Nutrition Uncategorized

Metabolic flux and nutrient partitioning

Phil Learney is a performance coach specialising in strength, conditioning and nutrition. He is based in London.

Metabolic flux is the term used to describe the rate in which molecules and metabolites are turned over in a metabolic pathway. Metabolic regulation is the term used for the actions of food and its transference into energy once ingested.

The three basic substrates we consume as energy are the macronutrients protein, carbohydrates and fats. Through destructive metabolism induced by being into a hypocaloric state (a catabolic process), we can break these down into smaller molecules. This process can only be accomplished with the presence of the relevant enzymes. The term we use for this transference of energy is ‘metabolic regulation’. This phrase is used for the actions of food and its transference into usable energy.

Once broken down and digested each substrate – demand dependent – can be oxidised as fuel or stored as metabolic fuel (metabolites) for later use in the forms of either muscle glycogen (carbohydrates) or triacylglycerol (fat).

Both protein and fat metabolites obtained from dietary sources and the body’s own stores have capacity for secondary fuel usage. This is one of the many reasons that these will be prioritised in any compositional diet.

Protein-sparing carbs
It is worth noting at this stage though that carbohydrates are ‘protein sparing’, because they take responsibility for providing glucose, taking the burden off protein and fats. This offsets the necessity for the body to utilise other substrates as a secondary glucose provider. This is great if the preservation of lean mass or temporal elevation of certain metabolic hormones is a priority or a necessity.

If fat loss is a priority this is counterproductive, so we run the risk of lean tissue becoming compromised. Fluctuations between a glucose-fed state and one of depletion would be best served for most successful strategies. This always remains a calorie-driven model but the fluctuations of protein and fats in a dietary strategy will be considerably less, in most cases, than that of carbohydrates, which we can use to manipulate energy systems easier.

During these processes, the energy released can be used or as previously mentioned stored as fuel by the muscle, fat, or liver cells. If the catabolic process of digestion is hindered then the availability of energy is also delayed. Anything that impairs digestion or absorption, or indeed increases the endogenous loss, will reduce the availability of immediate or sustained energy.

Net result
Therefore, two people who are on the same amount of macronutrients could be exposed to different amounts of useable energy available at different latency. The net amount, however, remains the same.
Because of the different thermodynamic properties of various macronutrients and the variance in efficiency, the notion that a calorie is a calorie means the laws of thermodynamics are and can be questioned.

Think of it like the bank putting a hold on your money. It is there, you just cannot spend it yet. It must be understood, however, that this does not detract from the basis of calories in versus calories out. It simply questions the efficiency, timing of availability and an individual’s efficiency in handling the substrate through its breakdown and absorption. It is often why you will observe different people responding at different stages to the same stimulus of nutrients.

Nutrient partitioning
Throughout the entire process of metabolism, metabolic pathways dictate the body’s requirements. In different physiological states, nutrients and metabolic substrates are divided or partitioned between tissues and organs through the expression of enzymes and proteins specific to that particular tissue. An example would be the one that provides the primary transport of glucose, GLUT-4.

GLUT-4 is found mainly in adipose tissue, skeletal and cardiac muscle tissue. When insulin is present, it stimulates glucose uptake into these tissues by mobilising GLUT-4 transporters from membranes within the cell to the membranes on the surface of the cell (plasma membrane).

This allows glucose from the bloodstream to bind to the GLUT-4 transporter and enter the cell. A decline in the insulin receptors that promote this uptake are hypothesised to be one of the major issues with insulin resistance, but it’s evident that in the presence of insulin resistance the glucose and amino acids that insulin may be carrying are unable to efficiently upload into the cell (like buffering on your computer).

In the physique development world, this pathway needs to be efficient if we are to efficiently build lean tissue and positively alter body-fat levels.

Recorded delivery
Think of your body like a recorded mail service sending relevant packages (substrates) to relevant addresses (areas). To ensure the package arrives there must be efficient transportation and reception. Upon arrival at its destination the receptors must be welcoming or sensitive to the material arriving if not the cells will be at a stage of what we call resistance.

In this case the metabolic substrate will be forced to stay outside the cell in the bloodstream until it either becomes welcoming. If not the substrate will eventually get packed up as a stored metabolite.
This is also how insulin got its unnecessary bad reputation as a ‘storage hormone’ because it does not just encourage the stimulation of GLUT-4 in muscle tissue but also fat tissue. A degree of insulin resistance means that the glucose that is unable to get into the cell has a high likelihood is will be converted into fatty acids and become stored fuel. Insulin is not to blame: it is simply just doing its job.

Insulin sensitivity
Insulin resistance is brought about largely by chronically poor dietary practices so this inefficient stage in partitioning must be improved by attempting to increase insulin sensitivity. The more efficient this becomes the easier it is to pack on muscle and keep body fat at bay. And the leaner someone becomes the more consistently efficient this process.

Nutrition partitioning, much like the management and production of insulin, is very specific to the individual and is one of the fundamental reasons why diets can never be generic in nature. The subtle variations between human beings in both their genetic makeup and the phenotype will influence the way in which substrates and calories are handled.

If we consider the androgen levels of an individual alongside chronic catecholamine (stress) responses each will respectively have an opposing effect yet be present at differing levels in an individual’s phenotype and genotype. Obesity, type 2 diabetes, and cardiovascular disease appear to share the same metabolic environment as insulin resistance and this chronic sub-acute inflammation.

Increase activity, reduce fat
At this stage, we could get engrossed in the scientific intricacies of these interactions. What we fundamentally aim to accomplish, aesthetically, with the majority of our clientele will give us the welcome by-products of decreased inflammation we desire.

Through the lowering of both subcutaneous and visceral fat, increasing overall activity levels, and the improvement of dietary practices, the sequela is improved insulin sensitivity, reduced inflammatory markers and a lowered risk of cardiovascular disease. Beyond these welcome by-products, we are undoubtedly looking at medical intervention that is beyond our scope of practice.

In amongst nutrient partitioning we must also consider the physiological parameter that dictates the compartmentalisation of fluid and the individual variability in this. Fluid can be broadly subdivided into two compartments: intracellular fluid (ICF) and extracellular fluid (ECF). These respectively amass 60–65% and 35–40% of total body water.

The role of salts
The electrolyte concentration, blood circulation, and the metabolism of tissue will all be factors in these levels. When considering electrolytes, sodium and potassium in particular will play an active role in these ratios because they are responsible partly for transporting fluid into cells, through osmosis. The areas of concern here will be the level of sodium in processed foods and perhaps a lack of good quality sodium in the diet.

Again, in reference to salt, we are up against a generic demonising of a critical component of an individual’s diet. The use of high quality non-refined salts within a non-processed diet is generally a good practice. The physical appearance of an individual may well be influenced by this and when considering more complex nutritional strategies these should be considered; however, they are beyond the realm of this article.

A key element missing from even the most basic of nutritional strategies, particularly when considering training is the inclusion of electrolytes and hydration. In and around the workout window, consuming fluid and electrolytes can serve to assist neural communication between cells and overall cellular recovery. The distribution of sodium and potassium across these different compartments plays a role in polarisation and the membrane potential of cells.