Can I ask a few questions about malate:
- Would it be used when a person is in ketosis?
- Does it have have a caloric value of its own or is it considered as a catalyst/co-factor for burning energy?
- Are there any substances that might be used disproportionately a) when relying on malic acid or b) when malate is in short supply?
Sure! I’ll do my best to answer—as I’ve already stated, I’m not an expert on metabolism, I just have been doing quite a deep dive for a few months:
1) Malate is pretty much always being used, though there might be some shifts in various organ systems depending on factors such as nutrient availability. It will still play its role of transferring reducing agents to the mitochondria regardless.
From what I remember, malate is also involved in gluconeogenesis and it’s been suggested that it would be preferentially used for this in situations where fatty acid oxidation is increased (i.e. in carbohydrate deprivation).
However, this is something that would be happening only in the liver—it seems to be unknown how exactly malate might be affected more broadly in that situation.
2) The only time it ends up as a “fuel source” is when it is part of gluconeogenesis and eventually becomes a glucose molecule. Still, even in that scenario, it’s not malate itself that is being used.
Malate is really just one transient form of a molecule that’s constantly being changed as it goes through metabolic processes. (True for all molecules, obviously, but this is an example with a particularly high amount of constant flux and transformation). What’s special about the “malate form” is that this specific structure can cross the mitochondrial membrane.
That makes it an effective shuttle—once it crosses the barrier, one of the H atoms gets chopped off and passed off to NAD (becoming NADH) to be used in the electron transport chain. With one less H, the molecule-formerly-known-as-malate is now called oxaloacetate.
3) the second part can be answered much more readily than the first. In an absence of malic acid, what you would expect is an upregulation of shuttling through the G3P shuttle as well as increased glycolysis as compensatory mechanisms. You’d also probably expect increased fatty acid oxidation as compensation for ATP that is not being made through OxPhos.
For the first part, I don’t know of any examples in the literature where cells are forced to use malate when they wouldn’t otherwise. Something to that effect could possibly be happening in some cells—it’s sort of what I suspect from my experiences of extra brain fog with the malic acid. But there’s just nothing I can specifically draw on from the literature to answer that definitively.
Thanks for your questions!