How essential is vitamin B3 to the diet?

According to one study:

It was calculated that if the excretory percentage of niacin metabolites in the urine were 60%, of the tryptophan ingested, the conversion factor would be a value of 67, meaning that is 67 mg of tryptophan is equal to 1 mg of niacin.

However, the study has a pretty low power with only 10 participants and an indirect measure of niacin.

Is it possible that a person could get all of their niacin from tryptophan? Probably - you just need to eat enough tryptophan, which is also an Essential Amino Acid. The body may preferentially keep Tryptophan (which it does not store) until Niacin runs out (which also cannot be stored).

organic chemistry - What is the effect of flash pasteurisation on Maillard reactions or production of AGEs in milk?

This is only an partial answer, as I do not have the time now to look for other references.

In the chapter about milk, Harold McGee's beautiful book "On Food & Cooking" says (pg. 21 of the 2004 edition, bold is mine):

Flavors from cooking
Low-temperature pasteurization slightly modifies milk flavor by driving off some of the more delicate aromas, but stabilizes it by inactivating enzymes and bacteria, and adds slightly sulfury and green-leaf notes (dimethyl sulfide, hexanal). High-temperature pasteurization or brief cooking - heating milk above 170°F/76°C - generates traces of many flavorful substances, including those characteristic of vanilla, almonds and cultured butter, as well as hydrogen sulfide. Prolonged boiling encourages browning or Maillard reactions between lactose and milk proteins, and generates molecules that combine to give the flavor of butterscotch.

He continues by speaking about the different types of pasteurization, and says (pg.22-23)

The third method of pasteurizing milk is the ultra-high temperature (UHT) method which involves heating milk at 265-300°F/130-150°C either instantaneously or for 1 to 3 seconds, and produces milk that, if packaged under strictly sterile conditions can be stored for months without refrigeration. The longer UHT treatment imparts a cooked flavor and slightly brown color to milk; cream contains less lactose and protein, so its color and flavor are less affected.
Sterilized milk has been heated at 230-250°F/110-121°C for 8 to 30 minutes; it is even darker and stronger in flavor, and keeps indefinitely at room temperature.

So, it appears as if indeed there may be some Maillard reaction going on in UHT, although it is probably minor (otherwise the milk would be much darker in color, as it is the case for sterilized milk).

The book also speaks about homogenization, but it does not mention anything specific about Maillard reaction there (homogenization is not carried on at high temperatures, so it does makes sense not to have it).

To answer your question:

Does this effect the nutritional value of the milk?

Well, according to this presentation (but, be warned, it does not cite any reference!) it seems that it does, although I don't think the Maillard reaction is specially involved.

http://www.slideshare.net/Aslal_saja/effect-of-uht-treatment

Specifically it says that the irreversible changes that occur are:

• Whey protein denaturation (35 to 100% of denaturation of $\beta$-lactoglobulin)


• Slight increase in the size of casein micelles


• Whey protein-casein micelle interaction


• Maillard reactions


• Fat globule composition changes (although no changes in nutritional value of milk fat)


• Vitamin content changes (20-30% loss in vitamin B1 and B12. Markedly reduced content in vitamin C and folic acid if high level of oxygen were present during the treatment, no changes in vitamins A, D, E and beta-carotene).

It also says that:

Biological value and net protein utilization of UHT milk were lower for stored milk than directly used milk. Lysine loss during UHT is high but not significant to affect milk nutritional values.

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I don't know whether the fact that you have problems when drinking UHT milk has anything to do with Maillard reaction or any of the other modifications brought by UHT treatment, and I will not venture in there, as I am not an MD and prefer not to give any sort of medical advice.

cardiology - How quickly can the human heart rate rise and fall?

How quickly can the human heart rate rise and lower?

For example lets say a human heart rate is rested and is at 60BPM and that person is suddenly scared to trigger their fight or flight reaction. Lets say their heart rate rises to double (120BPM).

