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No food Carbohydrate 6% Protein 12% Protein 24% Protein 40% Protein only

Meal content

Figure 2 The effect in rats of no meal, a carbohydrate meal with no protein, and meals with increasing amounts of protein on the plasma ratio of tryptophan to the large neutral amino-acids with which tryptophan competes for entry across the blood-brain barrier (cross-hatched bars), levels of tryptophan in the hypothalamus of the rat brain (hatched bars, expressed in mmolg"1), and levels of 5-hydroxytryptophan, an intermediate precursor of serotonin synthesis, in the hypothalamus (open bars, expressed in 0.1 mgg"1). The rise in tryptophan entering the brain after a carbohydrate meal drives increased serotonin synthesis, but this effect is progressively inhibited by increasing protein content.

No food Carbohydrate 6% Protein 12% Protein 24% Protein 40% Protein only

Meal content

Figure 2 The effect in rats of no meal, a carbohydrate meal with no protein, and meals with increasing amounts of protein on the plasma ratio of tryptophan to the large neutral amino-acids with which tryptophan competes for entry across the blood-brain barrier (cross-hatched bars), levels of tryptophan in the hypothalamus of the rat brain (hatched bars, expressed in mmolg"1), and levels of 5-hydroxytryptophan, an intermediate precursor of serotonin synthesis, in the hypothalamus (open bars, expressed in 0.1 mgg"1). The rise in tryptophan entering the brain after a carbohydrate meal drives increased serotonin synthesis, but this effect is progressively inhibited by increasing protein content.

5-HT, relative to fasted levels, in both rats and people (see Figure 2). Also, even pure carbohydrate does not appear to induce sleepiness in everyone.

Another difficulty in comparing the effects of carbohydrate and protein intakes is that relative changes in mood and performance might be due to a protein-induced increase in plasma tyrosine, the precursor amino-acid for synthesis of the catechol-amine neurotransmitters (adrenaline, noradrenaline, dopamine), which also competes with LNAA for entry into the brain. In catecholamine systems where the neurones are firing rapidly, acute physiological increases in brain tyrosine (e.g., by feeding a high-protein diet) can raise the tyrosine hydroxyla-tion rate and catecholamine turnover. Such systems include dopaminergic neurones involved in arousal, attention, and motivation. Nevertheless, high-protein meals in humans do not always raise the plasma tyrosine-LNAA ratio; the effect depends on nutritional status and time of day, for example.

Differential effects on performance have been seen with less extreme variations in protein and carbohydrate intakes. For example, a lunch of 55% energy as protein and 15% as carbohydrate produced faster responses to peripheral stimuli, but greater susceptibility to distraction, than eating the reverse proportions of protein and carbohydrate. Sleepiness was not affected by macronutrient composition in that study. However, with these protein-carbohydrate ratios, the plasma tryptophan-LNAA ratio could still be lowered by the protein-rich meal relative to the ratio after the carbohydrate-rich meal, even if tryptophan/LNAA does not rise from pre-meal levels after a carbohydrate-rich meal with much more than 5% protein (Figure 2).

A delay of at least 2h after eating may be necessary for changes in neurotransmitter precursors to influence behavior. Earlier effects may be related to changes in glucose availability and levels of insulin and counter-regulatory hormones such as adrenaline, glucagon, and cortisol. These changes could underlie recent results after breakfasts of 20:80, 50:50, and 80:20 protein-carbohydrate ratios (1.67MJ, 400kcal). A measure of central attention initially improved after the carbohydrate-rich breakfast, but later improved after the protein-rich ones; the opposite was found for peripheral attention. This study also found that the 80% protein breakfast produced the best short-term memory performance about 1-2 h after eating, but not at 3.5 h.

Effects of dietary fat Most studies of the effects of fat have varied its level together with that of carbohydrate, while keeping protein constant and so allowing equicaloric meals. Comparisons have been made of low-fat (e.g., 11-29% of energy as fat), medium-fat (e.g., 45% of energy as fat), and high-fat (e.g., 56-74% of energy as fat) breakfasts, mid-morning and midday meals, and intraduodenal infusions of lipid or saline. On balance, high-fat meals appear to increase subsequent fatigue and reduce alertness and attention, relative to high-carbohydrate/low-fat meals. However, there are inconsistencies in changes in specific moods and the effects of meal timing: for instance, feelings of drowsiness, confusion, and uncertainty were found to increase after both low- and high-fat lunches but not after a medium-fat lunch. One possibility is that mood may be adversely affected by meals that differ substantially from habitual ones in macronutrient composition. An alternative is that similar mood effects could be induced (albeit by different mechanisms) by high carbohydrate in one meal and high fat in the other: for example, 1.67 MJ (400kcal) drinks of pure fat or carbohydrate taken in the morning both increased an objective measure of fatigue relative to a mixed-macronutrient drink, although the two single-nutrient drinks had opposite effects on plasma tryptophan/LNAA ratios.

In many of these studies, the meals were designed to disguise variation in fat level from the participants. It is therefore possible that effects on mood may have resulted from discrepancies between subjects' expectations of certain post-ingestive effects and the actual effects that resulted from neurohormonal responses to the detection of specific nutrients in the duodenum and liver. A case in point may be the increase in tension, 90 min after lunch, with increasing fat intake reported by predominately female subjects: this might reflect an aversive reaction to (unexpected) fat-related post-ingestive sensations.

Postprandial declines in arousal can be quite noticeable 2.5-3 h after high-fat meals, but fat in mid-morning meals seems to be more sedating than fat ingested at lunchtime, which might relate to expectations. By comparison, when lipid was infused directly into the duodenum, a decline in alertness was apparent much sooner, by 30-90 min after the meal. These effects of fat may result from increased release of the gastric regulatory hormone cholecystokinin. However, in a study comparing ingestion of pure fat, carbohydrate, and protein (1.67 MJ (400kcal) at breakfast), measures of memory, attention, and reaction time deteriorated more after carbohydrate and protein than after fat. This beneficial effect of fat was attributed to the demonstrated relative absence of glycemic and hormonal (insulin, glucagon, and cortisol) perturbations in the 3 h following fat ingestion.

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Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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