissues of the brain, liver, kidneys and muscles. The brain activity of a newborn is estimated at a whopping 80% of her BMR, dropping to about 60% through the first year as activity increases and muscle becomes more metabolically relevant. In addition to organs and muscles there is new evidence that some of the fat mass in babies and children is in form of BAT = brown adipose tissue which is more metabolically active and itself participates in energy expenditure. (Adults have some BAT too but in much smaller quantities).
So what have we learned so far?
The energy intake in babies and children has to account for their high relative BMR, allow for activity and also provide the energy for the process of synthesis of new tissues PLUS the energy that is deposited in new tissues. Once again:
Intake = Basal Metabolic Rate + Activity + Growth *
*For science geeks, I have left out the Diet Induced Thermogenesis for the sake of simplicity and uncertain contribution to energy balance in children
It’s official. Babies and children are energy vampires. This might sound very daunting for the food providers, the parents. How do you know how much to feed a child to ensure all these processes perform without a hitch? At the same time the growth of most infants follows a fairly predictable pattern. Whether they are breastfed on demand or formula fed by the hour, whether they are weaned onto rice cereal or meat, save for small variations in fat mass their FFM (fat free mass) will be very similar.
Lucky for us, parents, we do not have to hunt around for scientific papers estimating energy expenditure, calculate our child’s activity and measure out every spoonful. Under normal circumstances, babies seem to do quite well without science.
That is when everything goes well. What if it doesn’t? For the first time in human history we are way more concerned with OVERnutrition rather than UNDERnutrition. But it is probably too simplistic to completely separate these two. Next post I will look at malnutrition and the fascinating subject of catch-up growth and what it can teach us about childhood obesity.
Veldhuis JD et al 2005 Endocrine control of body composition in infancy, childhood and puberty. Endocrine Reviews 26(1): 114-146
Activity, energy expenditure and energy requirements of infants and children, Proceedings of an IDECG workshop held in Campbridge, USA, 1989
Holliday 1986 Body composition and energy needs during growth. In: Human growth: a comprehensive treatise, 2nd ed. Plenum press, NY 1986