From the above example their rate has gone from 1000ms between beats to 500ms between beats. Can the human heart instantly in one heart beat go from the 1000ms to 500ms between beats or does it need to ramp up? If yes how quickly can the heart rate ramp up?

I understand that each human heart is different, and that the speed increase and decrease will be different from person to person. What I'm looking for is a value that I can safely say the human heart won't exceed.

Similarly the same question goes to your heart going lower.

genetics - Knockdown of long noncoding RNA (lncRNA) - how is it done?

Knockdown of lncRNA in mammals is not done via RNAi. Instead, one transfers antisense DNA oligos which bind to the RNA. This triggers the action of the RNase H enzyme, which degrades RNA-DNA duplexes. It degrades the lncRNA.

UPDATE: For reference, I learned about this from a seminar, and it is not very well documented, but after some literature search I found this paper to quote:

Although we (Fig. 3A) and others (42, 49) have used siRNA to knock down NEAT1 lncRNA and although the knockdown of strictly nuclear RNAs has been well described (53, 54), we remained concerned that because NEAT1 is mostly a nuclear RNA (55), it may not be very efficiently targeted by the RNAi machinery. Nuclear RNAs, however, are proficiently targeted using complementary antisense (AS) oligodeoxynucleotides that recruit nuclear RNase H activity to degrade the RNA (56). Hence, we also employed complementary AS oligodeoxynucleotides to knock down NEAT1 lncRNA in HIV-1 NL4-3-infected Jurkat cells (Fig. 4C, left). The AS approach reduced NEAT1 (Fig. 4C, left) and increased HIV-1 p24 production (Fig. 4C, right), providing results consistent with those from siRNA-mediated knockdown of NEAT1 (Fig. 4A and B).

neuroscience - What happens during a Raynauds episode?

Raynaud's phenomenon can be a serious health issue, as the blood flow to the extremities, mainly the fingers is compromised, causing fingers to blanche, and then turn blue. Severe Raynaud's can cause gangrene in illnesses like progressive/ diffuse scleroderma.

I am not asking about connective tissue disease per se, but what occurs during an actual Raynaud event (for a better way of putting it), not what precipitates the condition to occur within a particular person and not another.

what are the underlying biological processes that cause the blood flow to be compromised like this?

zoology - Is there such thing as "meters per calorie" for living organisms?

Halsey & White (2012) Comparative energetics of mammalian locomotion: Humans are not different. Journal of Human Evolution 63:718–722

This paper presents a comparison of the metabolic cost of walking and running in humans, Australopithecus and other mammals. They use a parameter NCOT (net cost of transport), whose units are ml O₂ consumed m^-1. The inverse of this parameter would be related to your metres per calorie.

The authors quote an NCOT of 12.77 ml O₂ m^-1 for a walking human. Using a value of 4 kcal g^-1 for glucose, I calculate that this NCOT is equivalent to 14.6 m kcal^-1.

Here I found an energy consumption value of 5.8 kcal min^-1 (for a 73 kg man) walking at 6.4 km h^-1, which translates to 18.5 m kcal^-1.

Incidentally, according to the authors of the paper:

The predicted net cost of transport (NCOT, ml O₂ m⁻¹) of mammals is related to mass (M, kg) according to 0.54M^0.70.

human biology - Does the brain and the body use the same energy source (glucose/ATP)?

While in general the answer is "yes", the brain does use the same type of resources as other parts of the body, you cannot from that alone derive conclusions about the brain competing with other organs. Also, I think that cognitive performance is not a function of availability of resources. In fact, the moment that the brain lacks crucial resources like oxygen, it simply shuts down. But you don't faint when you start running (if you do, go see a doctor). "Power-saving mode" for the brain is not "reduce IQ by 20 points", but "just make sure the body is breathing and shut down everything else".

Also, I do not think that digesting food requires any comparable amount of energy that it would require the brain to give up on some of the oxygen it needs.