OK, hang on. I KNOW the length of the diet affects things, but I'm wondering what affects leptin more - the deficit or the length of time dieting.
Alright - edited to include results of search on PubMed: for anyone wondering why a smaller caloric deficit is better than a BIG one:
Clinical implications of leptin and its potential humoral regulators in long-term
low-calorie diet therapy for obese humans.
Miyawaki T, Masuzaki H, Ogawa Y, Hosoda K, Nishimura H, Azuma N, Sugawara A,
Masuda I, Murata M, Matsuo T, Hayashi T, Inoue G, Yoshimasa Y, Nakao K.
Department of Medicine and Clinical Science, Kyoto University Graduate School of Medicine,
Kyoto, Japan.
OBJECTIVE: To address the clinical implications of leptin and to re-examine the relationship
between leptin and its potential humoral regulators such as insulin, nonesterified fatty acids (NEFA)
and triiodothyronine (T3) in low-calorie diet (LCD) for obese humans. DESIGN: Longitudinal study.
SETTING: University and foundation hospitals. SUBJECTS: Ten obese men and 10 premenopausal
obese women. INTERVENTIONS: Five men and five women took 800 kcal/day LCD and another five
men and five women took 1400 kcal/day balanced deficit diet (BDD) during 4 weeks. RESULTS:
Plasma leptin levels in the LCD group decreased more markedly (46.2+/-14.6 to 13.2+/-3.6 ng/ml)
than that expected for the decrement in percentage fat (39.0+/-1.7 to 35.9+/-1.7%) and body mass
index (BMI; 35.4+/-2.4 to 33.1+/-2.2 kg/m(2)), while that in the BDD group did not decrease
significantly (14.9+/-3.5 to 13.4+/-2.8 ng/ml). The ratio of the decrease in leptin levels to that of
BMI during the first week was significantly greater than that during the following 3 weeks
(39.5+/-2.7 vs 29.3+/-2.1%, P=0.017). The plasma insulin and T3 levels also fell substantially in the
first week and continued to decrease during the entire course. Plasma leptin levels measured
weekly in each subject were correlated well with insulin (r=0.586, P=0.0003) and T3 (r=0.785,
P=0.0004). Multiple regression analyses after adjustment for the time course and BMI revealed that
serum levels of T3 were independently correlated with plasma leptin levels (r=0.928, P<0.0001). The
plasma NEFA level was markedly elevated during the first 2 weeks and decreased thereafter.
CONCLUSIONS: A rapid fall in leptin during the first week of LCD, coordinated by insulin, T3 and
NEFA, should be beneficial for responding to decreased energy intake. Inversely, in view of the
powerful effect of leptin on energy dissipation, the present findings suggest the potential usefulness
of leptin in combination with caloric restriction for the treatment of obesity. SPONSORSHIP: The
Ministry of Education, Culture, Sports, Science and Technology of Japan and the Ministry of
Health, Labour and Welfare of Japan. doi:10.1038/sj.ejcn.1601363