Using Body Fat for Energy

Since I got back my Bod Pod number of 7.5% body fat I’ve been concerned about having sufficient energy for exercise. Due to the Low Carb diet, the only fuel source I have is dietary and body fat. I have always noticed that my performance during CrossFit is low compared to other athletes, and the little bit of carbohydrates I eat are being used to make glucose for my brain and those other cells which require glucose.

From this study (Journal of applied physiology (Bethesda, Md. : 1985). 88. 1707-14. Substrate metabolism during different exercise intensities in endurance- trained women. Romijn, Johannes & Coyle, Edward & Sidossis, Labros & Rosenblatt, J & Wolfe, R.R. 2000.):

Our laboratory has previously shown that changes in the percent body fat result in altered lipolytic sensitivity that makes individuals with low body fat better able to mobilize fatty acids than might be anticipated otherwise. Thus lipolytic responsiveness is enhanced in subjects with low body fat and decreased in obese subjects.

This may also affect the Alpert study numbers (Hypophagia – How much fat can I lose in a day?).

Lower Performance When Fat Fueled (and Higher Heart Rate)

This one goes against the Low Carb/High Fat diet for athletic performance (J Physiol. 1996 Apr 1; 492(Pt 1): 293–306. Interaction of training and diet on metabolism and endurance during exercise in man. J W Helge, E A Richter, and B Kiens).

Ten untrained young men ingested a carbohydrate-rich diet (65 energy percent (E%) carbohydrate, T-CHO) and ten similar subjects a fat-rich diet (62 E% fat, T-FAT) while endurance training was performed 3-4 times a week for 7 weeks. For another 8th week of training both groups ingested the carbohydrate-rich diet (T-CHO and T-FAT/CHO). 2. Maximal oxygen uptake increased by 11% (P < 0.05) in both groups after 7 and 8 weeks. Time to exhaustion at 81% of pre-training maximal oxygen uptake increased significantly from a mean (+/- S.E.M.) of 35 +/- 4 min to 102 +/- 5 and 65 +/- 7 min in T-CHO and T-FAT, respectively, after 7 weeks (P < 0.05, T-CHO vs. T-FAT).

Evidence that the High Fat group was using fat as their fuel:

During exercise after 7 weeks, the respiratory exchange ratio (RER) was unchanged in T-CHO (0.88 +/- 0.01) compared with pre-training but decreased (P < 0.05) to 0.82 +/- 0.02 in T-FAT.

Heart rate is of particular interest to me:

During exercise, plasma noradrenaline concentration and heart rate were higher in T-FAT than in T-CHO both at 7 and at 8 weeks.

This matches my own experiences with exercise where I see high heart rates.

Heart rate increased progressively in both groups during exercise
to 191 +/- 6 beats/min in T-FAT and 181 +/- 6 beats/min
in T-CHO at exhaustion.


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