Meal Timing, Glycemic Index & Load: Human Study Probes Whether "Hitting Your Macros" Really is All That Counts
|High or low GI, carbs in the morning or in the evning, cookies and dingdongs or all bran. So many questions and way too many answers from rodent studies or studies in obese diabetics... but what are Mr. and Mrs. Healthy Average Joe supposed to do?|
- Will a large evening energy and carbohydrate load cause an increase in postprandial glucose that is comparable to the same amount of energy and carbohydrates in the morning?
- Will a high glycaemic excursions in the evening be ameliorated by decreasing the glycaemic index (GI) of the meal?
White bread king or all-bran pauper - is that the question?
To answer this world-shattering question and actually prove their hypothesis that both, i.e. having carbs in the evening and having those in the form of high glycemic index foods, will have negative consequences on postprandial glycemia, the scientists picked six healthy volunteers (four females, two males; mean age 30 +/- 4.3 years, BMI 21·6 +/- 1.3 kg/m²) and randomly assigned them to a follow one of the four following dietary protocols:
- Low GI (average GI = 34), with the majority of energy load consumed in the morning (LGI-am)
- Low GI, with the majority of energy load consumed in the evening (LGI-pm)
- High GI (average GI = 84), with the majority of energy load consumed in the morning (HGI-am)
- High GI, with the majority of energy load consumed in the evening (HGI-pm)
|Figure 1: Composition of the two test diets (low GI, blue; high GI read) and individual macronutrient breakdown of the test meals the subjects consumed on two seperate occasions (based on Morgan. 2012)|
*note: The scientists probably chose similar foods for breakfast and dinner, because the study design required those to be exchangeable.Against that background it is still astonishing how much of a difference...
- 99% higher fiber content,
- -60% lower glycemic index (GI), and
- -63% lower glycemic load (GL)
"Glucose and insulin responses showed broadly similar patterns. Both meal timing and quality of carbohydrate affected postprandial glucose and insulin responses (P < 0.01). The area under the glucose and insulin response curves was greatest for the HGI-pm meal regimen. The HGI-pm meal regimen produced a significantly greater postprandial area under the glucose curve than for any of the other three meal regimens (P < 0.05). The postprandial area under the insulin curve was significantly greater than both the LGI regimens (P < 0•05). Postprandial insulin resistance measured by homeostatic model assessment was also significantly greater for the HGI-pm meal than for the two LGI meals (P < 0•05)."However, since Morgan, Shi, Hampton and Frost also state that "[p]ostprandial TAG and NEFA levels were not affected by meal timing or carbohydrate quality", I do suspect that there is a typo in table 3 of the original study, where it says that the TAG would be 5.04 mmol/l x h (probably is 6.04) and thus more than 15% lower than the average (TAG levels and insulin resistance usually go hand in hand, so it is really very unlikely that the 5.04 mmol/l x h is correct).
So what's the take home message here?
The only question that still has to be answered would be "King or pauper? At least with regard to the former, the best thing I can to is to suggest you read both the posts on "Breaking the Fast" and the "Carbs Past 6PM Posts" (Part 1 & Part 2). When you have done that your perspective on the importance of breakfast and the purported fallacy of having a large dinner should already have changed. The things that are still left to do is not fool yourself into the false belief that you can pound whatever junk you want (as long as it fits your macros). As the glucose curve of the high GI arms (light color) in the figure above goes to show you, your body won't be happy when you get your "carb macros" from sugary junk.So if we assume that my assumption with respect to the triglyceride values in the originally published study are correct and we are simply dealing with a typo here, the next questions which arise here, are...
Stick to starchy (or "save carbs", if you will) and fruit. Use veggies to fill you up. Use coconut & olive oil and the fats that are already in your meats, fish and dairy products to achieve baseline fat intake of at least 40-50g (all together). Aim for a 100-120g carbohydrate basis, diverge towards the lower side, when your body fat is high, you can't train or you're dieting and towards the higher side, when you are already very lean, have a high training volume, or are trying to build muscle. Complement that with min. 20g of quality protein with each meal. Don't deprive yourself on any nutrient completely and ramp up the total amount of food (at the given ratio) to fulfill your energy requirements.In that, avoid processed food sand rely on whole foods, whenever possible (>90%),. Use food supplements* only where it makes sense, e.g. a protein shake post workout (*creatine for example would not be a "food supplement", since you can NEVER get the amounts that are necessary to supersaturate your stores from meat alone) and don't forget to live about all that "dieting" and thinking about the best ways to eat, please!
- What is / are the reason/s that the lipid metabolism did not suffer?
- How reliable is the HOMA-PP, i.e. the postprandial assessment of insulin sensitivity via the homeostasis model assessment?
- What does all this mean for you? Does meal timing not make a difference and are macros all that counts?
The absence of changes in lipid metabolism after one day on high vs. low GI diets w/ different meal timing patterns yields answer #1 to question (3): If you are healthy the occasional day with junk food won't hurt you as long as you keep the total amount of energy at bay and jump back on the "healthy diet" wagon the very next day.
On the other hand, if only a single day of high GI food consumption can have such a pronounced impact on the postprandial HOMA levels, this raises the question how reliable this "long term measure" of glucose sensitivity actually is. Obviously, you should not go to the doctor's office and have your HOMA measured, at a morning after a day with three SuperSize Meals from McDonalds (even if you have been fasting after supper at night before, as the participants in the study at hand did) - unless you want a prescription for meformin, of course ;-)
It would however be likewise unwise to "do everything right" for three (maybe even just one day) before you head to the doctor to get blood drawn, just to be able to rejoice over a HOMA reading that does by no means represent your "normal" insulin sensitivity. This may make your doctor happy and spare you getting ticked off, but could have you run around pre-diabetic unnoticed for months if not years - maybe so long until the first irreversible damage has already been done.
The high susceptibility of HOMA measures to acute dietary modifications yields answer #2 to question (3): If you want know where you stand, don't make last minute changes to your diet before you get blood drawn. After all, the 90:10 rule (better 95:5 rule ;-) applies both ways - the 90/95 days of consistent eating patterns will decide whether you are lean, muscular and above all healthy or fat, undermuscled and sick.
- Morgan LM, Shi JW, Hampton SM, Frost G. Effect of meal timing and glycaemic index on glucose control and insulin secretion in healthy volunteers. Br J Nutr. 2012 Oct;108(7):1286-91.